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33 Commits

Author SHA1 Message Date
nobody
7cbf652c85 This commit was manufactured by cvs2svn to create tag
'Version_1_29_0'.

[SVN r15904]
2002-10-11 15:17:55 +00:00
Dave Abrahams
0e7e64c4a9 Note about broken 2.95.3 EH implementation :(
[SVN r15890]
2002-10-11 04:19:04 +00:00
Dave Abrahams
dae2e27a32 tweak
[SVN r15876]
2002-10-10 18:11:05 +00:00
Dave Abrahams
3744f8b4c1 tweak
[SVN r15874]
2002-10-10 18:09:37 +00:00
Dave Abrahams
e9a3d020f3 tweak
[SVN r15873]
2002-10-10 18:06:08 +00:00
Dave Abrahams
77b6388723 Move joel's updates over from the main trunk (finish)
[SVN r15870]
2002-10-10 17:52:30 +00:00
Dave Abrahams
a4912cebe0 Acknowledge Paul M and Aleksey G
[SVN r15868]
2002-10-10 17:04:57 +00:00
Dave Abrahams
956b0c5038 doc tweaks; added known-working-platforms page
[SVN r15867]
2002-10-10 16:45:45 +00:00
Dave Abrahams
682f175412 GCC 2.96 workaround
[SVN r15866]
2002-10-10 16:44:47 +00:00
Dave Abrahams
0dc693a9e3 Clean up RC branch some more
[SVN r15862]
2002-10-10 14:14:27 +00:00
Dave Abrahams
fa61758f32 Move joel's updates over from the main trunk
[SVN r15861]
2002-10-10 14:01:40 +00:00
Dave Abrahams
53cebbf024 Moved trunk revision over
[SVN r15857]
2002-10-10 13:10:51 +00:00
Dave Abrahams
64035277dc clean up v1 flotsam
[SVN r15856]
2002-10-10 13:05:52 +00:00
nobody
a8d37ebc4a This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15847]
2002-10-10 08:59:20 +00:00
Dave Abrahams
d5cfa0483a Clean up Boost.Python v1 flotsam, update documentation, prepare for release
[SVN r15829]
2002-10-09 16:58:35 +00:00
Dave Abrahams
9e21275595 Bug fix (thanks to Leonardo Rochael Almeida <leo@hiper.com.br>).
[SVN r15828]
2002-10-09 16:14:19 +00:00
nobody
0faabfb091 This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15827]
2002-10-09 14:31:40 +00:00
Dave Abrahams
ee6e678c95 Workaround GCC 3.x problem
[SVN r15791]
2002-10-07 20:13:00 +00:00
Ralf W. Grosse-Kunstleve
66358ecfd8 adjustment for Mac OS 10.2 c++
[SVN r15781]
2002-10-07 17:51:31 +00:00
Dave Abrahams
dfdf805106 Make AIX work again
[SVN r15743]
2002-10-05 20:11:57 +00:00
Dave Abrahams
281a5271da Make AIX work again
[SVN r15742]
2002-10-05 20:02:08 +00:00
Dave Abrahams
6eaa0827e5 Patches for CWPro8.3
[SVN r15734]
2002-10-05 16:39:34 +00:00
Dave Abrahams
c33ac6b47a Merge Trunk changes to RC_1_29_0
[SVN r15732]
2002-10-05 12:53:06 +00:00
Ralf W. Grosse-Kunstleve
0462c4d2f4 changes from branch merged to enable cxx 6.51 compilations
[SVN r15716]
2002-10-04 16:44:38 +00:00
nobody
1b35d5d3df This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15703]
2002-10-04 00:45:30 +00:00
nobody
512b912a9c This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15698]
2002-10-03 23:21:49 +00:00
Joel de Guzman
7da5d6344d Removing V1 Artifacts
[SVN r15592]
2002-10-01 00:31:57 +00:00
Joel de Guzman
f4ba587b19 reverting mistake
[SVN r15591]
2002-09-30 23:46:05 +00:00
Joel de Guzman
f1612c4294 Removing more V1 artifacts
[SVN r15590]
2002-09-30 23:44:55 +00:00
Joel de Guzman
f1b376776e merged from HEAD
[SVN r15589]
2002-09-30 23:20:11 +00:00
Joel de Guzman
eb6994e6b4 Removing BPL V1 Artifacts
[SVN r15569]
2002-09-30 07:18:46 +00:00
nobody
bd0b01c049 This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15564]
2002-09-30 03:35:54 +00:00
nobody
628573eb58 This commit was manufactured by cvs2svn to create branch 'RC_1_29_0'.
[SVN r15460]
2002-09-19 20:49:39 +00:00
718 changed files with 13286 additions and 42878 deletions

BIN
build/Attic/python_v1.zip Normal file

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66
build/Jamfile Normal file
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@@ -0,0 +1,66 @@
# (C) Copyright David Abrahams 2001. Permission to copy, use, modify, sell and
# distribute this software is granted provided this copyright notice appears
# in all copies. This software is provided "as is" without express or implied
# warranty, and with no claim as to its suitability for any purpose.
#
# Boost.Python library Jamfile
# declare the location of this subproject relative to the root
subproject libs/python/build ;
# bring in the rules for python
SEARCH on <module@>python.jam = $(BOOST_BUILD_PATH) ;
include <module@>python.jam ;
if [ check-python-config ]
{
local bpl-linkflags ;
if $(UNIX) && ( $(OS) = AIX )
{
bpl-linkflags = <linkflags>"-e initlibboost_python" ;
}
dll boost_python
:
../src/numeric.cpp
../src/list.cpp
../src/long.cpp
../src/dict.cpp
../src/tuple.cpp
../src/str.cpp
../src/aix_init_module.cpp
../src/converter/from_python.cpp
../src/converter/registry.cpp
../src/converter/type_id.cpp
../src/object/enum.cpp
../src/object/class.cpp
../src/object/function.cpp
../src/object/inheritance.cpp
../src/object/life_support.cpp
../src/object/pickle_support.cpp
../src/errors.cpp
../src/module.cpp
../src/converter/builtin_converters.cpp
../src/converter/arg_to_python_base.cpp
../src/object/iterator.cpp
../src/object_protocol.cpp
../src/object_operators.cpp
:
$(BOOST_PYTHON_V2_PROPERTIES)
<define>BOOST_PYTHON_SOURCE
$(bpl-linkflags)
;
stage bin-stage : <dll>boost_python
:
<tag><debug>"_debug"
<tag><debug-python>"_pydebug"
:
debug release
;
}

View File

@@ -1,77 +0,0 @@
# Copyright David Abrahams 2001-2006. Distributed under the Boost
# Software License, Version 1.0. (See accompanying
# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
import os ;
import modules ;
import python ;
if [ python.configured ] {
project boost/python
: source-location ../src
;
lib boost_python
: # sources
numeric.cpp
list.cpp
long.cpp
dict.cpp
tuple.cpp
str.cpp
slice.cpp
aix_init_module.cpp
converter/from_python.cpp
converter/registry.cpp
converter/type_id.cpp
object/enum.cpp
object/class.cpp
object/function.cpp
object/inheritance.cpp
object/life_support.cpp
object/pickle_support.cpp
errors.cpp
module.cpp
converter/builtin_converters.cpp
converter/arg_to_python_base.cpp
object/iterator.cpp
object/stl_iterator.cpp
object_protocol.cpp
object_operators.cpp
wrapper.cpp
import.cpp
exec.cpp
: # requirements
<link>static:<define>BOOST_PYTHON_STATIC_LIB
<define>BOOST_PYTHON_SOURCE
# On Windows, all code using Python has to link to the Python
# import library.
#
# On *nix we never link libboost_python to libpython. When
# extending Python, all Python symbols are provided by the
# Python interpreter executable. When embedding Python, the
# client executable is expected to explicitly link to
# /python//python (the target representing libpython) itself.
#
# python_for_extensions is a target defined by Boost.Build to
# provide the Python include paths, and on Windows, the Python
# import library, as usage requirements.
<library>/python//python_for_extensions
: # default build
<link>shared
: # usage requirements
<link>static:<define>BOOST_PYTHON_STATIC_LIB
<link>shared:<define>BOOST_PYTHON_DYNAMIC_LIB
;
}
else
{
ECHO "warning: Python location is not configured" ;
ECHO "warning: the Boost.Python library won't be built" ;
}

View File

@@ -1,910 +0,0 @@
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# Copyright David Abrahams 2006. Distributed under the Boost
# Software License, Version 1.0. (See accompanying
# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
import docutils ;
import path ;
sources = building.rst ;
bases = $(sources:S=) ;
# This is a path relative to the html/ subdirectory where the
# generated output will eventually be moved.
stylesheet = "--stylesheet=../../../rst.css" ;
for local b in $(bases)
{
html $(b) : $(b).rst :
<docutils-html>"-gdt --source-url="./$(b).rst" --link-stylesheet --traceback --trim-footnote-reference-space --footnote-references=superscript "$(stylesheet)
;
}
alias htmls : $(bases) ;
stage . : $(bases) ;

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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta name="generator" content=
"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
<meta http-equiv="refresh" content="0; URL=http://www.boost-consulting.com/writing/bpl.html">
<title>Loading: &ldquo;Building Hybrid Systems With Boost.Python&rdquo;</title>
</head>
<body>
Loading...; if nothing happens, please go to <a href= "http://www.boost-consulting.com/writing/bpl.html">http://www.boost-consulting.com/writing/bpl.html</a>.
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.. Copyright David Abrahams 2006. Distributed under the Boost
.. Software License, Version 1.0. (See accompanying
.. file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
This file has been moved to http://www.boost-consulting.com/writing/bpl.txt.

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Copyright David Abrahams 2006. Distributed under the Boost
Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
.. This is a comment. Note how any initial comments are moved by
transforms to after the document title, subtitle, and docinfo.
.. Need intro and conclusion
.. Exposing classes
.. Constructors
.. Overloading
.. Properties and data members
.. Inheritance
.. Operators and Special Functions
.. Virtual Functions
.. Call Policies
++++++++++++++++++++++++++++++++++++++++++++++
Introducing Boost.Python (Extended Abstract)
++++++++++++++++++++++++++++++++++++++++++++++
.. bibliographic fields (which also require a transform):
:Author: David Abrahams
:Address: 45 Walnut Street
Somerville, MA 02143
:Contact: dave@boost-consulting.com
:organization: `Boost Consulting`_
:date: $Date$
:status: This is a "work in progress"
:version: 1
:copyright: Copyright David Abrahams 2002. All rights reserved
:Dedication:
For my girlfriend, wife, and partner Luann
:abstract:
This paper describes the Boost.Python library, a system for
C++/Python interoperability.
.. meta::
:keywords: Boost,python,Boost.Python,C++
:description lang=en: C++/Python interoperability with Boost.Python
.. contents:: Table of Contents
.. section-numbering::
.. _`Boost Consulting`: http://www.boost-consulting.com
==============
Introduction
==============
Python and C++ are in many ways as different as two languages could
be: while C++ is usually compiled to machine-code, Python is
interpreted. Python's dynamic type system is often cited as the
foundation of its flexibility, while in C++ static typing is the
cornerstone of its efficiency. C++ has an intricate and difficult
meta-language to support compile-time polymorphism, while Python is
a uniform language with convenient runtime polymorphism.
Yet for many programmers, these very differences mean that Python and
C++ complement one another perfectly. Performance bottlenecks in
Python programs can be rewritten in C++ for maximal speed, and
authors of powerful C++ libraries choose Python as a middleware
language for its flexible system integration capabilities.
Furthermore, the surface differences mask some strong similarities:
* 'C'-family control structures (if, while, for...)
* Support for object-orientation, functional programming, and generic
programming (these are both *multi-paradigm* programming languages.)
* Comprehensive operator overloading facilities, recognizing the
importance of syntactic variability for readability and
expressivity.
* High-level concepts such as collections and iterators.
* High-level encapsulation facilities (C++: namespaces, Python: modules)
to support the design of re-usable libraries.
* Exception-handling for effective management of error conditions.
* C++ idioms in common use, such as handle/body classes and
reference-counted smart pointers mirror Python reference semantics.
Python provides a rich 'C' API for writers of 'C' extension modules.
Unfortunately, using this API directly for exposing C++ type and
function interfaces to Python is much more tedious than it should be.
This is mainly due to the limitations of the 'C' language. Compared to
C++ and Python, 'C' has only very rudimentary abstraction facilities.
Support for exception-handling is completely missing. One important
undesirable consequence is that 'C' extension module writers are
required to manually manage Python reference counts. Another unpleasant
consequence is a very high degree of repetition of similar code in 'C'
extension modules. Of course highly redundant code does not only cause
frustration for the module writer, but is also very difficult to
maintain.
The limitations of the 'C' API have lead to the development of a
variety of wrapping systems. SWIG_ is probably the most popular package
for the integration of C/C++ and Python. A more recent development is
the SIP_ package, which is specifically designed for interfacing Python
with the Qt_ graphical user interface library. Both SWIG and SIP
introduce a new specialized language for defining the inter-language
bindings. Of course being able to use a specialized language has
advantages, but having to deal with three different languages (Python,
C/C++ and the interface language) also introduces practical and mental
difficulties. The CXX_ package demonstrates an interesting alternative.
It shows that at least some parts of Python's 'C' API can be wrapped
and presented through a much more user-friendly C++ interface. However,
unlike SWIG and SIP, CXX does not include support for wrapping C++
classes as new Python types. CXX is also no longer actively developed.
In some respects Boost.Python combines ideas from SWIG and SIP with
ideas from CXX. Like SWIG and SIP, Boost.Python is a system for
wrapping C++ classes as new Python "built-in" types, and C/C++
functions as Python functions. Like CXX, Boost.Python presents Python's
'C' API through a C++ interface. Boost.Python goes beyond the scope of
other systems with the unique support for C++ virtual functions that
are overrideable in Python, support for organizing extensions as Python
packages with a central registry for inter-language type conversions,
and a convenient mechanism for tying into Python's serialization engine
(pickle). Importantly, all this is achieved without introducing a new
syntax. Boost.Python leverages the power of C++ meta-programming
techniques to introspect about the C++ type system, and presents a
simple, IDL-like C++ interface for exposing C/C++ code in extension
modules. Boost.Python is a pure C++ library, the inter-language
bindings are defined in pure C++, and other than a C++ compiler only
Python itself is required to get started with Boost.Python. Last but
not least, Boost.Python is an unrestricted open source library. There
are no strings attached even for commercial applications.
.. _SWIG: http://www.swig.org/
.. _SIP: http://www.riverbankcomputing.co.uk/sip/index.php
.. _Qt: http://www.trolltech.com/
.. _CXX: http://cxx.sourceforge.net/
===========================
Boost.Python Design Goals
===========================
The primary goal of Boost.Python is to allow users to expose C++
classes and functions to Python using nothing more than a C++
compiler. In broad strokes, the user experience should be one of
directly manipulating C++ objects from Python.
However, it's also important not to translate all interfaces *too*
literally: the idioms of each language must be respected. For
example, though C++ and Python both have an iterator concept, they are
expressed very differently. Boost.Python has to be able to bridge the
interface gap.
It must be possible to insulate Python users from crashes resulting
from trivial misuses of C++ interfaces, such as accessing
already-deleted objects. By the same token the library should
insulate C++ users from low-level Python 'C' API, replacing
error-prone 'C' interfaces like manual reference-count management and
raw ``PyObject`` pointers with more-robust alternatives.
Support for component-based development is crucial, so that C++ types
exposed in one extension module can be passed to functions exposed in
another without loss of crucial information like C++ inheritance
relationships.
Finally, all wrapping must be *non-intrusive*, without modifying or
even seeing the original C++ source code. Existing C++ libraries have
to be wrappable by third parties who only have access to header files
and binaries.
==========================
Hello Boost.Python World
==========================
And now for a preview of Boost.Python, and how it improves on the raw
facilities offered by Python. Here's a function we might want to
expose::
char const* greet(unsigned x)
{
static char const* const msgs[] = { "hello", "Boost.Python", "world!" };
if (x > 2)
throw std::range_error("greet: index out of range");
return msgs[x];
}
To wrap this function in standard C++ using the Python 'C' API, we'd
need something like this::
extern "C" // all Python interactions use 'C' linkage and calling convention
{
// Wrapper to handle argument/result conversion and checking
PyObject* greet_wrap(PyObject* args, PyObject * keywords)
{
int x;
if (PyArg_ParseTuple(args, "i", &x)) // extract/check arguments
{
char const* result = greet(x); // invoke wrapped function
return PyString_FromString(result); // convert result to Python
}
return 0; // error occurred
}
// Table of wrapped functions to be exposed by the module
static PyMethodDef methods[] = {
{ "greet", greet_wrap, METH_VARARGS, "return one of 3 parts of a greeting" }
, { NULL, NULL, 0, NULL } // sentinel
};
// module initialization function
DL_EXPORT init_hello()
{
(void) Py_InitModule("hello", methods); // add the methods to the module
}
}
Now here's the wrapping code we'd use to expose it with Boost.Python::
#include <boost/python.hpp>
using namespace boost::python;
BOOST_PYTHON_MODULE(hello)
{
def("greet", greet, "return one of 3 parts of a greeting");
}
and here it is in action::
>>> import hello
>>> for x in range(3):
... print hello.greet(x)
...
hello
Boost.Python
world!
Aside from the fact that the 'C' API version is much more verbose than
the BPL one, it's worth noting that it doesn't handle a few things
correctly:
* The original function accepts an unsigned integer, and the Python
'C' API only gives us a way of extracting signed integers. The
Boost.Python version will raise a Python exception if we try to pass
a negative number to ``hello.greet``, but the other one will proceed
to do whatever the C++ implementation does when converting an
negative integer to unsigned (usually wrapping to some very large
number), and pass the incorrect translation on to the wrapped
function.
* That brings us to the second problem: if the C++ ``greet()``
function is called with a number greater than 2, it will throw an
exception. Typically, if a C++ exception propagates across the
boundary with code generated by a 'C' compiler, it will cause a
crash. As you can see in the first version, there's no C++
scaffolding there to prevent this from happening. Functions wrapped
by Boost.Python automatically include an exception-handling layer
which protects Python users by translating unhandled C++ exceptions
into a corresponding Python exception.
* A slightly more-subtle limitation is that the argument conversion
used in the Python 'C' API case can only get that integer ``x`` in
*one way*. PyArg_ParseTuple can't convert Python ``long`` objects
(arbitrary-precision integers) which happen to fit in an ``unsigned
int`` but not in a ``signed long``, nor will it ever handle a
wrapped C++ class with a user-defined implicit ``operator unsigned
int()`` conversion. The BPL's dynamic type conversion registry
allows users to add arbitrary conversion methods.
==================
Library Overview
==================
This section outlines some of the library's major features. Except as
necessary to avoid confusion, details of library implementation are
omitted.
-------------------------------------------
The fundamental type-conversion mechanism
-------------------------------------------
XXX This needs to be rewritten.
Every argument of every wrapped function requires some kind of
extraction code to convert it from Python to C++. Likewise, the
function return value has to be converted from C++ to Python.
Appropriate Python exceptions must be raised if the conversion fails.
Argument and return types are part of the function's type, and much of
this tedium can be relieved if the wrapping system can extract that
information through introspection.
Passing a wrapped C++ derived class instance to a C++ function
accepting a pointer or reference to a base class requires knowledge of
the inheritance relationship and how to translate the address of a base
class into that of a derived class.
------------------
Exposing Classes
------------------
C++ classes and structs are exposed with a similarly-terse interface.
Given::
struct World
{
void set(std::string msg) { this->msg = msg; }
std::string greet() { return msg; }
std::string msg;
};
The following code will expose it in our extension module::
#include <boost/python.hpp>
BOOST_PYTHON_MODULE(hello)
{
class_<World>("World")
.def("greet", &World::greet)
.def("set", &World::set)
;
}
Although this code has a certain pythonic familiarity, people
sometimes find the syntax bit confusing because it doesn't look like
most of the C++ code they're used to. All the same, this is just
standard C++. Because of their flexible syntax and operator
overloading, C++ and Python are great for defining domain-specific
(sub)languages
(DSLs), and that's what we've done in BPL. To break it down::
class_<World>("World")
constructs an unnamed object of type ``class_<World>`` and passes
``"World"`` to its constructor. This creates a new-style Python class
called ``World`` in the extension module, and associates it with the
C++ type ``World`` in the BPL type conversion registry. We might have
also written::
class_<World> w("World");
but that would've been more verbose, since we'd have to name ``w``
again to invoke its ``def()`` member function::
w.def("greet", &World::greet)
There's nothing special about the location of the dot for member
access in the original example: C++ allows any amount of whitespace on
either side of a token, and placing the dot at the beginning of each
line allows us to chain as many successive calls to member functions
as we like with a uniform syntax. The other key fact that allows
chaining is that ``class_<>`` member functions all return a reference
to ``*this``.
So the example is equivalent to::
class_<World> w("World");
w.def("greet", &World::greet);
w.def("set", &World::set);
It's occasionally useful to be able to break down the components of a
Boost.Python class wrapper in this way, but the rest of this paper
will tend to stick to the terse syntax.
For completeness, here's the wrapped class in use:
>>> import hello
>>> planet = hello.World()
>>> planet.set('howdy')
>>> planet.greet()
'howdy'
Constructors
============
Since our ``World`` class is just a plain ``struct``, it has an
implicit no-argument (nullary) constructor. Boost.Python exposes the
nullary constructor by default, which is why we were able to write:
>>> planet = hello.World()
However, well-designed classes in any language may require constructor
arguments in order to establish their invariants. Unlike Python,
where ``__init__`` is just a specially-named method, In C++
constructors cannot be handled like ordinary member functions. In
particular, we can't take their address: ``&World::World`` is an
error. The library provides a different interface for specifying
constructors. Given::
struct World
{
World(std::string msg); // added constructor
...
we can modify our wrapping code as follows::
class_<World>("World", init<std::string>())
...
of course, a C++ class may have additional constructors, and we can
expose those as well by passing more instances of ``init<...>`` to
``def()``::
class_<World>("World", init<std::string>())
.def(init<double, double>())
...
Boost.Python allows wrapped functions, member functions, and
constructors to be overloaded to mirror C++ overloading.
Data Members and Properties
===========================
Any publicly-accessible data members in a C++ class can be easily
exposed as either ``readonly`` or ``readwrite`` attributes::
class_<World>("World", init<std::string>())
.def_readonly("msg", &World::msg)
...
and can be used directly in Python:
>>> planet = hello.World('howdy')
>>> planet.msg
'howdy'
This does *not* result in adding attributes to the ``World`` instance
``__dict__``, which can result in substantial memory savings when
wrapping large data structures. In fact, no instance ``__dict__``
will be created at all unless attributes are explicitly added from
Python. BPL owes this capability to the new Python 2.2 type system,
in particular the descriptor interface and ``property`` type.
In C++, publicly-accessible data members are considered a sign of poor
design because they break encapsulation, and style guides usually
dictate the use of "getter" and "setter" functions instead. In
Python, however, ``__getattr__``, ``__setattr__``, and since 2.2,
``property`` mean that attribute access is just one more
well-encapsulated syntactic tool at the programmer's disposal. BPL
bridges this idiomatic gap by making Python ``property`` creation
directly available to users. So if ``msg`` were private, we could
still expose it as attribute in Python as follows::
class_<World>("World", init<std::string>())
.add_property("msg", &World::greet, &World::set)
...
The example above mirrors the familiar usage of properties in Python
2.2+:
>>> class World(object):
... __init__(self, msg):
... self.__msg = msg
... def greet(self):
... return self.__msg
... def set(self, msg):
... self.__msg = msg
... msg = property(greet, set)
Operators and Special Functions
===============================
The ability to write arithmetic operators for user-defined types that
C++ and Python both allow the definition of has been a major factor in
the popularity of both languages for scientific computing. The
success of packages like NumPy attests to the power of exposing
operators in extension modules. In this example we'll wrap a class
representing a position in a large file::
class FilePos { /*...*/ };
// Linear offset
FilePos operator+(FilePos, int);
FilePos operator+(int, FilePos);
FilePos operator-(FilePos, int);
// Distance between two FilePos objects
int operator-(FilePos, FilePos);
// Offset with assignment
FilePos& operator+=(FilePos&, int);
FilePos& operator-=(FilePos&, int);
// Comparison
bool operator<(FilePos, FilePos);
The wrapping code looks like this::
class_<FilePos>("FilePos")
.def(self + int()) // __add__
.def(int() + self) // __radd__
.def(self - int()) // __sub__
.def(self - self) // __sub__
.def(self += int()) // __iadd__
.def(self -= int()) // __isub__
.def(self < self); // __lt__
;
The magic is performed using a simplified application of "expression
templates" [VELD1995]_, a technique originally developed by for
optimization of high-performance matrix algebra expressions. The
essence is that instead of performing the computation immediately,
operators are overloaded to construct a type *representing* the
computation. In matrix algebra, dramatic optimizations are often
available when the structure of an entire expression can be taken into
account, rather than processing each operation "greedily".
Boost.Python uses the same technique to build an appropriate Python
callable object based on an expression involving ``self``, which is
then added to the class.
Inheritance
===========
C++ inheritance relationships can be represented to Boost.Python by adding
an optional ``bases<...>`` argument to the ``class_<...>`` template
parameter list as follows::
class_<Derived, bases<Base1,Base2> >("Derived")
...
This has two effects:
1. When the ``class_<...>`` is created, Python type objects
corresponding to ``Base1`` and ``Base2`` are looked up in the BPL
registry, and are used as bases for the new Python ``Derived`` type
object [#mi]_, so methods exposed for the Python ``Base1`` and
``Base2`` types are automatically members of the ``Derived`` type.
Because the registry is global, this works correctly even if
``Derived`` is exposed in a different module from either of its
bases.
2. C++ conversions from ``Derived`` to its bases are added to the
Boost.Python registry. Thus wrapped C++ methods expecting (a
pointer or reference to) an object of either base type can be
called with an object wrapping a ``Derived`` instance. Wrapped
member functions of class ``T`` are treated as though they have an
implicit first argument of ``T&``, so these conversions are
necessary to allow the base class methods to be called for derived
objects.
Of course it's possible to derive new Python classes from wrapped C++
class instances. Because Boost.Python uses the new-style class
system, that works very much as for the Python built-in types. There
is one significant detail in which it differs: the built-in types
generally establish their invariants in their ``__new__`` function, so
that derived classes do not need to call ``__init__`` on the base
class before invoking its methods :
>>> class L(list):
... def __init__(self):
... pass
...
>>> L().reverse()
>>>
Because C++ object construction is a one-step operation, C++ instance
data cannot be constructed until the arguments are available, in the
``__init__`` function:
>>> class D(SomeBPLClass):
... def __init__(self):
... pass
...
>>> D().some_bpl_method()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: bad argument type for built-in operation
This happened because Boost.Python couldn't find instance data of type
``SomeBPLClass`` within the ``D`` instance; ``D``'s ``__init__``
function masked construction of the base class. It could be corrected
by either removing ``D``'s ``__init__`` function or having it call
``SomeBPLClass.__init__(...)`` explicitly.
Virtual Functions
=================
Deriving new types in Python from extension classes is not very
interesting unless they can be used polymorphically from C++. In
other words, Python method implementations should appear to override
the implementation of C++ virtual functions when called *through base
class pointers/references from C++*. Since the only way to alter the
behavior of a virtual function is to override it in a derived class,
the user must build a special derived class to dispatch a polymorphic
class' virtual functions::
//
// interface to wrap:
//
class Base
{
public:
virtual int f(std::string x) { return 42; }
virtual ~Base();
};
int calls_f(Base const& b, std::string x) { return b.f(x); }
//
// Wrapping Code
//
// Dispatcher class
struct BaseWrap : Base
{
// Store a pointer to the Python object
BaseWrap(PyObject* self_) : self(self_) {}
PyObject* self;
// Default implementation, for when f is not overridden
int f_default(std::string x) { return this->Base::f(x); }
// Dispatch implementation
int f(std::string x) { return call_method<int>(self, "f", x); }
};
...
def("calls_f", calls_f);
class_<Base, BaseWrap>("Base")
.def("f", &Base::f, &BaseWrap::f_default)
;
Now here's some Python code which demonstrates:
>>> class Derived(Base):
... def f(self, s):
... return len(s)
...
>>> calls_f(Base(), 'foo')
42
>>> calls_f(Derived(), 'forty-two')
9
Things to notice about the dispatcher class:
* The key element which allows overriding in Python is the
``call_method`` invocation, which uses the same global type
conversion registry as the C++ function wrapping does to convert its
arguments from C++ to Python and its return type from Python to C++.
* Any constructor signatures you wish to wrap must be replicated with
an initial ``PyObject*`` argument
* The dispatcher must store this argument so that it can be used to
invoke ``call_method``
* The ``f_default`` member function is needed when the function being
exposed is not pure virtual; there's no other way ``Base::f`` can be
called on an object of type ``BaseWrap``, since it overrides ``f``.
Admittedly, this formula is tedious to repeat, especially on a project
with many polymorphic classes; that it is necessary reflects
limitations in C++'s compile-time reflection capabilities. Several
efforts are underway to write front-ends for Boost.Python which can
generate these dispatchers (and other wrapping code) automatically.
If these are successful it will mark a move away from wrapping
everything directly in pure C++ for many of our users.
---------------
Serialization
---------------
*Serialization* is the process of converting objects in memory to a
form that can be stored on disk or sent over a network connection. The
serialized object (most often a plain string) can be retrieved and
converted back to the original object. A good serialization system will
automatically convert entire object hierarchies. Python's standard
``pickle`` module is such a system. It leverages the language's strong
runtime introspection facilities for serializing practically arbitrary
user-defined objects. With a few simple and unintrusive provisions this
powerful machinery can be extended to also work for wrapped C++ objects.
Here is an example::
#include <string>
struct World
{
World(std::string a_msg) : msg(a_msg) {}
std::string greet() const { return msg; }
std::string msg;
};
#include <boost/python.hpp>
using namespace boost::python;
struct World_picklers : pickle_suite
{
static tuple
getinitargs(World const& w) { return make_tuple(w.greet()); }
};
BOOST_PYTHON_MODULE(hello)
{
class_<World>("World", init<std::string>())
.def("greet", &World::greet)
.def_pickle(World_picklers())
;
}
Now let's create a ``World`` object and put it to rest on disk::
>>> import hello
>>> import pickle
>>> a_world = hello.World("howdy")
>>> pickle.dump(a_world, open("my_world", "w"))
In a potentially *different script* on a potentially *different
computer* with a potentially *different operating system*::
>>> import pickle
>>> resurrected_world = pickle.load(open("my_world", "r"))
>>> resurrected_world.greet()
'howdy'
Of course the ``cPickle`` module can also be used for faster
processing.
Boost.Python's ``pickle_suite`` fully supports the ``pickle`` protocol
defined in the standard Python documentation. There is a one-to-one
correspondence between the standard pickling methods (``__getinitargs__``,
``__getstate__``, ``__setstate__``) and the functions defined by the
user in the class derived from ``pickle_suite`` (``getinitargs``,
``getstate``, ``setstate``). The ``class_::def_pickle()`` member function
is used to establish the Python bindings for all user-defined functions
simultaneously. Correct signatures for these functions are enforced at
compile time. Non-sensical combinations of the three pickle functions
are also rejected at compile time. These measures are designed to
help the user in avoiding obvious errors.
Enabling serialization of more complex C++ objects requires a little
more work than is shown in the example above. Fortunately the
``object`` interface (see next section) greatly helps in keeping the
code manageable.
------------------
Object interface
------------------
Experienced extension module authors will be familiar with the 'C' view
of Python objects, the ubiquitous ``PyObject*``. Most if not all Python
'C' API functions involve ``PyObject*`` as arguments or return type. A
major complication is the raw reference counting interface presented to
the 'C' programmer. E.g. some API functions return *new references* and
others return *borrowed references*. It is up to the extension module
writer to properly increment and decrement reference counts. This
quickly becomes cumbersome and error prone, especially if there are
multiple execution paths.
Boost.Python provides a type ``object`` which is essentially a high
level wrapper around ``PyObject*``. ``object`` automates reference
counting as much as possible. It also provides the facilities for
converting arbitrary C++ types to Python objects and vice versa.
This significantly reduces the learning effort for prospective
extension module writers.
Creating an ``object`` from any other type is extremely simple::
object o(3);
``object`` has templated interactions with all other types, with
automatic to-python conversions. It happens so naturally that it's
easily overlooked.
The ``extract<T>`` class template can be used to convert Python objects
to C++ types::
double x = extract<double>(o);
All registered user-defined conversions are automatically accessible
through the ``object`` interface. With reference to the ``World`` class
defined in previous examples::
object as_python_object(World("howdy"));
World back_as_c_plus_plus_object = extract<World>(as_python_object);
If a C++ type cannot be converted to a Python object an appropriate
exception is thrown at runtime. Similarly, an appropriate exception is
thrown if a C++ type cannot be extracted from a Python object.
``extract<T>`` provides facilities for avoiding exceptions if this is
desired.
The ``object::attr()`` member function is available for accessing
and manipulating attributes of Python objects. For example::
object planet(World());
planet.attr("set")("howdy");
``planet.attr("set")`` returns a callable ``object``. ``"howdy"`` is
converted to a Python string object which is then passed as an argument
to the ``set`` method.
The ``object`` type is accompanied by a set of derived types
that mirror the Python built-in types such as ``list``, ``dict``,
``tuple``, etc. as much as possible. This enables convenient
manipulation of these high-level types from C++::
dict d;
d["some"] = "thing";
d["lucky_number"] = 13;
list l = d.keys();
This almost looks and works like regular Python code, but it is pure C++.
=================
Thinking hybrid
=================
For many applications runtime performance considerations are very
important. This is particularly true for most scientific applications.
Often the performance considerations dictate the use of a compiled
language for the core algorithms. Traditionally the decision to use a
particular programming language is an exclusive one. Because of the
practical and mental difficulties of combining different languages many
systems are written in just one language. This is quite unfortunate
because the price payed for runtime performance is typically a
significant overhead due to static typing. For example, our experience
shows that developing maintainable C++ code is typically much more
time-consuming and requires much more hard-earned working experience
than developing useful Python code. A related observation is that many
compiled packages are augmented by some type of rudimentary scripting
layer. These ad hoc solutions clearly show that many times a compiled
language alone does not get the job done. On the other hand it is also
clear that a pure Python implementation is too slow for numerically
intensive production code.
Boost.Python enables us to *think hybrid* when developing new
applications. Python can be used for rapidly prototyping a
new application. Python's ease of use and the large pool of standard
libraries give us a head start on the way to a first working system. If
necessary, the working procedure can be used to discover the
rate-limiting algorithms. To maximize performance these can be
reimplemented in C++, together with the Boost.Python bindings needed to
tie them back into the existing higher-level procedure.
Of course, this *top-down* approach is less attractive if it is clear
from the start that many algorithms will eventually have to be
implemented in a compiled language. Fortunately Boost.Python also
enables us to pursue a *bottom-up* approach. We have used this approach
very successfully in the development of a toolbox for scientific
applications (scitbx) that we will describe elsewhere. The toolbox
started out mainly as a library of C++ classes with Boost.Python
bindings, and for a while the growth was mainly concentrated on the C++
parts. However, as the toolbox is becoming more complete, more and more
newly added functionality can be implemented in Python. We expect this
trend to continue, as illustrated qualitatively in this figure:
.. image:: python_cpp_mix.png
This figure shows the ratio of newly added C++ and Python code over
time as new algorithms are implemented. We expect this ratio to level
out near 70% Python. The increasing ability to solve new problems
mostly with the easy-to-use Python language rather than a necessarily
more arcane statically typed language is the return on the investment
of learning how to use Boost.Python. The ability to solve some problems
entirely using only Python will enable a larger group of people to
participate in the rapid development of new applications.
=============
Conclusions
=============
The examples in this paper illustrate that Boost.Python enables
seamless interoperability between C++ and Python. Importantly, this is
achieved without introducing a third syntax: the Python/C++ interface
definitions are written in pure C++. This avoids any problems with
parsing the C++ code to be interfaced to Python, yet the interface
definitions are concise and maintainable. Freed from most of the
development-time penalties of crossing a language boundary, software
designers can take full advantage of two rich and complimentary
language environments. In practice it turns out that some things are
very difficult to do with pure Python/C (e.g. an efficient array
library with an intuitive interface in the compiled language) and
others are very difficult to do with pure C++ (e.g. serialization).
If one has the luxury of being able to design a software system as a
hybrid system from the ground up there are many new ways of avoiding
road blocks in one language or the other.
.. I'm not ready to give up on all of this quite yet
.. Perhaps one day we'll have a language with the simplicity and
expressive power of Python and the compile-time muscle of C++. Being
able to take advantage of all of these facilities without paying the
mental and development-time penalties of crossing a language barrier
would bring enormous benefits. Until then, interoperability tools
like Boost.Python can help lower the barrier and make the benefits of
both languages more accessible to both communities.
===========
Footnotes
===========
.. [#mi] For hard-core new-style class/extension module writers it is
worth noting that the normal requirement that all extension classes
with data form a layout-compatible single-inheritance chain is
lifted for Boost.Python extension classes. Clearly, either
``Base1`` or ``Base2`` has to occupy a different offset in the
``Derived`` class instance. This is possible because the wrapped
part of BPL extension class instances is never assumed to have a
fixed offset within the wrapper.
===========
Citations
===========
.. [VELD1995] T. Veldhuizen, "Expression Templates," C++ Report,
Vol. 7 No. 5 June 1995, pp. 26-31.
http://osl.iu.edu/~tveldhui/papers/Expression-Templates/exprtmpl.html

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<title>Boost C++ Libraries: Boost.Python Build and Test HOWTO</title>
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<body>
<div class="document" id="logo-boost-python-build-and-test-howto">
<h1 class="title"><a class="reference" href="../index.htm"><img alt="Boost C++ Libraries:" class="boost-logo" src="../boost.png" /></a> Boost.Python Build and Test HOWTO</h1>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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<div class="contents sidebar small topic">
<p class="topic-title first"><a id="contents" name="contents">Contents</a></p>
<ul class="auto-toc simple">
<li><a class="reference" href="#requirements" id="id20" name="id20">1&nbsp;&nbsp;&nbsp;Requirements</a></li>
<li><a class="reference" href="#background" id="id21" name="id21">2&nbsp;&nbsp;&nbsp;Background</a></li>
<li><a class="reference" href="#getting-boost-python-binaries" id="id22" name="id22">3&nbsp;&nbsp;&nbsp;Getting Boost.Python Binaries</a><ul class="auto-toc">
<li><a class="reference" href="#no-install-quickstart" id="id23" name="id23">3.1&nbsp;&nbsp;&nbsp;No-Install Quickstart</a></li>
<li><a class="reference" href="#installing-boost-python-on-your-system" id="id24" name="id24">3.2&nbsp;&nbsp;&nbsp;Installing Boost.Python on your System</a></li>
</ul>
</li>
<li><a class="reference" href="#configuring-boost-build" id="id25" name="id25">4&nbsp;&nbsp;&nbsp;Configuring Boost.Build</a></li>
<li><a class="reference" href="#building-an-extension-module" id="id26" name="id26">5&nbsp;&nbsp;&nbsp;Building an Extension Module</a></li>
<li><a class="reference" href="#testing" id="id27" name="id27">6&nbsp;&nbsp;&nbsp;Testing</a></li>
<li><a class="reference" href="#advanced-configuration" id="id28" name="id28">7&nbsp;&nbsp;&nbsp;Advanced Configuration</a><ul class="auto-toc">
<li><a class="reference" href="#python-configuration-parameters" id="id29" name="id29">7.1&nbsp;&nbsp;&nbsp;Python Configuration Parameters</a></li>
<li><a class="reference" href="#examples" id="id30" name="id30">7.2&nbsp;&nbsp;&nbsp;Examples</a></li>
</ul>
</li>
<li><a class="reference" href="#choosing-a-boost-python-library-binary" id="id31" name="id31">8&nbsp;&nbsp;&nbsp;Choosing a Boost.Python Library Binary</a><ul class="auto-toc">
<li><a class="reference" href="#the-dynamic-binary" id="id32" name="id32">8.1&nbsp;&nbsp;&nbsp;The Dynamic Binary</a></li>
<li><a class="reference" href="#the-static-binary" id="id33" name="id33">8.2&nbsp;&nbsp;&nbsp;The Static Binary</a></li>
</ul>
</li>
<li><a class="reference" href="#notes-for-mingw-and-cygwin-with-mno-cygwin-gcc-users" id="id34" name="id34">9&nbsp;&nbsp;&nbsp;Notes for MinGW (and Cygwin with -mno-cygwin) GCC Users</a></li>
</ul>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id20" id="requirements" name="requirements">1&nbsp;&nbsp;&nbsp;Requirements</a></h1>
<p>Boost.Python requires <a class="reference" href="http://www.python.org/2.2">Python 2.2</a><a class="footnote-reference" href="#id16" id="id2" name="id2"><sup>1</sup></a> <em>or</em> <a class="reference" href="http://www.python.org"><em>newer</em></a>.</p>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id21" id="background" name="background">2&nbsp;&nbsp;&nbsp;Background</a></h1>
<p>There are two basic models for combining C++ and Python:</p>
<ul class="simple">
<li><a class="reference" href="http://www.python.org/doc/current/ext/intro.html">extending</a>, in which the end-user launches the Python interpreter
executable and imports Python “extension modules” written in C++.
Think of taking a library written in C++ and giving it a Python
interface so Python programmers can use it. From Python, these
modules look just like regular Python modules.</li>
<li><a class="reference" href="http://www.python.org/doc/current/ext/embedding.html">embedding</a>, in which the end-user launches a program written
in C++ that in turn invokes the Python interpreter as a library
subroutine. Think of adding scriptability to an existing
application.</li>
</ul>
<p>The key distinction between extending and embedding is the location
of C++' <tt class="docutils literal"><span class="pre">main()</span></tt> function: in the Python interpreter executable,
or in some other program, respectively. Note that even when
embedding Python in another program, <a class="reference" href="http://www.python.org/doc/current/ext/extending-with-embedding.html">extension modules are often
the best way to make C/C++ functionality accessible to Python
code</a>, so the use of extension modules is really at the heart of
both models.</p>
<p>Except in rare cases, extension modules are built as
dynamically-loaded libraries with a single entry point, which means
you can change them without rebuilding either the other extension
modules or the executable containing <tt class="docutils literal"><span class="pre">main()</span></tt>.</p>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id22" id="getting-boost-python-binaries" name="getting-boost-python-binaries">3&nbsp;&nbsp;&nbsp;Getting Boost.Python Binaries</a></h1>
<p>Since Boost.Python is a separately-compiled (as opposed to
<a class="reference" href="../../../more/getting_started/windows.html#header-only-libraries">header-only</a>) library, its user relies on the services of a
Boost.Python library binary.</p>
<div class="section">
<h2><a class="toc-backref" href="#id23" id="no-install-quickstart" name="no-install-quickstart">3.1&nbsp;&nbsp;&nbsp;No-Install Quickstart</a></h2>
<p>If you just want to get started quickly building and testing
Boost.Python extension modules, or embedding Python in an
executable, you don't need to worry about installing Boost.Python
binaries explicitly. These instructions use <a class="reference" href="../../../tools/build">Boost.Build</a> projects,
which will build those binaries as soon as they're needed. Your
first tests may take a little longer while you wait for
Boost.Python to build, but doing things this way will save you from
worrying about build intricacies like which library binaries to use
for a specific compiler configuration.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p>Of course it's possible to use other build systems to
build Boost.Python and its extensions, but they are not
officially supported by Boost and <strong>99% of all “I can't build
Boost.Python” problems come from trying to use another build
system</strong>.</p>
<p class="last">If you want to use another system anyway, we suggest that you
follow these instructions, and then invoke <tt class="docutils literal"><span class="pre">bjam</span></tt> with the
<tt class="docutils literal"><span class="pre">-a</span> <span class="pre">-o</span></tt><em>filename</em> option to dump the build commands it executes
to a file, so you can see what your build system needs to do.</p>
</div>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id24" id="installing-boost-python-on-your-system" name="installing-boost-python-on-your-system">3.2&nbsp;&nbsp;&nbsp;Installing Boost.Python on your System</a></h2>
<p>If you need a regular, installation of the Boost.Python library
binaries on your system, the Boost <a class="reference" href="../../../more/getting_started/index.html">Getting Started Guide</a> will
walk you through the steps of installing one. If building binaries
from source, you might want to supply the <tt class="docutils literal"><span class="pre">--with-python</span></tt>
argument to <tt class="docutils literal"><span class="pre">bjam</span></tt> (or the <tt class="docutils literal"><span class="pre">--with-libraries=python</span></tt> argument
to <tt class="docutils literal"><span class="pre">configure</span></tt>), so only the Boost.Python binary will be built,
rather than all the Boost binaries.</p>
</div>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id25" id="configuring-boost-build" name="configuring-boost-build">4&nbsp;&nbsp;&nbsp;Configuring Boost.Build</a></h1>
<p>As described in the <a class="reference" href="http://www.boost.orgdoc/html/bbv2/advanced.html#bbv2.advanced.configuration">Boost.Build reference manual</a>, a file called
<tt class="docutils literal"><span class="pre">user-config.jam</span></tt> in your home
directory<a class="footnote-reference" href="#home-dir" id="id5" name="id5"><sup>7</sup></a> is used to
describe the build resources available to the build system. You'll
need to tell it about your Python installation.</p>
<div class="admonition-users-of-unix-variant-oses admonition">
<p class="first admonition-title">Users of Unix-Variant OSes</p>
<p class="last">If you are using a unix-variant OS and you ran Boost's
<tt class="docutils literal"><span class="pre">configure</span></tt> script, it may have generated a
<tt class="docutils literal"><span class="pre">user-config.jam</span></tt> for you.<a class="footnote-reference" href="#overwrite" id="id7" name="id7"><sup>4</sup></a> If your <tt class="docutils literal"><span class="pre">configure</span></tt>/<tt class="docutils literal"><span class="pre">make</span></tt> sequence was successful and Boost.Python binaries
were built, your <tt class="docutils literal"><span class="pre">user-config.jam</span></tt> file is probably already
correct.</p>
</div>
<p>If you have a fairly “standard” python installation for your
platform, there's very little you need to do to describe it.
Simply having</p>
<pre class="literal-block">
import toolset : using ;
using python ;
</pre>
<p>in a <tt class="docutils literal"><span class="pre">user-config.jam</span></tt> file in your home directory<a class="footnote-reference" href="#home-dir" id="id8" name="id8"><sup>7</sup></a>
should be enough.<a class="footnote-reference" href="#user-config-jam" id="id9" name="id9"><sup>6</sup></a> For more complicated setups,
see <a class="reference" href="#advanced-configuration">Advanced Configuration</a>.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">You might want to pass the <tt class="docutils literal"><span class="pre">--debug-configuration</span></tt>
option to <tt class="docutils literal"><span class="pre">bjam</span></tt> the first few times you invoke it, to make
sure that Boost.Build is correctly locating all the parts of
your Python installation. If it isn't, consider passing some of
the optional <a class="reference" href="#python-configuration-parameters">Python configuration parameters</a> detailed below.</p>
</div>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id26" id="building-an-extension-module" name="building-an-extension-module">5&nbsp;&nbsp;&nbsp;Building an Extension Module</a></h1>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id27" id="testing" name="testing">6&nbsp;&nbsp;&nbsp;Testing</a></h1>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id28" id="advanced-configuration" name="advanced-configuration">7&nbsp;&nbsp;&nbsp;Advanced Configuration</a></h1>
<p>If you have several versions of Python installed, or Python is
installed in an unusual way, you may want to supply any or all of
the following optional parameters to <tt class="docutils literal"><span class="pre">using</span> <span class="pre">python</span></tt>.</p>
<div class="section">
<h2><a class="toc-backref" href="#id29" id="python-configuration-parameters" name="python-configuration-parameters">7.1&nbsp;&nbsp;&nbsp;Python Configuration Parameters</a></h2>
<dl class="docutils">
<dt>version</dt>
<dd>the version of Python to use. Should be in Major.Minor
format, for example, <tt class="docutils literal"><span class="pre">2.3</span></tt>. Do not include the subminor
version (i.e. <em>not</em> <tt class="docutils literal"><span class="pre">2.5.1</span></tt>). If you have multiple Python
versions installed, the version will usually be the only
additional argument required.</dd>
<dt>cmd-or-prefix</dt>
<dd>preferably, a command that invokes a Python
interpreter. Alternatively, the installation prefix for Python
libraries and header files. Use the alternative formulation if
there is no appropriate Python executable available.</dd>
<dt>includes</dt>
<dd>the <tt class="docutils literal"><span class="pre">#include</span></tt> path for Python headers.</dd>
<dt>libraries</dt>
<dd>the path to Python library binaries. On MacOS/Darwin,
you can also pass the path of the Python framework.</dd>
<dt>condition</dt>
<dd>if specified, should be a set of Boost.Build
properties that are matched against the build configuration when
Boost.Build selects a Python configuration to use.</dd>
<dt>extension-suffix</dt>
<dd>A string to append to the name of extension
modules before the true filename extension. You almost certainly
don't need to use this. Usually this suffix is only used when
targeting a Windows debug build of Python, and will be set
automatically for you based on the value of the
<tt class="docutils literal"><span class="pre">&lt;python-debugging&gt;</span></tt> feature. However, at least one Linux
distribution (Ubuntu Feisty Fawn) has a specially configured
<a class="reference" href="https://wiki.ubuntu.com/PyDbgBuilds">python-dbg</a> package that claims to use such a suffix.</dd>
</dl>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id30" id="examples" name="examples">7.2&nbsp;&nbsp;&nbsp;Examples</a></h2>
<p>Note that in the examples below, case and <em>especially whitespace</em> are
significant.</p>
<ul>
<li><p class="first">If you have both python 2.5 and python 2.4 installed,
<tt class="docutils literal"><span class="pre">user-config.jam</span></tt> might contain:</p>
<pre class="literal-block">
using python : 2.5 ; # Make both versions of Python available
<html>
<head>
<meta name="generator" content=
"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="boost.css">
using python : 2.4 ; # To build with python 2.4, add python=2.4
# to your command line.
</pre>
<p>The first version configured (2.5) becomes the default. To build
against python 2.4, add <tt class="docutils literal"><span class="pre">python=2.4</span></tt> to the <tt class="docutils literal"><span class="pre">bjam</span></tt> command line.</p>
</li>
<li><p class="first">If you have python installed in an unusual location, you might
supply the path to the interpreter in the <tt class="docutils literal"><span class="pre">cmd-or-prefix</span></tt>
parameter:</p>
<pre class="literal-block">
using python : : /usr/local/python-2.6-beta/bin/python ;
</pre>
</li>
<li><p class="first">If you have a separate build of Python for use with a particular
toolset, you might supply that toolset in the <tt class="docutils literal"><span class="pre">condition</span></tt>
parameter:</p>
<pre class="literal-block">
using python ; # use for most toolsets
<title>Boost.Python - Building and Testing</title>
</head>
# Use with Intel C++ toolset
using python
: # version
: c:\\Devel\\Python-2.5-IntelBuild\\PCBuild\\python # cmd-or-prefix
: # includes
: # libraries
: &lt;toolset&gt;intel # condition
;
</pre>
</li>
<li><p class="first">You can set up your user-config.jam so a bjam built under Windows
can build/test both Windows and <a class="reference" href="http://cygwin.com">Cygwin</a> python extensions. Just pass
<tt class="docutils literal"><span class="pre">&lt;target-os&gt;cygwin</span></tt> in the <tt class="docutils literal"><span class="pre">condition</span></tt> parameter
for the cygwin python installation:</p>
<pre class="literal-block">
# windows installation
using python ;
<body link="#0000ff" vlink="#800080">
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../c++boost.gif" border="0"></a></h3>
</td>
# cygwin installation
using python : : c:\\cygwin\\bin\\python2.5 : : : &lt;target-os&gt;cygwin ;
</pre>
<p>when you put target-os=cygwin in your build request, it should build
with the cygwin version of python:<a class="footnote-reference" href="#flavor" id="id11" name="id11"><sup>5</sup></a></p>
<blockquote>
<p>bjam target-os=cygwin toolset=gcc</p>
</blockquote>
<p>This is supposed to work the other way, too (targeting windows
python with a <a class="reference" href="http://cygwin.com">Cygwin</a> bjam) but it seems as though the support in
Boost.Build's toolsets for building that way is broken at the
time of this writing.</p>
</li>
<li><p class="first">Note that because of <a class="reference" href="http://zigzag.cs.msu.su/boost.build/wiki/AlternativeSelection">the way Boost.Build currently selects target
alternatives</a>, you might have be very explicit in your build
requests. For example, given:</p>
<pre class="literal-block">
using python : 2.5 ; # a regular windows build
using python : 2.4 : : : : &lt;target-os&gt;cygwin ;
</pre>
<p>building with</p>
<pre class="literal-block">
bjam target-os=cygwin
</pre>
<p>will yield an error. Instead, you'll need to write:</p>
<pre class="literal-block">
bjam target-os=cygwin/python=2.4
</pre>
</li>
</ul>
</div>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id31" id="choosing-a-boost-python-library-binary" name="choosing-a-boost-python-library-binary">8&nbsp;&nbsp;&nbsp;Choosing a Boost.Python Library Binary</a></h1>
<p>If—instead of letting Boost.Build construct and link withthe right
libraries automatically—you choose to use a pre-built Boost.Python
library, you'll need to think about which one to link with. The
Boost.Python binary comes in both static and dynamic flavors. Take
care to choose the right flavor for your application.<a class="footnote-reference" href="#naming" id="id13" name="id13"><sup>2</sup></a></p>
<div class="section">
<h2><a class="toc-backref" href="#id32" id="the-dynamic-binary" name="the-dynamic-binary">8.1&nbsp;&nbsp;&nbsp;The Dynamic Binary</a></h2>
<p>The dynamic library is the safest and most-versatile choice:</p>
<ul class="simple">
<li>A single copy of the library code is used by all extension
modules built with a given toolset.<a class="footnote-reference" href="#toolset-specific" id="id14" name="id14"><sup>3</sup></a></li>
<li>The library contains a type conversion registry. Because one
registry is shared among all extension modules, instances of a
class exposed to Python in one dynamically-loaded extension
module can be passed to functions exposed in another such module.</li>
</ul>
</div>
<div class="section">
<h2><a class="toc-backref" href="#id33" id="the-static-binary" name="the-static-binary">8.2&nbsp;&nbsp;&nbsp;The Static Binary</a></h2>
<p>It might be appropriate to use the static Boost.Python library in
any of the following cases:</p>
<ul class="simple">
<li>You are <a class="reference" href="http://www.python.org/doc/current/ext/intro.html">extending</a> python and the types exposed in your
dynamically-loaded extension module don't need to be used by any
other Boost.Python extension modules, and you don't care if the
core library code is duplicated among them.</li>
<li>You are <a class="reference" href="http://www.python.org/doc/current/ext/embedding.html">embedding</a> python in your application and either:<ul>
<li>You are targeting a Unix variant OS other than MacOS or AIX,
where the dynamically-loaded extension modules can “see” the
Boost.Python library symbols that are part of the executable.</li>
<li>Or, you have statically linked some Boost.Python extension
modules into your application and you don't care if any
dynamically-loaded Boost.Python extension modules are able to
use the types exposed by your statically-linked extension
modules (and vice-versa).</li>
</ul>
</li>
</ul>
</div>
</div>
<div class="section">
<h1><a class="toc-backref" href="#id34" id="notes-for-mingw-and-cygwin-with-mno-cygwin-gcc-users" name="notes-for-mingw-and-cygwin-with-mno-cygwin-gcc-users">9&nbsp;&nbsp;&nbsp;Notes for MinGW (and Cygwin with -mno-cygwin) GCC Users</a></h1>
<p>If you are using a version of Python prior to 2.4.1 with a MinGW
prior to 3.0.0 (with binutils-2.13.90-20030111-1), you will need to
create a MinGW-compatible version of the Python library; the one
shipped with Python will only work with a Microsoft-compatible
linker. Follow the instructions in the “Non-Microsoft” section of
the “Building Extensions: Tips And Tricks” chapter in <a class="reference" href="http://www.python.org/doc/current/inst/index.html">Installing
Python Modules</a> to create <tt class="docutils literal"><span class="pre">libpythonXX.a</span></tt>, where <tt class="docutils literal"><span class="pre">XX</span></tt>
corresponds to the major and minor version numbers of your Python
installation.</p>
<hr class="docutils" />
<table class="docutils footnote" frame="void" id="id16" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2" name="id16">[1]</a></td><td>Note that although we tested earlier versions of
Boost.Python with Python 2.2, and we don't <em>think</em> we've done
anything to break compatibility, this release of Boost.Python
may not have been tested with versions of Python earlier than
2.4, so we're not 100% sure that python 2.2 and 2.3 are
supported.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="naming" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id13" name="naming">[2]</a></td><td><p class="first">Information about how to identify the
static and dynamic builds of Boost.Python:</p>
<ul class="simple">
<li><a class="reference" href="../../../more/getting_started/windows.html#library-naming">on Windows</a></li>
<li><a class="reference" href="../../../more/getting_started/unix-variants.html#library-naming">on Unix variants</a></li>
</ul>
<p class="last">Be sure to read this section even if your compiler supports
auto-linking, as Boost.Python does not yet take advantage of
that feature.</p>
</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="toolset-specific" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id14" name="toolset-specific">[3]</a></td><td>Because of the way most *nix platforms
share symbols among dynamically-loaded objects, I'm not
certainextension modules built with different compiler toolsets
will always use different copies of the Boost.Python library
when loaded into the same Python instance. Not using different
libraries could be a good thing if the compilers have compatible
ABIs, because extension modules built with the two libraries
would be interoperable. Otherwise, it could spell disaster,
since an extension module and the Boost.Python library would
have different ideas of such things as class layout. I would
appreciate someone doing the experiment to find out what
happens.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="overwrite" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id7" name="overwrite">[4]</a></td><td><tt class="docutils literal"><span class="pre">configure</span></tt> overwrites the existing
<tt class="docutils literal"><span class="pre">user-config.jam</span></tt> in your home directory
(if any) after making a backup of the old version.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="flavor" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id11" name="flavor">[5]</a></td><td>Note that the <tt class="docutils literal"><span class="pre">&lt;target-os&gt;cygwin</span></tt> feature is
different from the <tt class="docutils literal"><span class="pre">&lt;flavor&gt;cygwin</span></tt> subfeature of the <tt class="docutils literal"><span class="pre">gcc</span></tt>
toolset, and you might need handle both explicitly if you also
have a MinGW GCC installed.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="user-config-jam" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id9" name="user-config-jam">[6]</a></td><td>Create the <tt class="docutils literal"><span class="pre">user-config.jam</span></tt> file if you don't
already have one.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="home-dir" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="home-dir">[7]</a></td><td><em>(<a class="fn-backref" href="#id5">1</a>, <a class="fn-backref" href="#id8">2</a>)</em> <p>Windows users, your home directory can be
found by typing:</p>
<pre class="literal-block">
ECHO %HOMEDRIVE%%HOMEPATH%
</pre>
<p class="last">into a <a class="reference" href="../../../more/getting_started/windows.html#or-build-from-the-command-prompt">Windows command prompt</a></p>
</td></tr>
</tbody>
</table>
</div>
</div>
<div class="footer">
<hr class="footer" />
<a class="reference" href="./building.rst">View document source</a>.
Generated on: 2007-04-05 20:04 UTC.
Generated by <a class="reference" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
<td valign="top">
<h1 align="center">Boost.Python</h1>
</div>
</body>
<h2 align="center">Building and Testing</h2>
</td>
</tr>
</table>
<hr>
<h2>Contents</h2>
<dl class="Reference">
<dt><a href="#requirements">Requirements</a></dt>
<dt><a href="#building">Building Boost.Python</a></dt>
<dd>
<dl class="index">
<dt><a href="#configuration">Configuration</a></dt>
<dt><a href="#results">Results</a></dt>
<dt><a href="#testing">Testing</a></dt>
</dl>
</dd>
<dt><a href="#building_ext">Building your Extension Module</a></dt>
<dt><a href="#variants">Build Variants</a></dt>
</dl>
<hr>
<h2><a name="requirements">Requirements</a></h2>
<b>Boost.Python</b> version 2 requires <a href=
"http://www.python.org/2.2">Python 2.2</a> <i>or <a href=
"http://www.python.org">newer</a></i>. An unsupported archive of
Boost.Python version 1, which works with versions of Python since 1.5.2,
is available <a href="../build/python_v1.zip">here</a>.
<h2><a name="building">Building Boost.Python</a></h2>
<p>Every Boost.Python extension module must be linked with the
<code>boost_python</code> shared library. To build
<code>boost_python</code>, use <a href=
"../../../tools/build/index.html">Boost.Build</a> in the usual way from
the <code>libs/python/build</code> subdirectory of your boost
installation (if you have already built boost from the top level this may
have no effect, since the work is already done).</p>
<h3><a name="configuration">Configuration</a></h3>
You may need to configure the following variables to point Boost.Build at
your Python installation:
<table border="1" summary="build configuration variables">
<tr>
<th>Variable Name</th>
<th>Semantics</th>
<th>Default</th>
<th>Notes</th>
</tr>
<tr>
<td><code>PYTHON_ROOT</code></td>
<td>The root directory of your Python installation</td>
<td>Windows:&nbsp;<code>c:/tools/python</code>
Unix:&nbsp;<code>/usr/local</code></td>
<td>On Unix, this is the <code>--with-prefix=</code> directory used
to configure Python</td>
</tr>
<tr>
<td><code>PYTHON_VERSION</code></td>
<td>The The 2-part python Major.Minor version number</td>
<td><code>2.2</code></td>
<td>Be sure not to include a third number, e.g. <b>not</b>
"<code>2.2.1</code>", even if that's the version you have.</td>
</tr>
<tr>
<td><code>PYTHON_INCLUDES</code></td>
<td>path to Python <code>#include</code> directories</td>
<td>Autoconfigured from <code>PYTHON_ROOT</code></td>
</tr>
<tr>
<td><code>PYTHON_LIB_PATH</code></td>
<td>path to Python library object.</td>
<td>Autoconfigured from <code>PYTHON_ROOT</code></td>
</tr>
<tr>
<td><code>PYTHON_STDLIB_PATH</code></td>
<td>path to Python standard library modules</td>
<td>Autoconfigured from <code>PYTHON_ROOT</code></td>
</tr>
<tr>
<td><code>CYGWIN_ROOT</code></td>
<td>path to the user's Cygwin installation</td>
<td>
</td>
<td><a href="http://www.cygwin.com">Cygwin</a> only. This and the
following two settings are useful when building with multiple
toolsets on Windows, since Cygwin requires a different build of
Python.</td>
</tr>
<tr>
<td><code>GCC_PYTHON_ROOT</code></td>
<td>path to the user's Cygwin Python installation</td>
<td><code>$(CYGWIN_ROOT)/usr/local</code></td>
<td><a href="http://www.cygwin.com">Cygwin</a> only</td>
</tr>
<tr>
<td><code>GCC_DEBUG_PYTHON_ROOT</code></td>
<td>path to the user's Cygwin <code><a href=
"#variants">pydebug</a></code> build</td>
<td><code>$(CYGWIN_ROOT)/usr/local/pydebug</code></td>
<td><a href="http://www.cygwin.com">Cygwin</a> only</td>
</tr>
</table>
<h3><a name="results">Results</a></h3>
<p>The build process will create a
<code>libs/python/build/bin-stage</code> subdirectory of the boost root
(or of <code>$(ALL_LOCATE_TARGET)</code>, if you have set that variable),
containing the built libraries. The libraries are actually built to
unique directories for each toolset and variant elsewhere in the
filesystem, and copied to the <code>bin-stage</code> directory as a
convenience, so if you build with multiple toolsets at once, the product
of later toolsets will overwrite that of earlier toolsets in
<code>bin-stage</code>.</p>
<h3><a name="testing">Testing</a></h3>
<p>To build and test Boost.Python, start from the
<code>libs/python/test</code> directory and invoke</p>
<blockquote>
<pre>
bjam -sTOOLS=<i><a href=
"../../../tools/build/index.html#Tools">toolset</a></i> test
</pre>
</blockquote>
This will update all of the Boost.Python v1 test and example targets. The
tests are relatively quiet by default. To get more-verbose output, you
might try
<blockquote>
<pre>
bjam -sTOOLS=<i><a href=
"../../../tools/build/index.html#Tools">toolset</a></i> -sPYTHON_TEST_ARGS=-v test
</pre>
</blockquote>
which will print each test's Python code with the expected output as it
passes.
<h2><a name="building_ext">Building your Extension Module</a></h2>
Though there are other approaches, the easiest way to build an extension
module using Boost.Python is with Boost.Build. Until Boost.Build v2 is
released, cross-project build dependencies are not supported, so it works
most smoothly if you add a new subproject to your boost installation. The
<code>libs/python/example</code> subdirectory of your boost installation
contains a minimal example (along with many extra sources). To copy the
example subproject:
<ol>
<li>Create a new subdirectory in, <code>libs/python</code>, say
<code>libs/python/my_project</code>.</li>
<li>Copy <code><a href=
"../example/Jamfile">libs/python/example/Jamfile</a></code> to your new
directory.</li>
<li>Edit the Jamfile as appropriate for your project. You'll want to
change the "<code>subproject</code>" rule invocation at the top, and
the names of some of the source files and/or targets.</li>
</ol>
If you can't modify or copy your boost installation, the alternative is
to create your own Boost.Build project. A similar example you can use as
a starting point is available in <code><a href=
"../example/project.zip">this archive</a></code>. You'll need to edit the
Jamfile and Jamrules files, depending on the relative location of your
Boost installation and the new project. Note that automatic testing of
extension modules is not available in this configuration.
<h2><a name="variants">Build Variants</a></h2>
Three <a href=
"../../../tools/build/build_system.htm#variants">variant</a>
configurations of all python-related targets are supported, and can be
selected by setting the <code><a href=
"../../../tools/build/build_system.htm#user_globals">BUILD</a></code>
variable:
<ul>
<li><code>release</code> (optimization, <tt>-DNDEBUG</tt>)</li>
<li><code>debug</code> (no optimization <tt>-D_DEBUG</tt>)</li>
<li><code>debug-python</code> (no optimization, <tt>-D_DEBUG
-DBOOST_DEBUG_PYTHON</tt>)</li>
</ul>
<p>The first two variants of the <code>boost_python</code> library are
built by default, and are compatible with the default Python
distribution. The <code>debug-python</code> variant corresponds to a
specially-built debugging version of Python. On Unix platforms, this
python is built by adding <code>--with-pydebug</code> when configuring
the Python build. On Windows, the debugging version of Python is
generated by the "Win32 Debug" target of the <code>PCBuild.dsw</code>
Visual C++ 6.0 project in the <code>PCBuild</code> subdirectory of your
Python distribution. Extension modules built with Python debugging
enabled are <b>not link-compatible</b> with a non-debug build of Python.
Since few people actually have a debug build of Python (it doesn't come
with the standard distribution), the normal <code>debug</code> variant
builds modules which are compatible with ordinary Python.</p>
<p>On many windows compilers, when extension modules are built with
<tt>-D_DEBUG</tt>, Python defaults to <i>force</i> linking with a special
debugging version of the Python DLL. Since this debug DLL isn't supplied
with the default Python installation for Windows, Boost.Python uses
<tt><a href=
"../../../boost/python/detail/wrap_python.hpp">boost/python/detail/wrap_python.hpp</a></tt>
to temporarily undefine <tt>_DEBUG</tt> when <tt>Python.h</tt> is
<tt>#include</tt>d - unless <code>BOOST_DEBUG_PYTHON</code> is
defined.</p>
<p>If you want the extra runtime checks available with the debugging
version of the library, <tt>#define BOOST_DEBUG_PYTHON</tt> to re-enable
python debuggin, and link with the <code>debug-python</code> variant of
<tt>boost_python</tt>.</p>
<p>If you do not <tt>#define BOOST_DEBUG_PYTHON</tt>, be sure that any
source files in your extension module <tt>#include&nbsp;&lt;<a href=
"../../../boost/python/detail/wrap_python.hpp">boost/python/detail/wrap_python.hpp</a>&gt;</tt>
instead of the usual <tt>Python.h</tt>, or you will have link
incompatibilities.<br>
</p>
<hr>
<p>&copy; Copyright David Abrahams 2002. Permission to copy, use, modify,
sell and distribute this document is granted provided this copyright
notice appears in all copies. This document is provided ``as is'' without
express or implied warranty, and with no claim as to its suitability for
any purpose.</p>
<p>Updated: O8 October, 2002 (David Abrahams)</p>
</body>
</html>

View File

@@ -1,404 +0,0 @@
.. Copyright David Abrahams 2006. Distributed under the Boost
.. Software License, Version 1.0. (See accompanying
.. file LICENSE_1_0.txt or copy at
.. http://www.boost.org/LICENSE_1_0.txt)
==============================================
|(logo)|__ Boost.Python Build and Test HOWTO
==============================================
.. |(logo)| image:: ../boost.png
:alt: Boost C++ Libraries:
:class: boost-logo
__ ../index.htm
.. section-numbering::
:depth: 2
.. contents:: Contents
:depth: 2
:class: sidebar small
.. |newer| replace:: *newer*
Requirements
============
Boost.Python requires `Python 2.2`_ [#2.2]_ *or* |newer|__.
.. _Python 2.2: http://www.python.org/2.2
__ http://www.python.org
No-Install Quickstart
=====================
There is no need to install Boost in order to get started using
Boost.Python. These instructions use Boost.Build_ projects,
which will build those binaries as soon as they're needed. Your
first tests may take a little longer while you wait for
Boost.Python to build, but doing things this way will save you from
worrying about build intricacies like which library binaries to use
for a specific compiler configuration.
.. Note:: Of course it's possible to use other build systems to
build Boost.Python and its extensions, but they are not
officially supported by Boost. Moreover **99% of all “I can't
build Boost.Python” problems come from trying to use another
build system**.
If you want to use another system anyway, we suggest that you
follow these instructions, and then invoke ``bjam`` with the
``-a -o``\ *filename* option to dump the build commands it executes
to a file, so you can see what your build system needs to do.
1. Get Boost; see sections 1 and 2 of the Boost `Getting Started Guide`_.
2. Get the ``bjam`` build driver. See sections 5.2.1-5.2.3 of the
Boost `Getting Started Guide`_.
3. cd into the ``libs/python/test/example`` directory.
.. _Getting Started Guide: ../../../more/getting_started/index.html
Background
==========
There are two basic models for combining C++ and Python:
- extending_, in which the end-user launches the Python interpreter
executable and imports Python “extension modules” written in C++.
Think of taking a library written in C++ and giving it a Python
interface so Python programmers can use it. From Python, these
modules look just like regular Python modules.
- embedding_, in which the end-user launches a program written
in C++ that in turn invokes the Python interpreter as a library
subroutine. Think of adding scriptability to an existing
application.
.. _extending: http://www.python.org/doc/current/ext/intro.html
.. _embedding: http://www.python.org/doc/current/ext/embedding.html
The key distinction between extending and embedding is the location
of C++' ``main()`` function: in the Python interpreter executable,
or in some other program, respectively. Note that even when
embedding Python in another program, `extension modules are often
the best way to make C/C++ functionality accessible to Python
code`__, so the use of extension modules is really at the heart of
both models.
__ http://www.python.org/doc/current/ext/extending-with-embedding.html
Except in rare cases, extension modules are built as
dynamically-loaded libraries with a single entry point, which means
you can change them without rebuilding either the other extension
modules or the executable containing ``main()``.
Getting Boost.Python Binaries
=============================
Since Boost.Python is a separately-compiled (as opposed to
`header-only`_) library, its user relies on the services of a
Boost.Python library binary.
.. _header-only: ../../../more/getting_started/windows.html#header-only-libraries
Installing Boost.Python on your System
--------------------------------------
If you need a regular, installation of the Boost.Python library
binaries on your system, the Boost `Getting Started Guide`_ will
walk you through the steps of installing one. If building binaries
from source, you might want to supply the ``--with-python``
argument to ``bjam`` (or the ``--with-libraries=python`` argument
to ``configure``), so only the Boost.Python binary will be built,
rather than all the Boost binaries.
Configuring Boost.Build
=======================
As described in the `Boost.Build reference manual`__, a file called
``user-config.jam`` in your home
directory [#home-dir]_ is used to
describe the build resources available to the build system. You'll
need to tell it about your Python installation.
__ http://www.boost.orgdoc/html/bbv2/advanced.html#bbv2.advanced.configuration
.. Admonition:: Users of Unix-Variant OSes
If you are using a unix-variant OS and you ran Boost's
``configure`` script, it may have generated a
``user-config.jam`` for you. [#overwrite]_ If your ``configure``\
/\ ``make`` sequence was successful and Boost.Python binaries
were built, your ``user-config.jam`` file is probably already
correct.
If you have a fairly “standard” python installation for your
platform, there's very little you need to do to describe it.
Simply having ::
import toolset : using ;
using python ;
in a ``user-config.jam`` file in your home directory [#home-dir]_
should be enough. [#user-config.jam]_ For more complicated setups,
see `Advanced Configuration`_.
.. Note:: You might want to pass the ``--debug-configuration``
option to ``bjam`` the first few times you invoke it, to make
sure that Boost.Build is correctly locating all the parts of
your Python installation. If it isn't, consider passing some of
the optional `Python configuration parameters`_ detailed below.
Building an Extension Module
============================
Testing
=======
Advanced Configuration
======================
If you have several versions of Python installed, or Python is
installed in an unusual way, you may want to supply any or all of
the following optional parameters to ``using python``.
Python Configuration Parameters
-------------------------------
version
the version of Python to use. Should be in Major.Minor
format, for example, ``2.3``. Do not include the subminor
version (i.e. *not* ``2.5.1``). If you have multiple Python
versions installed, the version will usually be the only
additional argument required.
cmd-or-prefix
preferably, a command that invokes a Python
interpreter. Alternatively, the installation prefix for Python
libraries and header files. Use the alternative formulation if
there is no appropriate Python executable available.
includes
the ``#include`` path for Python headers.
libraries
the path to Python library binaries. On MacOS/Darwin,
you can also pass the path of the Python framework.
condition
if specified, should be a set of Boost.Build
properties that are matched against the build configuration when
Boost.Build selects a Python configuration to use.
extension-suffix
A string to append to the name of extension
modules before the true filename extension. You almost certainly
don't need to use this. Usually this suffix is only used when
targeting a Windows debug build of Python, and will be set
automatically for you based on the value of the
``<python-debugging>`` feature. However, at least one Linux
distribution (Ubuntu Feisty Fawn) has a specially configured
`python-dbg`__ package that claims to use such a suffix.
__ https://wiki.ubuntu.com/PyDbgBuilds
Examples
--------
Note that in the examples below, case and *especially whitespace* are
significant.
- If you have both python 2.5 and python 2.4 installed,
``user-config.jam`` might contain::
using python : 2.5 ; # Make both versions of Python available
using python : 2.4 ; # To build with python 2.4, add python=2.4
# to your command line.
The first version configured (2.5) becomes the default. To build
against python 2.4, add ``python=2.4`` to the ``bjam`` command line.
- If you have python installed in an unusual location, you might
supply the path to the interpreter in the ``cmd-or-prefix``
parameter::
using python : : /usr/local/python-2.6-beta/bin/python ;
- If you have a separate build of Python for use with a particular
toolset, you might supply that toolset in the ``condition``
parameter::
using python ; # use for most toolsets
# Use with Intel C++ toolset
using python
: # version
: c:\\Devel\\Python-2.5-IntelBuild\\PCBuild\\python # cmd-or-prefix
: # includes
: # libraries
: <toolset>intel # condition
;
- You can set up your user-config.jam so a bjam built under Windows
can build/test both Windows and Cygwin_ python extensions. Just pass
``<target-os>cygwin`` in the ``condition`` parameter
for the cygwin python installation::
# windows installation
using python ;
# cygwin installation
using python : : c:\\cygwin\\bin\\python2.5 : : : <target-os>cygwin ;
when you put target-os=cygwin in your build request, it should build
with the cygwin version of python: [#flavor]_
bjam target-os=cygwin toolset=gcc
This is supposed to work the other way, too (targeting windows
python with a Cygwin_ bjam) but it seems as though the support in
Boost.Build's toolsets for building that way is broken at the
time of this writing.
- Note that because of `the way Boost.Build currently selects target
alternatives`__, you might have be very explicit in your build
requests. For example, given::
using python : 2.5 ; # a regular windows build
using python : 2.4 : : : : <target-os>cygwin ;
building with ::
bjam target-os=cygwin
will yield an error. Instead, you'll need to write::
bjam target-os=cygwin/python=2.4
.. _Cygwin: http://cygwin.com
__ http://zigzag.cs.msu.su/boost.build/wiki/AlternativeSelection
Choosing a Boost.Python Library Binary
======================================
If—instead of letting Boost.Build construct and link withthe right
libraries automatically—you choose to use a pre-built Boost.Python
library, you'll need to think about which one to link with. The
Boost.Python binary comes in both static and dynamic flavors. Take
care to choose the right flavor for your application. [#naming]_
The Dynamic Binary
------------------
The dynamic library is the safest and most-versatile choice:
- A single copy of the library code is used by all extension
modules built with a given toolset. [#toolset-specific]_
- The library contains a type conversion registry. Because one
registry is shared among all extension modules, instances of a
class exposed to Python in one dynamically-loaded extension
module can be passed to functions exposed in another such module.
The Static Binary
-----------------
It might be appropriate to use the static Boost.Python library in
any of the following cases:
- You are extending_ python and the types exposed in your
dynamically-loaded extension module don't need to be used by any
other Boost.Python extension modules, and you don't care if the
core library code is duplicated among them.
- You are embedding_ python in your application and either:
- You are targeting a Unix variant OS other than MacOS or AIX,
where the dynamically-loaded extension modules can “see” the
Boost.Python library symbols that are part of the executable.
- Or, you have statically linked some Boost.Python extension
modules into your application and you don't care if any
dynamically-loaded Boost.Python extension modules are able to
use the types exposed by your statically-linked extension
modules (and vice-versa).
Notes for MinGW (and Cygwin with -mno-cygwin) GCC Users
=======================================================
If you are using a version of Python prior to 2.4.1 with a MinGW
prior to 3.0.0 (with binutils-2.13.90-20030111-1), you will need to
create a MinGW-compatible version of the Python library; the one
shipped with Python will only work with a Microsoft-compatible
linker. Follow the instructions in the “Non-Microsoft” section of
the “Building Extensions: Tips And Tricks” chapter in `Installing
Python Modules`__ to create ``libpythonXX.a``, where ``XX``
corresponds to the major and minor version numbers of your Python
installation.
__ http://www.python.org/doc/current/inst/index.html
-----------------------------
.. [#2.2] Note that although we tested earlier versions of
Boost.Python with Python 2.2, and we don't *think* we've done
anything to break compatibility, this release of Boost.Python
may not have been tested with versions of Python earlier than
2.4, so we're not 100% sure that python 2.2 and 2.3 are
supported.
.. [#naming] Information about how to identify the
static and dynamic builds of Boost.Python:
* `on Windows`__
* `on Unix variants`__
__ ../../../more/getting_started/windows.html#library-naming
__ ../../../more/getting_started/unix-variants.html#library-naming
Be sure to read this section even if your compiler supports
auto-linking, as Boost.Python does not yet take advantage of
that feature.
.. [#toolset-specific] Because of the way most \*nix platforms
share symbols among dynamically-loaded objects, I'm not
certainextension modules built with different compiler toolsets
will always use different copies of the Boost.Python library
when loaded into the same Python instance. Not using different
libraries could be a good thing if the compilers have compatible
ABIs, because extension modules built with the two libraries
would be interoperable. Otherwise, it could spell disaster,
since an extension module and the Boost.Python library would
have different ideas of such things as class layout. I would
appreciate someone doing the experiment to find out what
happens.
.. [#overwrite] ``configure`` overwrites the existing
``user-config.jam`` in your home directory
(if any) after making a backup of the old version.
.. [#flavor] Note that the ``<target-os>cygwin`` feature is
different from the ``<flavor>cygwin`` subfeature of the ``gcc``
toolset, and you might need handle both explicitly if you also
have a MinGW GCC installed.
.. [#user-config.jam] Create the ``user-config.jam`` file if you don't
already have one.
.. [#home-dir] Windows users, your home directory can be
found by typing::
ECHO %HOMEDRIVE%%HOMEPATH%
into a `Windows command prompt`__
__ ../../../more/getting_started/windows.html#or-build-from-the-command-prompt

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@@ -1,14 +1,11 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta name="generator" content=
"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<link rel="stylesheet" type="text/css" href="boost.css">
<title>Boost.Python</title>
</head>
@@ -19,7 +16,7 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
@@ -27,60 +24,10 @@
<h2 align="center">Index</h2>
</td>
<td align="right">
<form method="get" action="http://www.google.com/custom">
<p>
<span id= "search-choice">
Search
<select name="hq" id="hq">
<option label="All Documentation" value=
"site:www.boost.org inurl:www.boost.org/libs/python/doc">
All Documentation
</option>
<option label="Tutorial" value=
"site:www.boost.org inurl:www.boost.org/libs/python/doc/tutorial">
Tutorial
</option>
<option label="Reference" value=
"site:www.boost.org inurl:www.boost.org/libs/python/doc/v2">
Reference
</option>
</select>
<br>
</span>
<span id="search-text">
<input type="text" name="q" id="q" size="31" maxlength="255" alt="Search Text" />
</span>
<br>
<span id= "google">
<a href= "http://www.google.com/search">
<img src="../../../more/google_logo_25wht.gif" alt="Google" border="0" /></a>Powered
</span>
<span id="go">
<input type="image" name="search" src="../../../more/space.gif" alt="Search" id="search-button" />
</span>
<br>
<input type="hidden" name="cof" value= "LW:277;L:http://www.boost.org/boost.png;LH:86;AH:center;GL:0;S:http://www.boost.org;AWFID:9b83d16ce652ed5a;" />
<input type="hidden" name="sa" value= "Google Search" />
<input type="hidden" name= "domains" value= "www.boost.org;mail.python.org" /></p>
</form>
</td>
</tr>
<tr>
</tr>
</table>
<hr>
<h2>Synopsis</h2>
Welcome to version 2 of <b>Boost.Python</b>, a C++ library which enables
seamless interoperability between C++ and the <a href=
@@ -109,12 +56,6 @@
<li>Documentation Strings</li>
</ul>
The development of these features was funded in part by grants to <a
href="http://www.boost-consulting.com">Boost Consulting</a> from the <a
href="http://www.llnl.gov/">Lawrence Livermore National Laboratories</a>
and by the <a href="http://cci.lbl.gov/">Computational Crystallography
Initiative</a> at Lawrence Berkeley National Laboratories.
<hr>
<h2>Contents</h2>
@@ -124,15 +65,7 @@
<dt><a href="building.html">Building and Testing</a></dt>
<dt><a href="v2/reference.html">Reference Manual</a></dt>
<dt>Suites:</dt>
<dd>
<ul>
<li><a href="v2/pickle.html">Pickle</a></li>
<li><a href="v2/indexing.html">Indexing</a></li>
</ul>
</dd>
<dt><a href="v2/reference.html">Reference</a></dt>
<dt><a href="v2/configuration.html">Configuration Information</a></dt>
@@ -141,42 +74,22 @@
<dt><a href="v2/definitions.html">Definitions</a></dt>
<dt><a href="projects.html">Projects using Boost.Python</a></dt>
<dt><a href="support.html">Support Resources</a></dt>
<dt><a href="v2/faq.html">Frequently Asked Questions (FAQs)</a></dt>
<dt><a href="../pyste/index.html">Pyste (Boost.Python code generator)</a></dt>
<dt><a href="internals.html">Internals Documentation</a></dt>
<dt><a href="news.html">News/Change Log</a></dt>
<dt><a href="../todo.html">TODO list</a></dt>
<dt><a href="v2/progress_reports.html">LLNL Progress Reports</a></dt>
<dt><a href="v2/progress_reports.html">Progress Reports</a></dt>
<dt><a href="v2/acknowledgments.html">Acknowledgments</a></dt>
</dl>
<hr>
<h2>Articles</h2>
&quot;<a href="PyConDC_2003/bpl.html">Building Hybrid
Systems With Boost Python</a>&quot;, by Dave Abrahams and Ralf
W. Grosse-Kunstleve (<a href="PyConDC_2003/bpl.pdf">PDF</a>)
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
26 August, 2003
08 October, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002-2003.</i></p>
<p><i>&copy; Copyright <a href="../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. All Rights Reserved.</i></p>
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@@ -1,186 +0,0 @@
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<body>
<div class="document" id="boost-python-internals-logo">
<h1 class="title"><a class="reference" href="index.html">Boost.Python</a> Internals <a class="reference" href="../../../index.htm"><img alt="Boost" src="../../../boost.png" /></a></h1>
<div class="section" id="a-conversation-between-brett-calcott-and-david-abrahams">
<h1><a name="a-conversation-between-brett-calcott-and-david-abrahams">A conversation between Brett Calcott and David Abrahams</a></h1>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams and Brett Calcott 2003. See
accompanying <a class="reference" href="../../../LICENSE_1_0.txt">license</a> for terms of use.</td>
</tr>
</tbody>
</table>
<p>In both of these cases, I'm quite capable of reading code - but the
thing I don't get from scanning the source is a sense of the
architecture, both structurally, and temporally (er, I mean in what
order things go on).</p>
<ol class="arabic">
<li><p class="first">What happens when you do the following:</p>
<pre class="literal-block">
struct boring {};
...etc...
class_&lt;boring&gt;(&quot;boring&quot;)
;
</pre>
</li>
</ol>
<p>There seems to be a fair bit going on.</p>
<blockquote>
<ul class="simple">
<li>Python needs a new ClassType to be registered.</li>
<li>We need to construct a new type that can hold our boring struct.</li>
<li>Inward and outward converters need to be registered for the type.</li>
</ul>
</blockquote>
<p>Can you gesture in the general direction where these things are done?</p>
<blockquote>
<p>I only have time for a &quot;off-the-top-of-my-head&quot; answer at the moment;
I suggest you step through the code with a debugger after reading this
to see how it works, fill in details, and make sure I didn't forget
anything.</p>
<blockquote>
<p>A new (Python) subclass of Boost.Python.Instance (see
libs/python/src/object/class.cpp) is created by invoking
Boost.Python.class, the metatype:</p>
<pre class="literal-block">
&gt;&gt;&gt; boring = Boost.Python.class(
... 'boring'
... , bases_tuple # in this case, just ()
... , {
... '__module__' : module_name
... , '__doc__' : doc_string # optional
... }
... )
</pre>
<p>A handle to this object is stuck in the m_class_object field
of the registration associated with <tt class="literal"><span class="pre">typeid(boring)</span></tt>. The
registry will keep that object alive forever, even if you
wipe out the 'boring' attribute of the extension module
(probably not a good thing).</p>
<p>Because you didn't specify <tt class="literal"><span class="pre">class&lt;boring,</span> <span class="pre">non_copyable,</span>
<span class="pre">...&gt;</span></tt>, a to-python converter for boring is registered which
copies its argument into a value_holder held by the the
Python boring object.</p>
<p>Because you didn't specify <tt class="literal"><span class="pre">class&lt;boring</span> <span class="pre">...&gt;(no_init)</span></tt>,
an <tt class="literal"><span class="pre">__init__</span></tt> function object is added to the class
dictionary which default-constructs a boring in a
value_holder (because you didn't specify some smart pointer
or derived wrapper class as a holder) held by the Python
boring object.</p>
<p><tt class="literal"><span class="pre">register_class_from_python</span></tt> is used to register a
from-python converter for <tt class="literal"><span class="pre">shared_ptr&lt;boring&gt;</span></tt>.
<tt class="literal"><span class="pre">boost::shared_ptr</span></tt>s are special among smart pointers
because their Deleter argument can be made to manage the
whole Python object, not just the C++ object it contains, no
matter how the C++ object is held.</p>
<p>If there were any <tt class="literal"><span class="pre">bases&lt;&gt;</span></tt>, we'd also be registering the
relationship between these base classes and boring in the
up/down cast graph (<tt class="literal"><span class="pre">inheritance.[hpp/cpp]</span></tt>).</p>
<p>In earlier versions of the code, we'd be registering lvalue
from-python converters for the class here, but now
from-python conversion for wrapped classes is handled as a
special case, before consulting the registry, if the source
Python object's metaclass is the Boost.Python metaclass.</p>
<p>Hmm, that from-python converter probably ought to be handled
the way class converters are, with no explicit conversions
registered.</p>
</blockquote>
</blockquote>
<ol class="arabic" start="2">
<li><p class="first">Can you give a brief overview of the data structures that are
present in the registry</p>
<blockquote>
<p>The registry is simple: it's just a map from typeid -&gt;
registration (see boost/python/converter/registrations.hpp).
<tt class="literal"><span class="pre">lvalue_chain</span></tt> and <tt class="literal"><span class="pre">rvalue_chain</span></tt> are simple endogenous
linked lists.</p>
<p>If you want to know more, just ask.</p>
<p>If you want to know about the cast graph, ask me something specific in
a separate message.</p>
</blockquote>
<p>and an overview of the process that happens as a type makes its
way from c++ to python and back again.</p>
</li>
</ol>
<blockquote>
<p>Big subject. I suggest some background reading: look for relevant
info in the LLNL progress reports and the messages they link to.
Also,</p>
<blockquote>
<p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-May/001023.html">http://mail.python.org/pipermail/c++-sig/2002-May/001023.html</a></p>
<p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-December/003115.html">http://mail.python.org/pipermail/c++-sig/2002-December/003115.html</a></p>
<p><a class="reference" href="http://aspn.activestate.com/ASPN/Mail/Message/1280898">http://aspn.activestate.com/ASPN/Mail/Message/1280898</a></p>
<p><a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-July/001755.html">http://mail.python.org/pipermail/c++-sig/2002-July/001755.html</a></p>
</blockquote>
<p>from c++ to python:</p>
<blockquote>
<p>It depends on the type and the call policies in use or, for
<tt class="literal"><span class="pre">call&lt;&gt;(...)</span></tt>, <tt class="literal"><span class="pre">call_method&lt;&gt;(...)</span></tt>, or <tt class="literal"><span class="pre">object(...)</span></tt>, if
<tt class="literal"><span class="pre">ref</span></tt> or <tt class="literal"><span class="pre">ptr</span></tt> is used. There are also two basic
categories to to-python conversion, &quot;return value&quot; conversion
(for Python-&gt;C++ calls) and &quot;argument&quot; conversion (for
C++-&gt;Python calls and explicit <tt class="literal"><span class="pre">object()</span></tt> conversions). The
behavior of these two categories differs subtly in various ways
whose details I forget at the moment. You can probably find
the answers in the above references, and certainly in the code.</p>
<p>The &quot;default&quot; case is by-value (copying) conversion, which uses
to_python_value as a to-python converter.</p>
<blockquote>
<p>Since there can sensibly be only one way to convert any type
to python (disregarding the idea of scoped registries for the
moment), it makes sense that to-python conversions can be
handled by specializing a template. If the type is one of
the types handled by a built-in conversion
(builtin_converters.hpp), the corresponding template
specialization of to_python_value gets used.</p>
<p>Otherwise, to_python_value uses the <tt class="literal"><span class="pre">m_to_python</span></tt>
function in the registration for the C++ type.</p>
</blockquote>
<p>Other conversions, like by-reference conversions, are only
available for wrapped classes, and are requested explicitly by
using <tt class="literal"><span class="pre">ref(...)</span></tt>, <tt class="literal"><span class="pre">ptr(...)</span></tt>, or by specifying different
CallPolicies for a call, which can cause a different to-python
converter to be used. These conversions are never registered
anywhere, though they do need to use the registration to find
the Python class corresponding to the C++ type being referred
to. They just build a new Python instance and stick the
appropriate Holder instance in it.</p>
</blockquote>
<p>from python to C++:</p>
<blockquote>
<p>Once again I think there is a distinction between &quot;return value&quot;
and &quot;argument&quot; conversions, and I forget exactly what that is.</p>
<p>What happens depends on whether an lvalue conversion is needed
(see <a class="reference" href="http://mail.python.org/pipermail/c++-sig/2002-May/001023.html">http://mail.python.org/pipermail/c++-sig/2002-May/001023.html</a>)
All lvalue conversions are also registered in a type's rvalue
conversion chain, since when an rvalue will do, an lvalue is
certainly good enough.</p>
<p>An lvalue conversion can be done in one step (just get me the
pointer to the object - it can be <tt class="literal"><span class="pre">NULL</span></tt> if no conversion is
possible) while an rvalue conversion requires two steps to
support wrapped function overloading and multiple converters for
a given C++ target type: first tell me if a conversion is
possible, then construct the converted object as a second step.</p>
</blockquote>
</blockquote>
</div>
</div>
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===================================
Boost.Python_ Internals |(logo)|__
===================================
.. |(logo)| image:: ../../../boost.png
:alt: Boost
:class: boost-logo
__ ../../../index.htm
.. _`Boost.Python`: index.html
.. _license: ../../../LICENSE_1_0.txt
-------------------------------------------------------
A conversation between Brett Calcott and David Abrahams
-------------------------------------------------------
:copyright: Copyright David Abrahams and Brett Calcott 2003. See
accompanying license_ for terms of use.
In both of these cases, I'm quite capable of reading code - but the
thing I don't get from scanning the source is a sense of the
architecture, both structurally, and temporally (er, I mean in what
order things go on).
1) What happens when you do the following::
struct boring {};
...etc...
class_<boring>("boring")
;
There seems to be a fair bit going on.
- Python needs a new ClassType to be registered.
- We need to construct a new type that can hold our boring struct.
- Inward and outward converters need to be registered for the type.
Can you gesture in the general direction where these things are done?
I only have time for a "off-the-top-of-my-head" answer at the moment;
I suggest you step through the code with a debugger after reading this
to see how it works, fill in details, and make sure I didn't forget
anything.
A new (Python) subclass of Boost.Python.Instance (see
libs/python/src/object/class.cpp) is created by invoking
Boost.Python.class, the metatype::
>>> boring = Boost.Python.class(
... 'boring'
... , bases_tuple # in this case, just ()
... , {
... '__module__' : module_name
... , '__doc__' : doc_string # optional
... }
... )
A handle to this object is stuck in the m_class_object field
of the registration associated with ``typeid(boring)``. The
registry will keep that object alive forever, even if you
wipe out the 'boring' attribute of the extension module
(probably not a good thing).
Because you didn't specify ``class<boring, non_copyable,
...>``, a to-python converter for boring is registered which
copies its argument into a value_holder held by the the
Python boring object.
Because you didn't specify ``class<boring ...>(no_init)``,
an ``__init__`` function object is added to the class
dictionary which default-constructs a boring in a
value_holder (because you didn't specify some smart pointer
or derived wrapper class as a holder) held by the Python
boring object.
``register_class_from_python`` is used to register a
from-python converter for ``shared_ptr<boring>``.
``boost::shared_ptr``\ s are special among smart pointers
because their Deleter argument can be made to manage the
whole Python object, not just the C++ object it contains, no
matter how the C++ object is held.
If there were any ``bases<>``, we'd also be registering the
relationship between these base classes and boring in the
up/down cast graph (``inheritance.[hpp/cpp]``).
In earlier versions of the code, we'd be registering lvalue
from-python converters for the class here, but now
from-python conversion for wrapped classes is handled as a
special case, before consulting the registry, if the source
Python object's metaclass is the Boost.Python metaclass.
Hmm, that from-python converter probably ought to be handled
the way class converters are, with no explicit conversions
registered.
2) Can you give a brief overview of the data structures that are
present in the registry
The registry is simple: it's just a map from typeid ->
registration (see boost/python/converter/registrations.hpp).
``lvalue_chain`` and ``rvalue_chain`` are simple endogenous
linked lists.
If you want to know more, just ask.
If you want to know about the cast graph, ask me something specific in
a separate message.
and an overview of the process that happens as a type makes its
way from c++ to python and back again.
Big subject. I suggest some background reading: look for relevant
info in the LLNL progress reports and the messages they link to.
Also,
http://mail.python.org/pipermail/c++-sig/2002-May/001023.html
http://mail.python.org/pipermail/c++-sig/2002-December/003115.html
http://aspn.activestate.com/ASPN/Mail/Message/1280898
http://mail.python.org/pipermail/c++-sig/2002-July/001755.html
from c++ to python:
It depends on the type and the call policies in use or, for
``call<>(...)``, ``call_method<>(...)``, or ``object(...)``, if
``ref`` or ``ptr`` is used. There are also two basic
categories to to-python conversion, "return value" conversion
(for Python->C++ calls) and "argument" conversion (for
C++->Python calls and explicit ``object()`` conversions). The
behavior of these two categories differs subtly in various ways
whose details I forget at the moment. You can probably find
the answers in the above references, and certainly in the code.
The "default" case is by-value (copying) conversion, which uses
to_python_value as a to-python converter.
Since there can sensibly be only one way to convert any type
to python (disregarding the idea of scoped registries for the
moment), it makes sense that to-python conversions can be
handled by specializing a template. If the type is one of
the types handled by a built-in conversion
(builtin_converters.hpp), the corresponding template
specialization of to_python_value gets used.
Otherwise, to_python_value uses the ``m_to_python``
function in the registration for the C++ type.
Other conversions, like by-reference conversions, are only
available for wrapped classes, and are requested explicitly by
using ``ref(...)``, ``ptr(...)``, or by specifying different
CallPolicies for a call, which can cause a different to-python
converter to be used. These conversions are never registered
anywhere, though they do need to use the registration to find
the Python class corresponding to the C++ type being referred
to. They just build a new Python instance and stick the
appropriate Holder instance in it.
from python to C++:
Once again I think there is a distinction between "return value"
and "argument" conversions, and I forget exactly what that is.
What happens depends on whether an lvalue conversion is needed
(see http://mail.python.org/pipermail/c++-sig/2002-May/001023.html)
All lvalue conversions are also registered in a type's rvalue
conversion chain, since when an rvalue will do, an lvalue is
certainly good enough.
An lvalue conversion can be done in one step (just get me the
pointer to the object - it can be ``NULL`` if no conversion is
possible) while an rvalue conversion requires two steps to
support wrapped function overloading and multiple converters for
a given C++ target type: first tell me if a conversion is
possible, then construct the converted object as a second step.

326
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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<title>A New Type Conversion Mechanism for Boost.Python</title>
</head>
<body bgcolor="#FFFFFF" text="#000000">
<p><img border="0" src="../../../c++boost.gif" width="277" height="86"
alt="boost logo"></p>
<h1>A New Type Conversion Mechanism for Boost.Python</h1>
<p>By <a href="../../../people/dave_abrahams.htm">David Abrahams</a>.
<h2>Introduction</h2>
This document describes a redesign of the mechanism for automatically
converting objects between C++ and Python. The current implementation
uses two functions for any type <tt>T</tt>:
<blockquote><pre>
U from_python(PyObject*, type&lt;T&gt;);
void to_python(V);
</pre></blockquote>
where U is convertible to T and T is convertible to V. These functions
are at the heart of C++/Python interoperability in Boost.Python, so
why would we want to change them? There are many reasons:
<h3>Bugs</h3>
<p>Firstly, the current mechanism relies on a common C++ compiler
bug. This is not just embarrassing: as compilers get to be more
conformant, the library stops working. The issue, in detail, is the
use of inline friend functions in templates to generate
conversions. It is a very powerful, and legal technique as long as
it's used correctly:
<blockquote><pre>
template &lt;class Derived&gt;
struct add_some_functions
{
friend <i>return-type</i> some_function1(..., Derived <i>cv-*-&amp;-opt</i>, ...);
friend <i>return-type</i> some_function2(..., Derived <i>cv-*-&amp;-opt</i>, ...);
};
template &lt;class T&gt;
struct some_template : add_some_functions&lt;some_template&lt;T&gt; &gt;
{
};
</pre></blockquote>
The <tt>add_some_functions</tt> template generates free functions
which operate on <tt>Derived</tt>, or on related types. Strictly
speaking the related types are not just cv-qualified <tt>Derived</tt>
values, pointers and/or references. Section 3.4.2 in the standard
describes exactly which types you must use as parameters to these
functions if you want the functions to be found
(there is also a less-technical description in section 11.5.1 of
C++PL3 <a href="#ref_1">[1]</a>). Suffice it to say that
with the current design, the <tt>from_python</tt> and
<tt>to_python</tt> functions are not supposed to be callable under any
conditions!
<h3>Compilation and Linking Time</h3>
The conversion functions generated for each wrapped class using the
above technique are not function templates, but regular functions. The
upshot is that they must <i>all</i> be generated regardless of whether
they are actually used. Generating all of those functions can slow
down module compilation, and resolving the references can slow down
linking.
<h3>Efficiency</h3>
The conversion functions are primarily used in (member) function
wrappers to convert the arguments and return values. Being functions,
converters have no interface which allows us to ask &quot;will the
conversion succeed?&quot; without calling the function. Since the
return value of the function must be the object to be passed as an
argument, Boost.Python currently uses C++ exception-handling to detect
an unsuccessful conversion. It's not a particularly good use of
exception-handling, since the failure is not handled very far from
where it occurred. More importantly, it means that C++ exceptions are
thrown during overload resolution as we seek an overload that matches
the arguments passed. Depending on the implementation, this approach
can result in significant slowdowns.
<p>It is also unclear that the current library generates a minimal
amount of code for any type conversion. Many of the conversion
functions are nontrivial, and partly because of compiler limitations,
they are declared <tt>inline</tt>. Also, we could have done a better
job separating the type-specific conversion code from the code which
is type-independent.
<h3>Cross-module Support</h3>
The current strategy requires every module to contain the definition
of conversions it uses. In general, a new module can never supply
conversion code which is used by another module. Ralf Grosse-Kunstleve
designed a clever system which imports conversions directly from one
library into another using some explicit declarations, but it has some
disadvantages also:
<ol>
<li>The system Ullrich Koethe designed for implicit conversion between
wrapped classes related through inheritance does not currently work if
the classes are defined in separate modules.
<li>The writer of the importing module is required to know the name of
the module supplying the imported conversions.
<li>There can be only one way to extract any given C++ type from a
Python object in a given module.
</ol>
The first item might be addressed by moving Boost.Python into a shared
library, but the other two cannot. Ralf turned the limitation in item
two into a feature: the required module is loaded implicitly when a
conversion it defines is invoked. We will probably want to provide
that functionality anyway, but it's not clear that we should require
the declaration of all such conversions. The final item is a more
serious limitation. If, for example, new numeric types are defined in
separate modules, and these types can all be converted to
<tt>double</tt>s, we have to choose just one conversion method.
<h3>Ease-of-use</h3>
One persistent source of confusion for users of Boost.Python has been
the fact that conversions for a class are not be visible at
compile-time until the declaration of that class has been seen. When
the user tries to expose a (member) function operating on or returning
an instance of the class in question, compilation fails...even though
the user goes on to expose the class in the same translation unit!
<p>
The new system lifts all compile-time checks for the existence of
particular type conversions and replaces them with runtime checks, in
true Pythonic style. While this might seem cavalier, the compile-time
checks are actually not much use in the current system if many classes
are wrapped in separate modules, since the checks are based only on
the user's declaration that the conversions exist.
<h2>The New Design</h2>
<h3>Motivation</h3>
The new design was heavily influenced by a desire to generate as
little code as possible in extension modules. Some of Boost.Python's
clients are enormous projects where link time is proportional to the
amount of object code, and there are many Python extension modules. As
such, we try to keep type-specific conversion code out of modules
other than the one the converters are defined in, and rely as much as
possible on centralized control through a shared library.
<h3>The Basics</h3>
The library contains a <tt>registry</tt> which maps runtime type
identifiers (actually an extension of <tt>std::type_info</tt> which
preserves references and constness) to entries containing type
converters. An <tt>entry</tt> can contain only one converter from C++ to Python
(<tt>wrapper</tt>), but many converters from Python to C++
(<tt>unwrapper</tt>s). <font color="#ff0000">What should happen if
multiple modules try to register wrappers for the same type?</font>. Wrappers
and unwrappers are known as <tt>body</tt> objects, and are accessed
by the user and the library (in its function-wrapping code) through
corresponding <tt>handle</tt> (<tt>wrap&lt;T&gt;</tt> and
<tt>unwrap&lt;T&gt;</tt>) objects. The <tt>handle</tt> objects are
extremely lightweight, and delegate <i>all</i> of their operations to
the corresponding <tt>body</tt>.
<p>
When a <tt>handle</tt> object is constructed, it accesses the
registry to find a corresponding <tt>body</tt> that can convert the
handle's constructor argument. Actually the registry record for any
type
<tt>T</tt>used in a module is looked up only once and stored in a
static <tt>registration&lt;T&gt;</tt> object for efficiency. For
example, if the handle is an <tt>unwrap&lt;Foo&amp;&gt;</tt> object,
the <tt>entry</tt> for <tt>Foo&amp;</tt> is looked up in the
<tt>registry</tt>, and each <tt>unwrapper</tt> it contains is queried
to determine if it can convert the
<tt>PyObject*</tt> with which the <tt>unwrap</tt> was constructed. If
a body object which can perform the conversion is found, a pointer to
it is stored in the handle. A body object may at any point store
additional data in the handle to speed up the conversion process.
<p>
Now that the handle has been constructed, the user can ask it whether
the conversion can be performed. All handles can be tested as though
they were convertible to <tt>bool</tt>; a <tt>true</tt> value
indicates success. If the user forges ahead and tries to do the
conversion without checking when no conversion is possible, an
exception will be thrown as usual. The conversion itself is performed
by the body object.
<h3>Handling complex conversions</h3>
<p>Some conversions may require a dynamic allocation. For example,
when a Python tuple is converted to a <tt>std::vector&lt;double&gt;
const&amp;</tt>, we need some storage into which to construct the
vector so that a reference to it can be formed. Furthermore, multiple
conversions of the same type may need to be &quot;active&quot;
simultaneously, so we can't keep a single copy of the storage
anywhere. We could keep the storage in the <tt>body</tt> object, and
have the body clone itself in case the storage is used, but in that
case the storage in the body which lives in the registry is never
used. If the storage was actually an object of the target type (the
safest way in C++), we'd have to find a way to construct one for the
body in the registry, since it may not have a default constructor.
<p>
The most obvious way out of this quagmire is to allocate the object using a
<i>new-expression</i>, and store a pointer to it in the handle. Since
the <tt>body</tt> object knows everything about the data it needs to
allocate (if any), it is also given responsibility for destroying that
data. When the <tt>handle</tt> is destroyed it asks the <tt>body</tt>
object to tear down any data it may have stored there. In many ways,
you can think of the <tt>body</tt> as a &quot;dynamically-determined
vtable&quot; for the handle.
<h3>Eliminating Redundancy</h3>
If you look at the current Boost.Python code, you'll see that there
are an enormous number of conversion functions generated for each
wrapped class. For a given class <tt>T</tt>, functions are generated
to extract the following types <tt>from_python</tt>:
<blockquote><pre>
T*
T const*
T const* const&amp;
T* const&amp;
T&amp;
T const&amp;
T
std::auto_ptr&lt;T&gt;&amp;
std::auto_ptr&lt;T&gt;
std::auto_ptr&lt;T&gt; const&amp;
boost::shared_ptr&lt;T&gt;&amp;
boost::shared_ptr&lt;T&gt;
boost::shared_ptr&lt;T&gt; const&amp;
</pre></blockquote>
Most of these are implemented in terms of just a few conversions, and
<t>if you're lucky</t>, they will be inlined and cause no extra
overhead. In the new system, however, a significant amount of data
will be associated with each type that needs to be converted. We
certainly don't want to register a separate unwrapper object for all
of the above types.
<p>Fortunately, much of the redundancy can be eliminated. For example,
if we generate an unwrapper for <tt>T&</tt>, we don't need an
unwrapper for <tt>T const&</tt> or <tt>T</tt>. Accordingly, the user's
request to wrap/unwrap a given type is translated at compile-time into
a request which helps to eliminate redundancy. The rules used to
<tt>unwrap</tt> a type are:
<ol>
<li> Treat built-in types specially: when unwrapping a value or
constant reference to one of these, use a value for the target
type. It will bind to a const reference if neccessary, and more
importantly, avoids having to dynamically allocate room for
an lvalue of types which can be cheaply copied.
<li>
Reduce everything else to a reference to an un-cv-qualified type
where possible. Since cv-qualification is lost on Python
anyway, there's no point in trying to convert to a
<tt>const&amp;</tt>. <font color="#ff0000">What about conversions
to values like the tuple-&gt;vector example above? It seems to me
that we don't want to make a <tt>vector&lt;double&gt;&amp;</tt>
(non-const) converter available for that case. We may need to
rethink this slightly.</font>
</ol>
<p>To handle the problem described above in item 2, we modify the
procedure slightly. To unwrap any non-scalar <tt>T</tt>, we seek an
unwrapper for <tt>add_reference&lt;T&gt;::type</tt>. Unwrappers for
<tt>T&nbsp;const&amp;</tt> always return <tt>T&amp;</tt>, and are
registered under both <tt>T&nbsp;&amp;</tt> and
<tt>T&nbsp;const&amp;</tt>.
<p>For compilers not supporting partial specialization, unwrappers for
<tt>T&nbsp;const&amp;</tt> must return <tt>T&nbsp;const&amp;</tt>
(since constness can't be stripped), but a separate unwrapper object
need to be registered for <tt>T&nbsp;&amp;</tt> and
<tt>T&nbsp;const&amp;</tt> anyway, for the same reasons.
<font color="#ff0000">We may want to make it possible to compile as
though partial specialization were unavailable even on compilers where
it is available, in case modules could be compiled by different
compilers with compatible ABIs (e.g. Intel C++ and MSVC6).</font>
<h3>Efficient Argument Conversion</h3>
Since type conversions are primarily used in function wrappers, an
optimization is provided for the case where a group of conversions are
used together. Each <tt>handle</tt> class has a corresponding
&quot;<tt>_more</tt>&quot; class which does the same job, but has a
trivial destructor. Instead of asking each &quot;<tt>_more</tt>&quot;
handle to destroy its own body, it is linked into an endogenous list
managed by the first (ordinary) handle. The <tt>wrap</tt> and
<tt>unwrap</tt> destructors are responsible for traversing that list
and asking each <tt>body</tt> class to tear down its
<tt>handle</tt>. This mechanism is also used to determine if all of
the argument/return-value conversions can succeed with a single
function call in the function wrapping code. <font color="#ff0000">We
might need to handle return values in a separate step for Python
callbacks, since the availablility of a conversion won't be known
until the result object is retrieved.</font>
<br>
<hr>
<h2>References</h2>
<p><a name="ref_1">[1]</a>B. Stroustrup, The C++ Programming Language
Special Edition Addison-Wesley, ISBN 0-201-70073-5.
<hr>
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan -->19 December 2001<!--webbot bot="Timestamp" endspan i-checksum="31283" --></p>
<p>© Copyright David Abrahams, 2001</p>
</body>
</html>

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This hierarchy contains converter handle classes.
+-------------+
| noncopyable |
+-------------+
^
| A common base class used so that
+--------+--------+ conversions can be linked into a
| conversion_base | chain for efficient argument
+-----------------+ conversion
^
|
+---------+-----------+
| |
+-----------+----+ +------+-------+ only used for
| unwrap_more<T> | | wrap_more<T> | chaining, and don't manage any
+----------------+ +--------------+ resources.
^ ^
| |
+-----+-----+ +-------+-+ These converters are what users
| unwrap<T> | | wrap<T> | actually touch, but they do so
+-----------+ +---------+ through a type generator which
minimizes the number of converters
that must be generated, so they
Each unwrap<T>, unwrap_more<T>, wrap<T>, wrap_more<T> converter holds
a reference to an appropriate converter object
This hierarchy contains converter body classes
Exposes use/release which
are needed in case the converter
+-----------+ in the registry needs to be
| converter | cloned. That occurs when a
+-----------+ unwrap target type is not
^ contained within the Python object.
|
+------------------+-----+
| |
+--------+-------+ Exposes |
| unwrapper_base | convertible() |
+----------------+ |
^ |
| |
+--------+----+ +-----+-----+
| unwrapper<T>| | wrapper<T>|
+-------------+ +-----------+
Exposes T convert(PyObject*) Exposes PyObject* convert(T)
unwrap:
constructed with a PyObject*, whose reference count is
incremented.
find the registry entry for the target type
look in the collection of converters for one which claims to be
able to convert the PyObject to the target type.
stick a pointer to the unwrapper in the unwrap object
when unwrap is queried for convertibility, it checks to see
if it has a pointer to an unwrapper.
on conversion, the unwrapper is asked to allocate an
implementation if the unwrap object isn't already holding
one. The unwrap object "takes ownership" of the unwrapper's
implementation. No memory allocation will actually take place
unless this is a value conversion.
on destruction, the unwrapper is asked to free any implementation
held by the unwrap object. No memory deallocation actually
takes place unless this is a value conversion
on destruction, the reference count on the held PyObject is
decremented.
We need to make sure that by default, you can't instantiate
callback<> for reference and pointer return types: although the
unwrappers may exist, they may convert by-value, which would cause
the referent to be destroyed upon return.
wrap:
find the registry entry for the source type
see if there is a converter. If found, stick a pointer to it in
the wrap object.
when queried for convertibility, it checks to see if it has a
pointer to a converter.
on conversion, a reference to the target PyObject is held by the
converter. Generally, the PyObject will have been created by the
converter, but in certain cases it may be a pre-existing object,
whose reference count will have been incremented.
when a wrap<T> x is used to return from a C++ function,
x.release() is returned so that x no longer holds a reference to
the PyObject when destroyed.
Otherwise, on destruction, any PyObject still held has its
reference-count decremented.
When a converter is created by the user, the appropriate element must
be added to the registry; when it is destroyed, it must be removed
from the registry.

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<h1 align="center"><a href="index.html">Boost.Python</a></h1>
<h2 align="center">News/Change Log</h2>
</td>
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</table>
<hr>
<dl class="page-index">
<dt>Current CVS</dt>
<dd>
<ul>
<li>C++ signatures are now automatically appended to the
docstrings.
<li>New <a href="v2/docstring_options.html"
><code>docstring_options.hpp</code></a> header to
control the content of docstrings.
<li>Support for converting <code>void*</code> to/from python,
with <code><a
href="v2/opaque.html">opaque_pointer_converter</a></code>
as the return value policy. Thanks to Niall Douglas for the
initial patch.
</ul>
</dd>
<dt>19 October 2005 - 1.33.1 release</dt>
<dd>
<ul>
<li><code>wrapper&lt;T&gt;</code> can now be used as expected with a
held type of <i>some-smart-pointer</i><code>&lt;T&gt;</code></li>
<li>The build now assumes Python 2.4 by default, rather than 2.2</li>
<li>Support Python that's built without Unicode support</li>
<li>Support for wrapping classes with overloaded address-of
(<code>&amp;</code>) operators</li>
</ul>
</dd>
<dt>14 August 2005 - 1.33 release</dt>
<dd>
<ul>
<li>Support for docstrings on nonstatic properties.</li>
<li>We now export the client-provided docstrings for
<code>init&lt;optional&lt;&gt; &gt;</code> and
<i>XXX</i><code>_FUNCTION_OVERLOADS()</code> for only the last
overload.</li>
<li>Fixed some support for Embedded VC++ 4</li>
<li>Better support for rvalue from-python conversions of shared_ptr:
always return a pointer that holds the owning python object *unless*
the python object contains a NULL shared_ptr holder of the right
type.</li>
<li>Support for exposing <code>vector&lt;T*&gt;</code> with the
indexing suite.</li>
<li>Support for GCC-3.3 on MacOS.</li>
<li>updated visual studio project build file to include two new files
(slice.cpp and wrapper.cpp)</li>
<li>Added search feature to the index page.</li>
<li>Numerous fixes to the tutorial</li>
<li>Numerous workarounds for MSVC 6 and 7, GCC 2.96, and EDG
2.45</li>
</ul>
</dd>
<dt>11 March 2005</dt>
<dd>
<ul>
<li>Added a hack that will fool PyDoc into working with Boost.Python,
thanks to Nick Rasmussen</li>
</ul>
</dd>
<dt>19 November 2004 - 1.32 release</dt>
<dd>
<ul>
<li>Updated to use the Boost Software License.</li>
<li>A new, <a href=
"tutorial/doc/html/python/exposing.html#python.class_virtual_functions">
better method of wrapping classes with virtual functions</a> has been
implemented.</li>
<li>Support for upcoming GCC symbol export control features have been
folded in, thanks to Niall Douglas.</li>
<li>Improved support for <code>std::auto_ptr</code>-like types.</li>
<li>The Visual C++ bug that makes top-level <i>cv-qualification</i>
of function parameter types part of the function type has been worked
around.</li>
<li>Components used by other libraries have been moved out of
<code>python/detail</code> and into <code>boost/detail</code> to
improve dependency relationships.</li>
<li>Miscellaneous bug fixes and compiler workarounds.</li>
</ul>
</dd>
<dt>8 Sept 2004</dt>
<dd>Support for Python's Bool type, thanks to <a href=
"mailto:dholth-at-fastmail.fm">Daniel Holth</a>.</dd>
<dt>11 Sept 2003</dt>
<dd>
<ul>
<li>Changed the response to multiple to-python converters being
registered for the same type from a hard error into warning;
Boost.Python now reports the offending type in the message.</li>
<li>Added builtin <code>std::wstring</code> conversions</li>
<li>Added <code>std::out_of_range</code> =&gt; Python
<code>IndexError</code> exception conversion, thanks to <a href=
"mailto:RaoulGough-at-yahoo.co.uk">Raoul Gough</a></li>
</ul>
</dd>
<dt>9 Sept 2003</dt>
<dd>Added new <code><a href="v2/str.html#str-spec">str</a></code></dd>
<dt>constructors which take a range of characters, allowing strings
containing nul (<code>'\0'</code>) characters.</dt>
<dt>8 Sept 2003</dt>
<dd>Added the ability to create methods from function objects (with an
<code>operator()</code>); see the <a href=
"v2/make_function.html#make_function-spec">make_function</a> docs for
more info.</dd>
<dt>10 August 2003</dt>
<dd>Added the new <code>properties</code> unit tests contributed by
<a href="mailto:romany-at-actimize.com">Roman Yakovenko</a> and
documented <code>add_static_property</code> at his urging.</dd>
<dt>1 August 2003</dt>
<dd>
Added the new <code>arg</code> class contributed by <a href=
"mailto:nickm-at-sitius.com">Nikolay Mladenov</a> which supplies the
ability to wrap functions that can be called with ommitted arguments in
the middle:
<pre>
void f(int x = 0, double y = 3.14, std::string z = std::string("foo"));
BOOST_PYTHON_MODULE(test)
{
def("f", f
, (arg("x", 0), arg("y", 3.14), arg("z", "foo")));
}
</pre>And in Python:
<pre>
&gt;&gt;&gt; import test
&gt;&gt;&gt; f(0, z = "bar")
&gt;&gt;&gt; f(z = "bar", y = 0.0)
</pre>Thanks, Nikolay!
</dd>
<dt>22 July 2003</dt>
<dd>Killed the dreaded "bad argument type for builtin operation" error.
Argument errors now show the actual and expected argument types!</dd>
<dt>19 July 2003</dt>
<dd>Added the new <code><a href=
"v2/return_arg.html">return_arg</a></code> policy from <a href=
"mailto:nickm-at-sitius.com">Nikolay Mladenov</a>. Thanks, Nikolay!</dd>
<dt>18 March, 2003</dt>
<dd><a href="mailto:Gottfried.Ganssauge-at-haufe.de">Gottfried
Gan&szlig;auge</a> has contributed <a href=
"v2/opaque.html">opaque pointer support</a>.<br>
<a href="mailto:nicodemus-at-globalite.com.br">Bruno da Silva de
Oliveira</a> has contributed the exciting <a href=
"../pyste/index.html">Pyste</a> ("Pie-steh") package.</dd>
<dt>24 February 2003</dt>
<dd>Finished improved support for <code>boost::shared_ptr</code>. Now any
wrapped object of C++ class <code>X</code> can be converted automatically
to <code>shared_ptr&lt;X&gt;</code>, regardless of how it was wrapped.
The <code>shared_ptr</code> will manage the lifetime of the Python object
which supplied the <code>X</code>, rather than just the <code>X</code>
object itself, and when such a <code>shared_ptr</code> is converted back
to Python, the original Python object will be returned.</dd>
<dt>19 January 2003</dt>
<dd>Integrated <code>staticmethod</code> support from <a href=
"mailto:nickm-at-sitius.com">Nikolay Mladenov</a>. Thanks, Nikolay!</dd>
<dt>29 December 2002</dt>
<dd>Added Visual Studio project file and instructions from Brett Calcott.
Thanks, Brett!</dd>
<dt>20 December 2002</dt>
<dd>Added automatic downcasting for pointers, references, and smart
pointers to polymorphic class types upon conversion to python</dd>
<dt>18 December 2002</dt>
<dd>Optimized from_python conversions for wrapped classes by putting the
conversion logic in the shared library instead of registering separate
converters for each class in each extension module</dd>
<dt>19 November 2002</dt>
<dd>Removed the need for users to cast base class member function
pointers when used as arguments to <a href=
"v2/class.html#class_-spec-modifiers">add_property</a></dd>
<dt>13 December 2002</dt>
<dd>Allow exporting of <a href=
"v2/enum.html#enum_-spec"><code>enum_</code></a> values into enclosing
<a href="v2/scope.html#scope-spec"><code>scope</code></a>.<br>
Fixed unsigned integer conversions to deal correctly with numbers that
are out-of-range of <code>signed long</code>.</dd>
<dt>14 November 2002</dt>
<dd>Auto-detection of class data members wrapped with <a href=
"v2/data_members.html#make_getter-spec"><code>make_getter</code></a></dd>
<dt>13 November 2002</dt>
<dd>Full Support for <code>std::auto_ptr&lt;&gt;</code> added.</dd>
<dt>October 2002</dt>
<dd>Ongoing updates and improvements to tutorial documentation</dd>
<dt>10 October 2002</dt>
<dd>Boost.Python V2 is released!</dd>
</dl>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
19 November 2004
<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i>&copy; Copyright <a href="../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002-2003.</i></p>
</body>
</html>

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.. Copyright David Abrahams 2006. Distributed under the Boost
.. Software License, Version 1.0. (See accompanying
.. file LICENSE_1_0.txt or copy at
.. http://www.boost.org/LICENSE_1_0.txt)
How Runtime Polymorphism is expressed in Boost.Python:
-----------------------------------------------------
struct A { virtual std::string f(); virtual ~A(); };
std::string call_f(A& x) { return x.f(); }
struct B { virtual std::string f() { return "B"; } };
struct Bcb : B
{
Bcb(PyObject* self) : m_self(self) {}
virtual std::string f() { return call_method<std::string>(m_sef, "f"); }
static std::string f_default(B& b) { return b.B::f(); }
PyObject* m_self;
};
struct C : B
{
virtual std::string f() { return "C"; }
};
>>> class D(B):
... def f():
... return 'D'
...
>>> class E(B): pass
...
When we write, "invokes B::f non-virtually", we mean:
void g(B& x) { x.B::f(); }
This will call B::f() regardless of the dynamic type of x. Any other
way of invoking B::f, including through a function pointer, is a
"virtual invocation", and will call the most-derived override of f().
Case studies
C++\Python class
\___A_____B_____C_____D____E___
|
A | 1
|
B | 2 3
|
Bcb | 4 5 6
|
C | 7 8
|
1. Simple case
2. Python A holds a B*. Probably won't happen once we have forced
downcasting.
Requires:
x.f() -> 'B'
call_f(x) -> 'B'
Implies: A.f invokes A::f() (virtually or otherwise)
3. Python B holds a B*.
Requires:
x.f() -> 'B'
call_f(x) -> 'B'
Implies: B.f invokes B::f (virtually or otherwise)
4. B constructed from Python
Requires:
x.f() -> 'B'
call_f(x) -> 'B'
Implies: B.f invokes B::f non-virtually. Bcb::f invokes B::f
non-virtually.
Question: Does it help if we arrange for Python B construction to
build a true B object? Then this case doesn't arise.
5. D is a Python class derived from B
Requires:
x.f() -> 'D'
call_f(x) -> 'D'
Implies: Bcb::f must invoke call_method to look up the Python
method override, otherwise call_f wouldn't work.
6. E is like D, but doesn't override f
Requires:
x.f() -> 'B'
call_f(x) -> 'B'
Implies: B.f invokes B::f non-virtually. If it were virtual, x.f()
would cause infinite recursion, because we've already
determined that Bcb::f must invoke call_method to look up
the Python method override.
7. Python B object holds a C*
Requires:
x.f() -> 'C'
call_f(x) -> 'C'
Implies: B.f invokes B::f virtually.
8. C object constructed from Python
Requires:
x.f() -> 'C'
call_f(x) -> 'C'
Implies: nothing new.
------
Total implications:
2: A.f invokes A::f() (virtually or otherwise)
3: B.f invokes B::f (virtually or otherwise)
4: B.f invokes B::f non-virtually. Bcb::f invokes B::f non-virtually
6: B.f invokes B::f non-virtually.
7: B.f invokes B::f virtually.
5: Bcb::f invokes call_method to look up the Python method
Though (4) is avoidable, clearly 6 and 7 are not, and they
conflict. The implication is that B.f must choose its behavior
according to the type of the contained C++ object. If it is Bcb, a
non-virtual call to B::f must occur. Otherwise, a virtual call to B::f
must occur. This is essentially the same scheme we had with
Boost.Python v1.
Note: in early versions of Boost.Python v1, we solved this problem by
introducing a new Python class in the hierarchy, so that D and E
actually derive from a B', and B'.f invokes B::f non-virtually, while
B.f invokes B::f virtually. However, people complained about the
artificial class in the hierarchy, which was revealed when they tried
to do normal kinds of Python introspection.
-------
Assumption: we will have a function which builds a virtual function
dispatch callable Python object.
make_virtual_function(pvmf, default_impl, call_policies, dispatch_type)
Pseudocode:
Get first argument from Python arg tuple
if it contains dispatch_type
call default_impl
else
call through pvmf
Open questions:
1. What about Python multiple inheritance? Do we have the right
check in the if clause above?
A: Not quite. The correct test looks like:
Deduce target type of pvmf, i.e. T in R(T::*)(A1...AN).
Find holder in first argument which holds T
if it holds dispatch_type...
2. Can we make this more efficient?
The current "returning" mechanism will look up a holder for T
again. I don't know if we know how to avoid that.
OK, the solution involves reworking the call mechanism. This is
neccesary anyway in order to enable wrapping of function objects.
It can result in a reduction in the overall amount of source code,
because returning<> won't need to be specialized for every
combination of function and member function... though it will still
need a void specialization. We will still need a way to dispatch to
member functions through a regular function interface. mem_fn is
almost the right tool, but it only goes up to 8
arguments. Forwarding is tricky if you don't want to incur copies.
I think the trick is to use arg_from_python<T>::result_type for each
argument to the forwarder.
Another option would be to use separate function, function object,
and member function dispatchers. Once you know you have a member
function, you don't need cv-qualified overloads to call it.
Hmm, while we're at this, maybe we should solve the write-back
converter problem. Can we do it? Maybe not. Ralf doesn't want to
write special write-back functions here, does he? He wants the
converter to do the work automatically. We could add
cleanup/destructor registration. That would relieve the client from
having accessible destructors for types which are being converted by
rvalue. I'm not sure that this will really save any code,
however. It rather depends on the linker, doesn't it? I wonder if
this can be done in a backwards-compatible fashion by generating the
delete function when it's not supplied?

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<h3><a href="../../../index.htm"><img height="86" width="277" alt=
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<td valign="top">
<h1 align="center"><a href="index.html">Boost.Python</a></h1>
<h2 align="center">Projects using Boost.Python</h2>
</td>
</tr>
</table>
<hr>
<h2>Introduction</h2>
<p>This is a partial list of projects using Boost.Python. If you are using
Boost.Python as your Python/C++ binding solution, we'd be proud to list
your project on this page. Just <a href=
"mailto:c++-sig@python.org">post</a> a short description of your project
and how Boost.Python helps you get the job done, and we'll add it to this
page .</p>
<hr>
<h3>Data Analysis</h3>
<dl class="page-index">
<dt><b><a href="http://www.neuralynx.com">NeuraLab</a></b></dt>
<dd>Neuralab is a data analysis environment specifically tailored for
neural data from <a href="http://www.neuralynx.com">Neuralynx</a>
acquisition systems. Neuralab combines presentation quality graphics, a
numerical analysis library, and the <a href=
"http://www.python.org">Python</a> scripting engine in a single
application. With Neuralab, Neuralynx users can perform common analysis
tasks with just a few mouse clicks. More advanced users can create custom
Python scripts, which can optionally be assigned to menus and mouse
clicks.</dd>
</dl>
<dl class="page-index">
<dt><b>TSLib</b> - <a href="http://www.fortressinv.com">Fortress
Investment Group LLC</a></dt>
<dd>
Fortress Investment Group has contracted <a href=
"http://www.boost-consulting.com">Boost Consulting</a> to develop core
internal financial analysis tools in C++ and to prepare Python bindings
for them using Boost.Python.
<p>Tom Barket of Fortress writes:</p>
<blockquote>
We have a large C++ analytical library specialized for research in
finance and economics, built for speed and mission critical
stability. Yet Python offers us the flexibility to test out new ideas
quickly and increase the productivity of our time versus working in
C++. There are several key features which make Python stand out. Its
elegance, stability, and breadth of resources on the web are all
valuable, but the most important is its extensibility, due to its
open source transparency. Boost.Python makes Python extensibility
extremely simple and straightforward, yet preserves a great deal of
power and control.
</blockquote>
</dd>
</dl>
<h3>Educational</h3>
<dl class="page-index">
<dt><a href="http://edu.kde.org/kig"><b>Kig</b></a></dt>
<dd>
<p>KDE Interactive Geometry is a high-school level educational tool,
built for the KDE desktop. It is a nice tool to let students work with
geometrical constructions. It is meant to be the most intuitive, yet
featureful application of its kind.</p>
<p>Versions after 0.6.x (will) support objects built by the user
himself in the Python language. The exporting of the relevant internal
API's were done using Boost.Python, which made the process very
easy.</p>
</dd>
</dl>
<h3>Enterprise Software</h3>
<dl class="page-index">
<dt><b><a href="http://openwbem.sourceforge.net">OpenWBEM</a></b></dt>
<dd>
The OpenWBEM project is an effort to develop an open-source
implementation of Web Based Enterprise Management suitable for
commercial and non-commercial application
<p><a href="mailto:dnuffer@sco.com">Dan Nuffer</a> writes:</p>
<blockquote>
I'm using Boost.Python to wrap the client API of OpenWBEM.This will
make it easier to do rapid prototyping, testing, and scripting when
developing management solutions that use WBEM.
</blockquote>
</dd>
<dt><b><a href="http://www.transversal.com">Metafaq</a></b></dt>
<dd>
Metafaq, from <a href="http://www.transversal.com">Transversal,
Inc.</a>, is an enterprise level online knowledge base management
system.
<p><a href="mailto:ben.young-at-transversal.com">Ben Young</a>
writes:</p>
<blockquote>
Boost.Python is used in an automated process to generate python
bindings to our api which is exposed though multiple backends and
frontends. This allows us to write quick tests and bespoke scripts to
perform one off tasks without having to go through the full
compilation cycle.
</blockquote>
</dd>
</dl>
<h3>Games</h3>
<dl>
<dt><b><a href="http://www.firaxis.com">Civilization IV</a></b></dt>
</dl>
<blockquote>
&ldquo;The fourth game in the PC strategy series that has sold over five
million copies, Sid Meier's Civilization IV is a bold step forward for
the franchise, with spectacular new 3D graphics and all-new single and
multiplayer content. Civilization IV will also set a new standard for
user-modification, allowing gamers to create their own add-ons using
Python and XML.
<p>Sid Meier's Civilization IV will be released for PC in late 2005. For
more information please visit <a href=
"http://www.firaxis.com">http://www.firaxis.com</a> or write <a href=
"mailto:kgilmore@firaxis.com">kgilmore@firaxis.com</a>&rdquo;</p>
</blockquote>
<p>Boost.Python is used as the interface layer between the C++ game code
and Python. Python is used for many purposes in the game, including map
generation, interface screens, game events, tools, tutorials, etc. Most
high-level game operations have been exposed to Python in order to give
modders the power they need to customize the game.</p>
<blockquote>
-Mustafa Thamer, Civ4 Lead Programmer
</blockquote>
<dl class="page-index">
<dt><b><a href="http://vegastrike.sourceforge.net">Vega
Strike</a></b></dt>
<dd>
<a href="http://vegastrike.sourceforge.net">Vega Strike</a> is the 3D
Space Simulator that allows you to trade and bounty hunt in a vast
universe. Players face dangers, decisions, piracy, and aliens.
<p><a href="http://vegastrike.sourceforge.net">Vega Strike</a> has
decided to base its scripting on python, using boost as the layer
between the class hierarchy in python and the class hierarchy in C++.
The result is a very flexible scripting system that treats units as
native python classes when designing missions or writing AI's.</p>
<p>A large economic and planetary simulation is currently being run in
the background in python and the results are returned back into C++ in
the form of various factions' spaceships appearing near worlds that
they are simulated to be near in python if the player is in the general
neighborhood.</p>
</dd>
</dl>
<h3>Graphics</h3>
<dl class="page-index">
<dt><b><a href="http://sourceforge.net/projects/pyosg">OpenSceneGraph
Bindings</a></b></dt>
<dd><a href="mailto:gideon@computer.org">Gideon May</a> has created a set
of bindings for <a href=
"http://www.openscenegraph.org">OpenSceneGraph</a>, a cross-platform
C++/OpenGL library for the real-time visualization.<br>
&nbsp;</dd>
<dt><b><a href=
"http://www.slac.stanford.edu/grp/ek/hippodraw/index.html">HippoDraw</a></b></dt>
<dd>
HippoDraw is a data analysis environment consisting of a canvas upon
which graphs such as histograms, scattter plots, etc, are prsented. It
has a highly interactive GUI interface, but some things you need to do
with scripts. HippoDraw can be run as Python extension module so that
all the manipulation can be done from either Python or the GUI.
<p>Before the web page came online, <a href=
"mailto:Paul_Kunz@SLAC.Stanford.EDU">Paul F. Kunz</a> wrote:</p>
<blockquote>
Don't have a web page for the project, but the organization's is
<a href=
"http://www.slac.stanford.edu">http://www.slac.stanford.edu</a> (the
first web server site in America, I installed it).
</blockquote>Which was just too cool a piece of trivia to omit.<br>
&nbsp;
</dd>
<dt><a href="http://www.iplt.org"><b>IPLT</b></a></dt>
<dd>
<a href="mailto:ansgar.philippsen-at-unibas.ch">Ansgar Philippsen</a>
writes:
<blockquote>
IPLT is an image processing library and toolbox for the structural
biology electron microscopy community. I would call it a
budding/evolving project, since it is currently not in production
stage, but rather under heavy development. Python is used as the main
scripting/interaction level, but also for rapid prototyping, since
the underlying C++ class library is pretty much fully exposed via
boost.python (at least the high-level interface). The combined power
of C++ and Python for this project turned out to be just awesome.
</blockquote><br>
&nbsp;
</dd>
<dt><a href=
"http://www.procoders.net/pythonmagick"><b>PythonMagick</b></a></dt>
<dd>PythonMagick binds the <a href=
"http://www.graphicsmagick.org">GraphicsMagick</a> image manipulation
library to Python.<br>
&nbsp;</dd>
<dt><a href="http://www.vpython.org"><b>VPython</b></a></dt>
<dd>
<a href="mailto:Bruce_Sherwood-at-ncsu.edu">Bruce Sherwood</a> writes:
<blockquote>
VPython is an extension for Python that makes it easy to create
navigable 3D animations, which are generated as a side effect of
computational code. VPython is used in education for various
purposes, including teaching physics and programming, but it has also
been used by research scientists to visualize systems or data in 3D.
</blockquote><br>
&nbsp;
</dd>
</dl>
<h3>Scientific Computing</h3>
<dl class="page index">
<dt><a href="http://camfr.sourceforge.net"><b>CAMFR</b></a></dt>
<dd>
CAMFR is a photonics and electromagnetics modelling tool. Python is
used for computational steering.
<p><a href="mailto:Peter.Bienstman@rug.ac.be">Peter Bienstman</a>
writes:</p>
<blockquote>
Thanks for providing such a great tool!
</blockquote>
</dd>
<dt><a href="http://cctbx.sourceforge.net"><b>cctbx - Computational
Crystallography Toolbox</b></a></dt>
<dd>
Computational Crystallography is concerned with the derivation of
atomic models of crystal structures, given experimental X-ray
diffraction data. The cctbx is an open-source library of fundamental
algorithms for crystallographic computations. The core algorithms are
implemented in C++ and accessed through higher-level Python interfaces.
<p>The cctbx grew together with Boost.Python and is designed from the
ground up as a hybrid Python/C++ system. With one minor exception,
run-time polymorphism is completely handled by Python. C++ compile-time
polymorphism is used to implement performance critical algorithms. The
Python and C++ layers are seamlessly integrated using Boost.Python.</p>
<p>The SourceForge cctbx project is organized in modules to facilitate
use in non-crystallographic applications. The scitbx module implements
a general purpose array family for scientific applications and pure C++
ports of FFTPACK and the L-BFGS quasi-Newton minimizer.</p>
</dd>
<dt><a href="http://www.llnl.gov/CASC/emsolve"><b>EMSolve</b></a></dt>
<dd>EMSolve is a provably stable, charge conserving, and energy
conserving solver for Maxwell's equations.<br>
&nbsp;</dd>
<dt><b><a href="http://cern.ch/gaudi">Gaudi</a></b> and <b><a href=
"http://cern.ch/Gaudi/RootPython/">RootPython</a></b></dt>
<dd>
Gaudi is a framework for particle physics collision data processing
applications developed in the context of the LHCb and ATLAS experiments
at CERN.
<p><a href="mailto:Pere.Mato@cern.ch">Pere Mato Vila</a> writes:</p>
<blockquote>
We are using Boost.Python to provide scripting/interactive capability
to our framework. We have a module called "GaudiPython" implemented
using Boost.Python that allows the interaction with any framework
service or algorithm from python. RootPython also uses Boost.Python
to provide a generic "gateway" between the <a href=
"http://root.cern.ch">ROOT</a> framework and python
<p>Boost.Python is great. We managed very quickly to interface our
framework to python, which is great language. We are trying to
facilitate to our physicists (end-users) a rapid analysis application
development environment based on python. For that, Boost.Python plays
and essential role.</p>
</blockquote>
</dd>
<dt><b><a href="http://www.esss.com.br">ESSS</a></b></dt>
<dd>
ESSS (Engineering Simulation and Scientific Software) is a company that
provides engineering solutions and acts in the brazilian and
south-american market providing products and services related to
Computational Fluid Dynamics and Image Analysis.
<p><a href="mailto:bruno@esss.com.br">Bruno da Silva de Oliveira</a>
writes:</p>
<blockquote>
Recently we moved our work from working exclusively with C++ to an
hybrid-language approach, using Python and C++, with Boost.Python
providing the layer between the two. The results are great so far!
</blockquote>
<p>Two projects have been developed so far with this technology:</p>
<p><b><a href=
"http://www.esss.com.br/index.php?pg=dev_projetos">Simba</a></b>
provides 3D visualization of geological formations gattered from the
simulation of the evolution of oil systems, allowing the user to
analyse various aspects of the simulation, like deformation, pressure
and fluids, along the time of the simulation.</p>
<p><b><a href=
"http://www.esss.com.br/index.php?pg=dev_projetos">Aero</a></b> aims to
construct a CFD with brazilian technology, which involves various
companies and universities. ESSS is responsible for various of the
application modules, including GUI and post-processing of results.</p>
</dd>
<dt><b><a href="http://www.rationaldiscovery.com">Rational Discovery
LLC</a></b></dt>
<dd>
Rational Discovery provides computational modeling, combinatorial
library design and custom software development services to the
pharmaceutical, biotech and chemical industries. We do a substantial
amount of internal research to develop new approaches for applying
machine-learning techniques to solve chemical problems. Because we're a
small organization and chemistry is a large and complex field, it is
essential that we be able to quickly and easily prototype and test new
algorithms.
<p>For our internal software, we implement core data structures in C
and expose them to Python using Boost.Python. Algorithm development is
done in Python and then translated to C if required (often it's not).
This hybrid development approach not only greatly increases our
productivity, but it also allows "non-developers" (people without C
experience) to take part in method development. Learning C is a
daunting task, but "Python fits your brain." (Thanks to Bruce Eckel for
the quote.)</p>
</dd>
</dl>
<h3>Systems Libraries</h3>
<dl>
<dt><a href="http://itamarst.org/software"><b>Fusion</b></a></dt>
<dd>
<p>Fusion is a library that supports implementing protocols in C++ for
use with Twisted, allowing control over memory allocation strategies,
fast method calls internally, etc.. Fusion supports TCP, UDP and
multicast, and is implemented using the Boost.Python python
bindings.</p>
<p>Fusion is licensed under the MIT license, and available for download
from <a href=
"http://itamarst.org/software">http://itamarst.org/software</a>.</p>
</dd>
</dl>
<h3>Tools</h3>
<dl>
<dt><a href="http://www.jayacard.org"><b>Jayacard</b></a></dt>
<dd>
Jayacard aims at developing a secure portable open source operating
system for contactless smart cards and a complete suite of high quality
development tools to ease smart card OS and application development.
<p>The core of the smart card reader management is written in C++ but
all the development tools are written in the friendly Python language.
Boost plays the fundamental role of binding the tools to our core smart
card reader library.</p>
</dd>
</dl>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
15 July, 2003</p>
<p><i>&copy; Copyright <a href="../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002-2003.</i></p>
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="boost.css">
<title>Boost.Python - Support Resources</title>
</head>
<body link="#0000ff" vlink="#800080">
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../boost.png" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="index.html">Boost.Python</a></h1>
<h2 align="center">Support Resources</h2>
</td>
</tr>
</table>
<hr>
<h2>Synopsis</h2>
<p>This is a list of available resources for support with Boost.Python
problems and feature requests. <b>Please try to resist emailing the
Boost.Python developers directly for support.</b> Use the following
resources instead; the developers are listening!</p>
<hr>
<dl class="page-index">
<dt><b><a href="http://www.boost-consulting.com">Boost
Consulting</a></b> - Commercial support, development, training, and
distribution for all the Boost libraries, from the people who brought
you Boost.Python.<br>
&nbsp;</dt>
<dt><b><a href=
"http://www.boost.org/more/mailing_lists.htm#cplussig">The Python
C++-sig</a></b> mailing list is a forum for discussing Python/C++
interoperability, and Boost.Python in particular. Post your
Boost.Python questions here.<br>
&nbsp;</dt>
<dt>The <b>Boost.Python <a href=
"http://www.python.org/cgi-bin/moinmoin/boost_2epython">Wiki
Pages</a></b> established by Mike Rovner as part of the <a href=
"http://www.python.org/cgi-bin/moinmoin">PythonInfo Wiki</a> serves as
a forum to gather peoples' experience and as a cookbook.<br>
&nbsp;</dt>
</dl>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
12 Sept, 2003 <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2003.</i></p>
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# Copyright Joel de Guzman 2006. Distributed under the Boost
# Software License, Version 1.0. (See accompanying
# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
project boost/libs/python/doc/tutorial/doc ;
import boostbook : boostbook ;
using quickbook ;
boostbook tutorial
:
tutorial.qbk
:
<xsl:param>boost.root=../../../../../..
<xsl:param>boost.libraries=../../../../../../libs/libraries.htm
;

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<html>
<head>
<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
<title>Basic Interface</title>
<link rel="stylesheet" href="theme/style.css" type="text/css">
<link rel="prev" href="object_interface.html">
<link rel="next" href="derived_object_types.html">
</head>
<body>
<table width="100%" height="48" border="0" cellspacing="2">
<tr>
<td><img src="theme/c%2B%2Bboost.gif">
</td>
<td width="85%">
<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Basic Interface</b></font>
</td>
</tr>
</table>
<br>
<table border="0">
<tr>
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="object_interface.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="20"><a href="derived_object_types.html"><img src="theme/r_arr.gif" border="0"></a></td>
</tr>
</table>
<p>
Class <tt>object</tt> wraps <tt>PyObject*</tt>. All the intricacies of dealing with
<tt>PyObject</tt>s such as managing reference counting are handled by the
<tt>object</tt> class. C++ object interoperability is seamless. Boost.Python C++
<tt>object</tt>s can in fact be explicitly constructed from any C++ object.</p>
<p>
To illustrate, this Python code snippet:</p>
<code><pre>
<span class=identifier>def </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>x</span><span class=special>, </span><span class=identifier>y</span><span class=special>):
</span><span class=keyword>if </span><span class=special>(</span><span class=identifier>y </span><span class=special>== </span><span class=literal>'foo'</span><span class=special>):
</span><span class=identifier>x</span><span class=special>[</span><span class=number>3</span><span class=special>:</span><span class=number>7</span><span class=special>] </span><span class=special>= </span><span class=literal>'bar'
</span><span class=keyword>else</span><span class=special>:
</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>items </span><span class=special>+= </span><span class=identifier>y</span><span class=special>(</span><span class=number>3</span><span class=special>, </span><span class=identifier>x</span><span class=special>)
</span><span class=keyword>return </span><span class=identifier>x
</span><span class=identifier>def </span><span class=identifier>getfunc</span><span class=special>():
</span><span class=keyword>return </span><span class=identifier>f</span><span class=special>;
</span></pre></code>
<p>
Can be rewritten in C++ using Boost.Python facilities this way:</p>
<code><pre>
<span class=identifier>object </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>object </span><span class=identifier>x</span><span class=special>, </span><span class=identifier>object </span><span class=identifier>y</span><span class=special>) </span><span class=special>{
</span><span class=keyword>if </span><span class=special>(</span><span class=identifier>y </span><span class=special>== </span><span class=string>&quot;foo&quot;</span><span class=special>)
</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>slice</span><span class=special>(</span><span class=number>3</span><span class=special>,</span><span class=number>7</span><span class=special>) </span><span class=special>= </span><span class=string>&quot;bar&quot;</span><span class=special>;
</span><span class=keyword>else
</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;items&quot;</span><span class=special>) </span><span class=special>+= </span><span class=identifier>y</span><span class=special>(</span><span class=number>3</span><span class=special>, </span><span class=identifier>x</span><span class=special>);
</span><span class=keyword>return </span><span class=identifier>x</span><span class=special>;
</span><span class=special>}
</span><span class=identifier>object </span><span class=identifier>getfunc</span><span class=special>() </span><span class=special>{
</span><span class=keyword>return </span><span class=identifier>object</span><span class=special>(</span><span class=identifier>f</span><span class=special>);
</span><span class=special>}
</span></pre></code>
<p>
Apart from cosmetic differences due to the fact that we are writing the
code in C++, the look and feel should be immediately apparent to the Python
coder.</p>
<table border="0">
<tr>
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="object_interface.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="20"><a href="derived_object_types.html"><img src="theme/r_arr.gif" border="0"></a></td>
</tr>
</table>
<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
no claim as to its suitability for any purpose. </font> </p>
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<head>
<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
<title>Building an Extension Module </title>
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<td><img src="theme/c%2B%2Bboost.gif">
</td>
<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><strong>Building
an Extension Module</strong></font> </td>
</tr>
</table>
<br>
<table border="0">
<tr>
<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><img src="theme/l_arr.gif" border="0"></td>
<td width="20"><img src="theme/r_arr.gif" border="0"></td>
</tr>
</table>
<h2>Building Boost.Python</h2>
<p>Every Boost.Python extension module must be linked with the boost_python shared
library. To build boost_python, use <a href="file:///C:/dev/boost/tools/build/index.html">Boost.Build</a>
in the usual way from the <tt>libs/python/build</tt> subdirectory of your boost
installation (if you have already built boost from the top level this may have
no effect, since the work is already done).</p>
<h2>Configuration</h2>
<p>You may need to configure the following variables to point Boost.Build at your
Python installation:</p>
<table width="95%" border="0" align="center">
<tr class="table_title">
<td width="24%">Variable Name</td>
<td width="20%">Semantics</td>
<td width="21%">Default</td>
<td width="35%">Notes</td>
</tr>
<tr class="table_cells">
<td><tt>PYTHON_ROOT</tt></td>
<td> The root directory of your Python installation</td>
<td>Windows: <tt><br>
c:/tools/python <br>
Unix: /usr/local</tt></td>
<td>On Unix, this is the <tt>--with-prefix=</tt> directory used to configure
Python</td>
</tr>
<tr class="table_cells">
<td><tt>PYTHON_VERSION</tt></td>
<td> The The 2-part python Major.Minor version number</td>
<td>Windows: 2.1 Unix: 1.5</td>
<td>Be sure not to include a third number, e.g. not &quot;2.2.1&quot;, even
if that's the version you have.</td>
</tr>
<tr class="table_cells">
<td><tt>PYTHON_INCLUDES</tt></td>
<td> path to Python <span class="preprocessor">#include</span> directories</td>
<td>Autoconfigured from <tt><br>
PYTHON_ROOT</tt></td>
<td>&nbsp;</td>
</tr>
<tr class="table_cells">
<td><tt>PYTHON_LIB_PATH</tt></td>
<td>path to Python library object.</td>
<td>Autoconfigured from <tt><br>
PYTHON_ROOT</tt></td>
<td>&nbsp;</td>
</tr>
<tr class="table_cells">
<td><tt>PYTHON_STDLIB_PATH</tt></td>
<td>path to Python standard library modules</td>
<td>Autoconfigured from <tt><br>
PYTHON_ROOT</tt></td>
<td>&nbsp;</td>
</tr>
<tr class="table_cells">
<td height="129"><tt>CYGWIN_ROOT</tt></td>
<td> path to the user's Cygwin installation</td>
<td>Autoconfigured from <tt><br>
PYTHON_ROOT</tt></td>
<td><a href="http://www.cygwin.com">Cygwin</a> only. This and the following
two settings are useful when building with multiple toolsets on Windows,
since Cygwin requires a different build of Python.</td>
</tr>
<tr class="table_cells">
<td height="21"><tt>GCC_PYTHON_ROOT</tt></td>
<td>path to the user's Cygwin Python installation</td>
<td><tt>$(CYGWIN_ROOT)<br>
/usr/local</tt></td>
<td> <a href="http://www.cygwin.com">Cygwin</a> only</td>
</tr>
<tr class="table_cells">
<td><tt>GCC_DEBUG_PYTHON_ROOT</tt></td>
<td> path to the user's Cygwin <a href="#variants">pydebug</a>
build</td>
<td><tt>$(CYGWIN_ROOT)<br>
/usr/local/pydebug</tt></td>
<td> <a href="http://www.cygwin.com">Cygwin</a> only</td>
</tr>
</table>
<h2>Results</h2>
<p>The build process will create a <tt>libs/python/build/bin-stage</tt> subdirectory
of the boost root (or of <tt>$(ALL_LOCATE_TARGET)</tt>, if you have set that
variable), containing the built libraries. The libraries are actually built
to unique directories for each toolset and variant elsewhere in the filesystem,
and copied to the bin-stage directory as a convenience, so if you build with
multiple toolsets at once, the product of later toolsets will overwrite that
of earlier toolsets in bin-stage.</p>
<h2>Testing</h2>
<p>To build and test Boost.Python from within the <tt>libs/python/build directory</tt>,
invoke</p>
<pre> bjam -sTOOLS=<a href="../../../tools/build/index.html">toolset</a> test</pre>
<p>This will update all of the Boost.Python v1 test and example targets. The tests
are relatively quiet by default. To get more-verbose output, you might try</p>
<pre> bjam -sTOOLS=<a href="../../../tools/build/index.html">toolset</a> -sPYTHON_TEST_ARGS=-v test</pre>
<p>which will print each test's Python code with the expected output as it passes.</p>
<h2>Building your Extension Module</h2>
<p>Though there are other approaches, the easiest way to build an extension module
using Boost.Python is with Boost.Build. Until Boost.Build v2 is released, cross-project
build dependencies are not supported, so it works most smoothly if you add a
new subproject to your boost installation. The <tt>libs/python/example</tt>
subdirectory of your boost installation contains a minimal example (along with
many extra sources). To copy the example subproject:</p>
<ol>
<li> Create a new subdirectory in,<tt> libs/python</tt>, say <tt>libs/python/my_project</tt>.</li>
<li> Copy <a href="../example/Jamfile"><tt>libs/python/example/Jamfile</tt></a>
to your new directory.</li>
<li> Edit the Jamfile as appropriate for your project. You'll want to change
the <tt>subproject</tt> rule invocation at the top, and the names of some
of the source files and/or targets.</li>
</ol>
<p>If you can't modify or copy your boost installation, the alternative is to
create your own Boost.Build project. A similar example you can use as a starting
point is available in <a href="../example/project.zip">this archive</a>. You'll
need to edit the Jamfile and Jamrules files, depending on the relative location
of your Boost installation and the new project. Note that automatic testing
of extension modules is not available in this configuration.</p>
<h2>Build Variants</h2>
<p>Three variant configurations of all python-related targets are supported, and
can be selected by setting the BUILD variable:</p>
<p> * <tt>release</tt> (optimization, <tt>-DNDEBUG</tt>)<br>
* <tt>debug</tt> (no optimization <tt>-D_DEBUG</tt>)<br>
* <tt>debug-python</tt> (no optimization, <tt>-D_DEBUG -DBOOST_DEBUG_PYTHON</tt>)</p>
<p>The first two variants of the boost_python library are built by default, and
are compatible with the default Python distribution. The debug-python variant
corresponds to a specially-built debugging version of Python. On Unix platforms,
this python is built by adding <tt>--with-pydebug</tt> when configuring the
Python build. On Windows, the debugging version of Python is generated by the
&quot;Win32 Debug&quot; target of the PCBuild.dsw Visual C++ 6.0 project in
the PCBuild subdirectory of your Python distribution. Extension modules built
with Python debugging enabled are not link-compatible with a non-debug build
of Python. Since few people actually have a debug build of Python (it doesn't
come with the standard distribution), the normal debug variant builds modules
which are compatible with ordinary Python.</p>
<p>On many windows compilers, when extension modules are built with <tt>-D_DEBUG</tt>,
Python defaults to force linking with a special debugging version of the Python
DLL. Since this debug DLL isn't supplied with the default Python installation
for Windows, Boost.Python uses <a href="../../../boost/python/detail/wrap_python.hpp"><tt>boost/python/detail/wrap_python.hpp</tt></a>
to temporarily undefine <tt>_DEBUG</tt> when <tt>Python.h</tt> is <span class="preprocessor">#included</span>
- unless <tt>BOOST_DEBUG_PYTHON</tt> is defined.</p>
<p>If you want the extra runtime checks available with the debugging version of
the library, <span class="preprocessor">#define</span> <tt>BOOST_DEBUG_PYTHON</tt>
to re-enable python debuggin, and link with the debug-python variant of boost_python.</p>
<p>If you do not <span class="preprocessor">#define</span> <tt>BOOST_DEBUG_PYTHON</tt>,
be sure that any source files in your extension module <span class="preprocessor">#include</span>
<tt>&lt;boost/python/detail/wrap_python.hpp&gt;</tt> instead of the usual <tt>Python.h</tt>,
or you will have link incompatibilities.</p>
<code></code>
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Building Hello World</b></font>
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<a name="from_start_to_finish"></a><h2>From Start To Finish</h2><p>
Now the first thing you'd want to do is to build the Hello World module and
try it for yourself in Python. In this section, we shall outline the steps
necessary to achieve that. We shall use the build tool that comes bundled
with every boost distribution: <b>bjam</b>.</p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box">
<img src="theme/lens.gif"></img> <b>Building without bjam</b><br><br>
Besides bjam, there are of course other ways to get your module built.
What's written here should not be taken as &quot;the one and only way&quot;.
There are of course other build tools apart from <tt>bjam</tt>.
</td>
</tr>
</table>
<p>
We shall skip over the details. Our objective will be to simply create the
hello world module and run it in Python. For a complete reference to
building Boost.Python, check out: <a href="../../building.html">
building.html</a>.
After this brief <i>bjam</i> tutorial, we should have built two DLLs:</p>
<ul><li>boost_python.dll</li><li>hello.pyd</li></ul><p>
if you are on Windows, and</p>
<ul><li>libboost_python.so</li><li>hello.so</li></ul><p>
if you are on Unix.</p>
<p>
The tutorial example can be found in the directory:
<tt>libs/python/example/tutorial</tt>. There, you can find:</p>
<ul><li>hello.cpp</li><li>Jamfile</li></ul><p>
The <tt>hello.cpp</tt> file is our C++ hello world example. The <tt>Jamfile</tt> is a
minimalist <i>bjam</i> script that builds the DLLs for us.</p>
<p>
Before anything else, you should have the bjam executable in your boost
directory or somewhere in your path such that <tt>bjam</tt> can be executed in
the command line. Pre-built Boost.Jam executables are available for most
platforms. For example, a pre-built Microsoft Windows bjam executable can
be downloaded <a href="http://boost.sourceforge.net/jam-executables/bin.ntx86/bjam.zip">
here</a>.
The complete list of bjam pre-built
executables can be found <a href="../../../../../tools/build/index.html#Jam">
here</a>.</p>
<a name="lets_jam_"></a><h2>Lets Jam!</h2><p>
<img src="theme/jam.png"></img></p>
<p>
Here is our minimalist Jamfile:</p>
<code><pre>
subproject libs/python/example/tutorial ;
SEARCH on python.jam = $(BOOST_BUILD_PATH) ;
include python.jam ;
extension hello # Declare a Python extension called hello
: hello.cpp # source
&lt;dll&gt;../../build/boost_python # dependencies
;
</pre></code><p>
First, we need to specify our location in the boost project hierarchy.
It so happens that the tutorial example is located in <tt>/libs/python/example/tutorial</tt>.
Thus:</p>
<code><pre>
subproject libs/python/example/tutorial ;
</pre></code><p>
Then we will include the definitions needed by Python modules:</p>
<code><pre>
SEARCH on python.jam = $(BOOST_BUILD_PATH) ;
include python.jam ;
</pre></code><p>
Finally we declare our <tt>hello</tt> extension:</p>
<code><pre>
extension hello # Declare a Python extension called hello
: hello.cpp # source
&lt;dll&gt;../../build/boost_python # dependencies
;
</pre></code><a name="running_bjam"></a><h2>Running bjam</h2><p>
<i>bjam</i> is run using your operating system's command line interpreter.</p>
<blockquote><p>Start it up.</p></blockquote><p>
Make sure that the environment is set so that we can invoke the C++
compiler. With MSVC, that would mean running the <tt>Vcvars32.bat</tt> batch
file. For instance:</p>
<code><pre>
<span class=identifier>C</span><span class=special>:\</span><span class=identifier>Program </span><span class=identifier>Files</span><span class=special>\</span><span class=identifier>Microsoft </span><span class=identifier>Visual </span><span class=identifier>Studio</span><span class=special>\</span><span class=identifier>VC98</span><span class=special>\</span><span class=identifier>bin</span><span class=special>\</span><span class=identifier>Vcvars32</span><span class=special>.</span><span class=identifier>bat
</span></pre></code>
<p>
Some environment variables will have to be setup for proper building of our
Python modules. Example:</p>
<code><pre>
<span class=identifier>set </span><span class=identifier>PYTHON_ROOT</span><span class=special>=</span><span class=identifier>c</span><span class=special>:/</span><span class=identifier>dev</span><span class=special>/</span><span class=identifier>tools</span><span class=special>/</span><span class=identifier>python
</span><span class=identifier>set </span><span class=identifier>PYTHON_VERSION</span><span class=special>=</span><span class=number>2.2
</span></pre></code>
<p>
The above assumes that the Python installation is in <tt>c:/dev/tools/python</tt>
and that we are using Python version 2.2. You'll have to tweak this path
appropriately. <img src="theme/note.gif"></img> Be sure not to include a third number, e.g. <b>not</b> &quot;2.2.1&quot;,
even if that's the version you have.</p>
<p>
Now we are ready... Be sure to <tt>cd</tt> to <tt>libs/python/example/tutorial</tt>
where the tutorial <tt>&quot;hello.cpp&quot;</tt> and the <tt>&quot;Jamfile&quot;</tt> is situated.</p>
<p>
Finally:</p>
<code><pre>
<span class=identifier>bjam </span><span class=special>-</span><span class=identifier>sTOOLS</span><span class=special>=</span><span class=identifier>msvc
</span></pre></code>
<p>
We are again assuming that we are using Microsoft Visual C++ version 6. If
not, then you will have to specify the appropriate tool. See
<a href="../../../../../tools/build/index.html">
Building Boost Libraries</a> for
further details.</p>
<p>
It should be building now:</p>
<code><pre>
cd C:\dev\boost\libs\python\example\tutorial
bjam -sTOOLS=msvc
...patience...
...found 1703 targets...
...updating 40 targets...
</pre></code><p>
And so on... Finally:</p>
<code><pre>
vc-C++ ..\..\..\..\libs\python\example\tutorial\bin\hello.pyd\msvc\debug\
runtime-link-dynamic\hello.obj
hello.cpp
vc-Link ..\..\..\..\libs\python\example\tutorial\bin\hello.pyd\msvc\debug\
runtime-link-dynamic\hello.pyd ..\..\..\..\libs\python\example\tutorial\bin\
hello.pyd\msvc\debug\runtime-link-dynamic\hello.lib
Creating library ..\..\..\..\libs\python\example\tutorial\bin\hello.pyd\
msvc\debug\runtime-link-dynamic\hello.lib and object ..\..\..\..\libs\python\
example\tutorial\bin\hello.pyd\msvc\debug\runtime-link-dynamic\hello.exp
...updated 40 targets...
</pre></code><p>
If all is well, you should now have:</p>
<ul><li>boost_python.dll</li><li>hello.pyd</li></ul><p>
if you are on Windows, and</p>
<ul><li>libboost_python.so</li><li>hello.so</li></ul><p>
if you are on Unix.</p>
<p>
<tt>boost_python.dll</tt> can be found somewhere in <tt>libs\python\build\bin</tt>
while <tt>hello.pyd</tt> can be found somewhere in
<tt>libs\python\example\tutorial\bin</tt>. After a successful build, you can just
link in these DLLs with the Python interpreter. In Windows for example, you
can simply put these libraries inside the directory where the Python
executable is.</p>
<p>
You may now fire up Python and run our hello module:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>import </span><span class=identifier>hello
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>print </span><span class=identifier>hello</span><span class=special>.</span><span class=identifier>greet</span><span class=special>()
</span><span class=identifier>hello</span><span class=special>, </span><span class=identifier>world
</span></pre></code>
<blockquote><p><b>There you go... Have fun!</b></p></blockquote><table border="0">
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Call Policies</b></font>
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<p>
In C++, we often deal with arguments and return types such as pointers
and references. Such primitive types are rather, ummmm, low level and
they really don't tell us much. At the very least, we don't know the
owner of the pointer or the referenced object. No wonder languages
such as Java and Python never deal with such low level entities. In
C++, it's usually considered a good practice to use smart pointers
which exactly describe ownership semantics. Still, even good C++
interfaces use raw references and pointers sometimes, so Boost.Python
must deal with them. To do this, it may need your help. Consider the
following C++ function:</p>
<code><pre>
<span class=identifier>X</span><span class=special>&amp; </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>Y</span><span class=special>&amp; </span><span class=identifier>y</span><span class=special>, </span><span class=identifier>Z</span><span class=special>* </span><span class=identifier>z</span><span class=special>);
</span></pre></code>
<p>
How should the library wrap this function? A naive approach builds a
Python X object around result reference. This strategy might or might
not work out. Here's an example where it didn't</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>y</span><span class=special>, </span><span class=identifier>z</span><span class=special>) </span>#<span class=identifier>x </span><span class=identifier>refers </span><span class=identifier>to </span><span class=identifier>some </span><span class=identifier>C</span><span class=special>++ </span><span class=identifier>X
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>del </span><span class=identifier>y
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>some_method</span><span class=special>() </span>#<span class=identifier>CRASH</span><span class=special>!
</span></pre></code>
<p>
What's the problem?</p>
<p>
Well, what if f() was implemented as shown below:</p>
<code><pre>
<span class=identifier>X</span><span class=special>&amp; </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>Y</span><span class=special>&amp; </span><span class=identifier>y</span><span class=special>, </span><span class=identifier>Z</span><span class=special>* </span><span class=identifier>z</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>y</span><span class=special>.</span><span class=identifier>z </span><span class=special>= </span><span class=identifier>z</span><span class=special>;
</span><span class=keyword>return </span><span class=identifier>y</span><span class=special>.</span><span class=identifier>x</span><span class=special>;
</span><span class=special>}
</span></pre></code>
<p>
The problem is that the lifetime of result X&amp; is tied to the lifetime
of y, because the f() returns a reference to a member of the y
object. This idiom is is not uncommon and perfectly acceptable in the
context of C++. However, Python users should not be able to crash the
system just by using our C++ interface. In this case deleting y will
invalidate the reference to X. We have a dangling reference.</p>
<p>
Here's what's happening:</p>
<ol><li><tt>f</tt> is called passing in a reference to <tt>y</tt> and a pointer to <tt>z</tt></li><li>A reference to <tt>y.x</tt> is returned</li><li><tt>y</tt> is deleted. <tt>x</tt> is a dangling reference</li><li><tt>x.some_method()</tt> is called</li><li><b>BOOM!</b></li></ol><p>
We could copy result into a new object:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>y</span><span class=special>, </span><span class=identifier>z</span><span class=special>).</span><span class=identifier>set</span><span class=special>(</span><span class=number>42</span><span class=special>) </span>#<span class=identifier>Result </span><span class=identifier>disappears
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>y</span><span class=special>.</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>get</span><span class=special>() </span>#<span class=identifier>No </span><span class=identifier>crash</span><span class=special>, </span><span class=identifier>but </span><span class=identifier>still </span><span class=identifier>bad
</span><span class=number>3.14
</span></pre></code>
<p>
This is not really our intent of our C++ interface. We've broken our
promise that the Python interface should reflect the C++ interface as
closely as possible.</p>
<p>
Our problems do not end there. Suppose Y is implemented as follows:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Y
</span><span class=special>{
</span><span class=identifier>X </span><span class=identifier>x</span><span class=special>; </span><span class=identifier>Z</span><span class=special>* </span><span class=identifier>z</span><span class=special>;
</span><span class=keyword>int </span><span class=identifier>z_value</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>z</span><span class=special>-&gt;</span><span class=identifier>value</span><span class=special>(); </span><span class=special>}
</span><span class=special>};
</span></pre></code>
<p>
Notice that the data member <tt>z</tt> is held by class Y using a raw
pointer. Now we have a potential dangling pointer problem inside Y:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>y</span><span class=special>, </span><span class=identifier>z</span><span class=special>) </span>#<span class=identifier>y </span><span class=identifier>refers </span><span class=identifier>to </span><span class=identifier>z
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>del </span><span class=identifier>z </span>#<span class=identifier>Kill </span><span class=identifier>the </span><span class=identifier>z </span><span class=identifier>object
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>y</span><span class=special>.</span><span class=identifier>z_value</span><span class=special>() </span>#<span class=identifier>CRASH</span><span class=special>!
</span></pre></code>
<p>
For reference, here's the implementation of <tt>f</tt> again:</p>
<code><pre>
<span class=identifier>X</span><span class=special>&amp; </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>Y</span><span class=special>&amp; </span><span class=identifier>y</span><span class=special>, </span><span class=identifier>Z</span><span class=special>* </span><span class=identifier>z</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>y</span><span class=special>.</span><span class=identifier>z </span><span class=special>= </span><span class=identifier>z</span><span class=special>;
</span><span class=keyword>return </span><span class=identifier>y</span><span class=special>.</span><span class=identifier>x</span><span class=special>;
</span><span class=special>}
</span></pre></code>
<p>
Here's what's happening:</p>
<ol><li><tt>f</tt> is called passing in a reference to <tt>y</tt> and a pointer to <tt>z</tt></li><li>A pointer to <tt>z</tt> is held by <tt>y</tt></li><li>A reference to <tt>y.x</tt> is returned</li><li><tt>z</tt> is deleted. <tt>y.z</tt> is a dangling pointer</li><li><tt>y.z_value()</tt> is called</li><li><tt>z-&gt;value()</tt> is called</li><li><b>BOOM!</b></li></ol><a name="call_policies"></a><h2>Call Policies</h2><p>
Call Policies may be used in situations such as the example detailed above.
In our example, <tt>return_internal_reference</tt> and <tt>with_custodian_and_ward</tt>
are our friends:</p>
<code><pre>
<span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=identifier>f</span><span class=special>,
</span><span class=identifier>return_internal_reference</span><span class=special>&lt;</span><span class=number>1</span><span class=special>,
</span><span class=identifier>with_custodian_and_ward</span><span class=special>&lt;</span><span class=number>1</span><span class=special>, </span><span class=number>2</span><span class=special>&gt; </span><span class=special>&gt;());
</span></pre></code>
<p>
What are the <tt>1</tt> and <tt>2</tt> parameters, you ask?</p>
<code><pre>
<span class=identifier>return_internal_reference</span><span class=special>&lt;</span><span class=number>1
</span></pre></code>
<p>
Informs Boost.Python that the first argument, in our case <tt>Y&amp; y</tt>, is the
owner of the returned reference: <tt>X&amp;</tt>. The &quot;<tt>1</tt>&quot; simply specifies the
first argument. In short: &quot;return an internal reference <tt>X&amp;</tt> owned by the
1st argument <tt>Y&amp; y</tt>&quot;.</p>
<code><pre>
<span class=identifier>with_custodian_and_ward</span><span class=special>&lt;</span><span class=number>1</span><span class=special>, </span><span class=number>2</span><span class=special>&gt;
</span></pre></code>
<p>
Informs Boost.Python that the lifetime of the argument indicated by ward
(i.e. the 2nd argument: <tt>Z* z</tt>) is dependent on the lifetime of the
argument indicated by custodian (i.e. the 1st argument: <tt>Y&amp; y</tt>).</p>
<p>
It is also important to note that we have defined two policies above. Two
or more policies can be composed by chaining. Here's the general syntax:</p>
<code><pre>
<span class=identifier>policy1</span><span class=special>&lt;</span><span class=identifier>args</span><span class=special>...,
</span><span class=identifier>policy2</span><span class=special>&lt;</span><span class=identifier>args</span><span class=special>...,
</span><span class=identifier>policy3</span><span class=special>&lt;</span><span class=identifier>args</span><span class=special>...&gt; </span><span class=special>&gt; </span><span class=special>&gt;
</span></pre></code>
<p>
Here is the list of predefined call policies. A complete reference detailing
these can be found <a href="../../v2/reference.html#models_of_call_policies">
here</a>.</p>
<ul><li><b>with_custodian_and_ward</b><br> Ties lifetimes of the arguments</li><li><b>with_custodian_and_ward_postcall</b><br> Ties lifetimes of the arguments and results</li><li><b>return_internal_reference</b><br> Ties lifetime of one argument to that of result</li><li><b>return_value_policy&lt;T&gt; with T one of:</b><br></li><li><b>reference_existing_object</b><br>naïve (dangerous) approach</li><li><b>copy_const_reference</b><br>Boost.Python v1 approach</li><li><b>copy_non_const_reference</b><br></li><li><b>manage_new_object</b><br> Adopt a pointer and hold the instance</li></ul><table width="80%" border="0" align="center">
<tr>
<td class="note_box">
<img src="theme/smiley.gif"></img> <b>Remember the Zen, Luke:</b><br><br>
&quot;Explicit is better than implicit&quot;<br>
&quot;In the face of ambiguity, refuse the temptation to guess&quot;<br> </td>
</tr>
</table>
<table border="0">
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
no claim as to its suitability for any purpose. </font> </p>
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<p>
Data members may also be exposed to Python so that they can be
accessed as attributes of the corresponding Python class. Each data
member that we wish to be exposed may be regarded as <b>read-only</b> or
<b>read-write</b>. Consider this class <tt>Var</tt>:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Var
</span><span class=special>{
</span><span class=identifier>Var</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>name</span><span class=special>) </span><span class=special>: </span><span class=identifier>name</span><span class=special>(</span><span class=identifier>name</span><span class=special>), </span><span class=identifier>value</span><span class=special>() </span><span class=special>{}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=keyword>const </span><span class=identifier>name</span><span class=special>;
</span><span class=keyword>float </span><span class=identifier>value</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<p>
Our C++ <tt>Var</tt> class and its data members can be exposed to Python:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Var</span><span class=special>&gt;(</span><span class=string>&quot;Var&quot;</span><span class=special>, </span><span class=identifier>init</span><span class=special>&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def_readonly</span><span class=special>(</span><span class=string>&quot;name&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>name</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def_readwrite</span><span class=special>(</span><span class=string>&quot;value&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>value</span><span class=special>);
</span></pre></code>
<p>
Then, in Python:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>Var</span><span class=special>(</span><span class=literal>'pi'</span><span class=special>)
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value </span><span class=special>= </span><span class=number>3.14
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>print </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>name</span><span class=special>, </span><span class=literal>'is around'</span><span class=special>, </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value
</span><span class=identifier>pi </span><span class=identifier>is </span><span class=identifier>around </span><span class=number>3.14
</span></pre></code>
<p>
Note that <tt>name</tt> is exposed as <b>read-only</b> while <tt>value</tt> is exposed
as <b>read-write</b>.</p>
<code><pre>
&gt;&gt;&gt; x.name = 'e' # can't change name
Traceback (most recent call last):
File &quot;&lt;stdin&gt;&quot;, line 1, in ?
AttributeError: can't set attribute
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<a name="python_operators"></a><h2>Python Operators</h2><p>
C is well known for the abundance of operators. C++ extends this to the
extremes by allowing operator overloading. Boost.Python takes advantage of
this and makes it easy to wrap C++ operator-powered classes.</p>
<p>
Consider a file position class <tt>FilePos</tt> and a set of operators that take
on FilePos instances:</p>
<code><pre>
<span class=keyword>class </span><span class=identifier>FilePos </span><span class=special>{ </span><span class=comment>/*...*/ </span><span class=special>};
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>+(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>+(</span><span class=keyword>int</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span><span class=keyword>int </span><span class=keyword>operator</span><span class=special>-(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>-(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>+=(</span><span class=identifier>FilePos</span><span class=special>&amp;, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>-=(</span><span class=identifier>FilePos</span><span class=special>&amp;, </span><span class=keyword>int</span><span class=special>);
</span><span class=keyword>bool </span><span class=keyword>operator</span><span class=special>&lt;(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span></pre></code>
<p>
The class and the various operators can be mapped to Python rather easily
and intuitively:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>FilePos</span><span class=special>&gt;(</span><span class=string>&quot;FilePos&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>+ </span><span class=keyword>int</span><span class=special>()) </span><span class=comment>// __add__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=keyword>int</span><span class=special>() </span><span class=special>+ </span><span class=identifier>self</span><span class=special>) </span><span class=comment>// __radd__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>- </span><span class=identifier>self</span><span class=special>) </span><span class=comment>// __sub__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>- </span><span class=keyword>int</span><span class=special>()) </span><span class=comment>// __rsub__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>+= </span><span class=keyword>int</span><span class=special>()) </span><span class=comment>// __iadd__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>-= </span><span class=identifier>other</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>&lt; </span><span class=identifier>self</span><span class=special>); </span><span class=comment>// __lt__
</span></pre></code>
<p>
The code snippet above is very clear and needs almost no explanation at
all. It is virtually the same as the operators' signatures. Just take
note that <tt>self</tt> refers to FilePos object. Also, not every class <tt>T</tt> that
you might need to interact with in an operator expression is (cheaply)
default-constructible. You can use <tt>other&lt;T&gt;()</tt> in place of an actual
<tt>T</tt> instance when writing &quot;self expressions&quot;.</p>
<a name="special_methods"></a><h2>Special Methods</h2><p>
Python has a few more <i>Special Methods</i>. Boost.Python supports all of the
standard special method names supported by real Python class instances. A
similar set of intuitive interfaces can also be used to wrap C++ functions
that correspond to these Python <i>special functions</i>. Example:</p>
<code><pre>
<span class=keyword>class </span><span class=identifier>Rational
</span><span class=special>{ </span><span class=keyword>operator </span><span class=keyword>double</span><span class=special>() </span><span class=keyword>const</span><span class=special>; </span><span class=special>};
</span><span class=identifier>Rational </span><span class=identifier>pow</span><span class=special>(</span><span class=identifier>Rational</span><span class=special>, </span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>Rational </span><span class=identifier>abs</span><span class=special>(</span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>ostream</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>&lt;&lt;(</span><span class=identifier>ostream</span><span class=special>&amp;,</span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Rational</span><span class=special>&gt;()
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>float_</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) </span><span class=comment>// __float__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>pow</span><span class=special>(</span><span class=identifier>self</span><span class=special>, </span><span class=identifier>other</span><span class=special>&lt;</span><span class=identifier>Rational</span><span class=special>&gt;)) </span><span class=comment>// __pow__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>abs</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) </span><span class=comment>// __abs__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>str</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) </span><span class=comment>// __str__
</span><span class=special>;
</span></pre></code>
<p>
Need we say more?</p>
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<td class="note_box">
<img src="theme/lens.gif"></img> What is the business of <tt>operator&lt;&lt;</tt> <tt>.def(str(self))</tt>?
Well, the method <tt>str</tt> requires the <tt>operator&lt;&lt;</tt> to do its work (i.e.
<tt>operator&lt;&lt;</tt> is used by the method defined by def(str(self)). </td>
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<p>
In C++, classes with public data members are usually frowned
upon. Well designed classes that take advantage of encapsulation hide
the class' data members. The only way to access the class' data is
through access (getter/setter) functions. Access functions expose class
properties. Here's an example:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Num
</span><span class=special>{
</span><span class=identifier>Num</span><span class=special>();
</span><span class=keyword>float </span><span class=identifier>get</span><span class=special>() </span><span class=keyword>const</span><span class=special>;
</span><span class=keyword>void </span><span class=identifier>set</span><span class=special>(</span><span class=keyword>float </span><span class=identifier>value</span><span class=special>);
</span><span class=special>...
</span><span class=special>};
</span></pre></code>
<p>
However, in Python attribute access is fine; it doesn't neccessarily break
encapsulation to let users handle attributes directly, because the
attributes can just be a different syntax for a method call. Wrapping our
<tt>Num</tt> class using Boost.Python:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Num</span><span class=special>&gt;(</span><span class=string>&quot;Num&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>&quot;rovalue&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>get</span><span class=special>)
</span><span class=special>.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>&quot;value&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>get</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>set</span><span class=special>);
</span></pre></code>
<p>
And at last, in Python:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>Num</span><span class=special>()
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value </span><span class=special>= </span><span class=number>3.14
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value</span><span class=special>, </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>rovalue
</span><span class=special>(</span><span class=number>3.14</span><span class=special>, </span><span class=number>3.14</span><span class=special>)
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>rovalue </span><span class=special>= </span><span class=number>2.17 </span>#<span class=identifier>error</span><span class=special>!
</span></pre></code>
<p>
Take note that the class property <tt>rovalue</tt> is exposed as <b>read-only</b>
since the <tt>rovalue</tt> setter member function is not passed in:</p>
<code><pre>
<span class=special>.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>&quot;rovalue&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Var</span><span class=special>::</span><span class=identifier>get</span><span class=special>)
</span></pre></code>
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<p>
In this section, we shall learn how to make functions behave
polymorphically through virtual functions. Continuing our example, let us
add a virtual function to our <tt>Base</tt> class:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Base
</span><span class=special>{
</span><span class=keyword>virtual </span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() </span><span class=special>= </span><span class=number>0</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<p>
Since <tt>f</tt> is a pure virtual function, <tt>Base</tt> is now an abstract
class. Given an instance of our class, the free function <tt>call_f</tt>
calls some implementation of this virtual function in a concrete
derived class:</p>
<code><pre>
<span class=keyword>int </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>Base</span><span class=special>&amp; </span><span class=identifier>b</span><span class=special>) </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>b</span><span class=special>.</span><span class=identifier>f</span><span class=special>(); </span><span class=special>}
</span></pre></code>
<p>
To allow this function to be implemented in a Python derived class, we
need to create a class wrapper:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>BaseWrap </span><span class=special>: </span><span class=identifier>Base
</span><span class=special>{
</span><span class=identifier>BaseWrap</span><span class=special>(</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self_</span><span class=special>)
</span><span class=special>: </span><span class=identifier>self</span><span class=special>(</span><span class=identifier>self_</span><span class=special>) </span><span class=special>{}
</span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>call_method</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;(</span><span class=identifier>self</span><span class=special>, </span><span class=string>&quot;f&quot;</span><span class=special>); </span><span class=special>}
</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box">
<img src="theme/lens.gif"></img> <b>member function and methods</b><br><br> Python, like
many object oriented languages uses the term <b>methods</b>. Methods
correspond roughly to C++'s <b>member functions</b> </td>
</tr>
</table>
<p>
Our class wrapper <tt>BaseWrap</tt> is derived from <tt>Base</tt>. Its overridden
virtual member function <tt>f</tt> in effect calls the corresponding method
of the Python object <tt>self</tt>, which is a pointer back to the Python
<tt>Base</tt> object holding our <tt>BaseWrap</tt> instance.</p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box">
<img src="theme/note.gif"></img> <b>Why do we need BaseWrap?</b><br><br>
<i>You may ask</i>, &quot;Why do we need the <tt>BaseWrap</tt> derived class? This could
have been designed so that everything gets done right inside of
Base.&quot;<br><br>
One of the goals of Boost.Python is to be minimally intrusive on an
existing C++ design. In principle, it should be possible to expose the
interface for a 3rd party library without changing it. To unintrusively
hook into the virtual functions so that a Python override may be called, we
must use a derived class.<br><br>
Note however that you don't need to do this to get methods overridden
in Python to behave virtually when called <i>from</i> <b>Python</b>. The only
time you need to do the <tt>BaseWrap</tt> dance is when you have a virtual
function that's going to be overridden in Python and called
polymorphically <i>from</i> <b>C++</b>. </td>
</tr>
</table>
<p>
Wrapping <tt>Base</tt> and the free function <tt>call_f</tt>:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Base</span><span class=special>, </span><span class=identifier>BaseWrap</span><span class=special>, </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>noncopyable</span><span class=special>&gt;(</span><span class=string>&quot;Base&quot;</span><span class=special>, </span><span class=identifier>no_init</span><span class=special>)
</span><span class=special>;
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;call_f&quot;</span><span class=special>, </span><span class=identifier>call_f</span><span class=special>);
</span></pre></code>
<p>
Notice that we parameterized the <tt>class_</tt> template with <tt>BaseWrap</tt> as the
second parameter. What is <tt>noncopyable</tt>? Without it, the library will try
to create code for converting Base return values of wrapped functions to
Python. To do that, it needs Base's copy constructor... which isn't
available, since Base is an abstract class.</p>
<p>
In Python, let us try to instantiate our <tt>Base</tt> class:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>base </span><span class=special>= </span><span class=identifier>Base</span><span class=special>()
</span><span class=identifier>AttributeError</span><span class=special>: </span><span class=special>...
</span></pre></code>
<p>
Why is it an error? <tt>Base</tt> is an abstract class. As such it is advisable
to define the Python wrapper with <tt>no_init</tt> as we have done above. Doing
so will disallow abstract base classes such as <tt>Base</tt> to be instantiated.</p>
<a name="deriving_a_python_class"></a><h2>Deriving a Python class</h2><p>
Now, at last, we can even derive from our base class <tt>Base</tt> in Python:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=keyword>class </span><span class=identifier>Derived</span><span class=special>(</span><span class=identifier>Base</span><span class=special>):
</span><span class=special>... </span><span class=identifier>def </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
</span><span class=special>... </span><span class=keyword>return </span><span class=number>42
</span><span class=special>...
</span></pre></code>
<p>
Cool eh? A Python class deriving from a C++ class!</p>
<p>
Let's now make an instance of our Python class <tt>Derived</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>derived </span><span class=special>= </span><span class=identifier>Derived</span><span class=special>()
</span></pre></code>
<p>
Calling <tt>derived.f()</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>derived</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
</span><span class=number>42
</span></pre></code>
<p>
Will yield the expected result. Finally, calling calling the free function
<tt>call_f</tt> with <tt>derived</tt> as argument:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>derived</span><span class=special>)
</span><span class=number>42
</span></pre></code>
<p>
Will also yield the expected result.</p>
<p>
Here's what's happening:</p>
<ol><li><tt>call_f(derived)</tt> is called in Python</li><li>This corresponds to <tt>def(&quot;call_f&quot;, call_f);</tt>. Boost.Python dispatches this call.</li><li><tt>int call_f(Base&amp; b) { return b.f(); }</tt> accepts the call.</li><li>The overridden virtual function <tt>f</tt> of <tt>BaseWrap</tt> is called.</li><li><tt>call_method&lt;int&gt;(self, &quot;f&quot;);</tt> dispatches the call back to Python.</li><li><tt>def f(self): return 42</tt> is finally called.</li></ol><p>
Rewind back to our <tt>Base</tt> class, if its member function <tt>f</tt> was not
declared as pure virtual:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Base
</span><span class=special>{
</span><span class=keyword>virtual </span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=number>0</span><span class=special>; </span><span class=special>}
</span><span class=special>};
</span></pre></code>
<p>
And instead is implemented to return <tt>0</tt>, as shown above.</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>BaseWrap </span><span class=special>: </span><span class=identifier>Base
</span><span class=special>{
</span><span class=identifier>BaseWrap</span><span class=special>(</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self_</span><span class=special>)
</span><span class=special>: </span><span class=identifier>self</span><span class=special>(</span><span class=identifier>self_</span><span class=special>) </span><span class=special>{}
</span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>call_method</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;(</span><span class=identifier>self</span><span class=special>, </span><span class=string>&quot;f&quot;</span><span class=special>); </span><span class=special>}
</span><span class=keyword>static </span><span class=keyword>int </span><span class=identifier>default_f</span><span class=special>(</span><span class=identifier>Base</span><span class=special>* </span><span class=identifier>b</span><span class=special>) </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>b</span><span class=special>-&gt;</span><span class=identifier>Base</span><span class=special>::</span><span class=identifier>f</span><span class=special>(); </span><span class=special>} </span><span class=comment>// &lt;&lt;=== added
</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<p>
then, our Boost.Python wrapper:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Base</span><span class=special>, </span><span class=identifier>BaseWrap</span><span class=special>&gt;(</span><span class=string>&quot;Base&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>BaseWrap</span><span class=special>::</span><span class=identifier>default_f</span><span class=special>)
</span><span class=special>;
</span></pre></code>
<p>
Note that we are allowing <tt>Base</tt> objects to be instantiated this time,
unlike before where we specifically defined the <tt>class_&lt;Base&gt;</tt> with
<tt>no_init</tt>.</p>
<p>
In Python, the results would be as expected:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>base </span><span class=special>= </span><span class=identifier>Base</span><span class=special>()
</span><span class=special>&gt;&gt;&gt; </span><span class=keyword>class </span><span class=identifier>Derived</span><span class=special>(</span><span class=identifier>Base</span><span class=special>):
</span><span class=special>... </span><span class=identifier>def </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
</span><span class=special>... </span><span class=keyword>return </span><span class=number>42
</span><span class=special>...
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>derived </span><span class=special>= </span><span class=identifier>Derived</span><span class=special>()
</span></pre></code>
<p>
Calling <tt>base.f()</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>base</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
</span><span class=number>0
</span></pre></code>
<p>
Calling <tt>derived.f()</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>derived</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
</span><span class=number>42
</span></pre></code>
<p>
Calling <tt>call_f</tt>, passing in a <tt>base</tt> object:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>base</span><span class=special>)
</span><span class=number>0
</span></pre></code>
<p>
Calling <tt>call_f</tt>, passing in a <tt>derived</tt> object:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>derived</span><span class=special>)
</span><span class=number>42
</span></pre></code>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Constructors</b></font>
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<p>
Our previous example didn't have any explicit constructors.
Since <tt>World</tt> is declared as a plain struct, it has an implicit default
constructor. Boost.Python exposes the default constructor by default,
which is why we were able to write</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>planet </span><span class=special>= </span><span class=identifier>hello</span><span class=special>.</span><span class=identifier>World</span><span class=special>()
</span></pre></code>
<p>
We may wish to wrap a class with a non-default constructor. Let us
build on our previous example:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>World
</span><span class=special>{
</span><span class=identifier>World</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>): </span><span class=identifier>msg</span><span class=special>(</span><span class=identifier>msg</span><span class=special>) </span><span class=special>{} </span><span class=comment>// added constructor
</span><span class=keyword>void </span><span class=identifier>set</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>) </span><span class=special>{ </span><span class=keyword>this</span><span class=special>-&gt;</span><span class=identifier>msg </span><span class=special>= </span><span class=identifier>msg</span><span class=special>; </span><span class=special>}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>greet</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>msg</span><span class=special>; </span><span class=special>}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<p>
This time <tt>World</tt> has no default constructor; our previous
wrapping code would fail to compile when the library tried to expose
it. We have to tell <tt>class_&lt;World&gt;</tt> about the constructor we want to
expose instead.</p>
<code><pre>
<span class=preprocessor>#include </span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>python</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;
</span><span class=keyword>using </span><span class=keyword>namespace </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>python</span><span class=special>;
</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>hello</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>World</span><span class=special>&gt;(</span><span class=string>&quot;World&quot;</span><span class=special>, </span><span class=identifier>init</span><span class=special>&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;greet&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>greet</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;set&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
</span><span class=special>;
</span><span class=special>}
</span></pre></code>
<p>
<tt>init&lt;std::string&gt;()</tt> exposes the constructor taking in a
<tt>std::string</tt> (in Python, constructors are spelled
&quot;<tt>&quot;__init__&quot;</tt>&quot;).</p>
<p>
We can expose additional constructors by passing more <tt>init&lt;...&gt;</tt>s to
the <tt>def()</tt> member function. Say for example we have another World
constructor taking in two doubles:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>World</span><span class=special>&gt;(</span><span class=string>&quot;World&quot;</span><span class=special>, </span><span class=identifier>init</span><span class=special>&lt;</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>init</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>, </span><span class=keyword>double</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;greet&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>greet</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;set&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
</span><span class=special>;
</span></pre></code>
<p>
On the other hand, if we do not wish to expose any constructors at
all, we may use <tt>no_init</tt> instead:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Abstract</span><span class=special>&gt;(</span><span class=string>&quot;Abstract&quot;</span><span class=special>, </span><span class=identifier>no_init</span><span class=special>)
</span></pre></code>
<p>
This actually adds an <tt>__init__</tt> method which always raises a
Python RuntimeError exception.</p>
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<p>
Boost.Python wraps (member) function pointers. Unfortunately, C++ function
pointers carry no default argument info. Take a function <tt>f</tt> with default
arguments:</p>
<code><pre>
<span class=keyword>int </span><span class=identifier>f</span><span class=special>(</span><span class=keyword>int</span><span class=special>, </span><span class=keyword>double </span><span class=special>= </span><span class=number>3.14</span><span class=special>, </span><span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=special>= </span><span class=string>&quot;hello&quot;</span><span class=special>);
</span></pre></code>
<p>
But the type of a pointer to the function <tt>f</tt> has no information
about its default arguments:</p>
<code><pre>
<span class=keyword>int</span><span class=special>(*</span><span class=identifier>g</span><span class=special>)(</span><span class=keyword>int</span><span class=special>,</span><span class=keyword>double</span><span class=special>,</span><span class=keyword>char </span><span class=keyword>const</span><span class=special>*) </span><span class=special>= </span><span class=identifier>f</span><span class=special>; </span><span class=comment>// defaults lost!
</span></pre></code>
<p>
When we pass this function pointer to the <tt>def</tt> function, there is no way
to retrieve the default arguments:</p>
<code><pre>
<span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=identifier>f</span><span class=special>); </span><span class=comment>// defaults lost!
</span></pre></code>
<p>
Because of this, when wrapping C++ code in earlier versions of
Boost.Python, we had to resort to writing thin wrappers:</p>
<code><pre>
<span class=comment>// write &quot;thin wrappers&quot;
</span><span class=keyword>int </span><span class=identifier>f1</span><span class=special>(</span><span class=keyword>int </span><span class=identifier>x</span><span class=special>) </span><span class=special>{ </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>x</span><span class=special>); </span><span class=special>}
</span><span class=keyword>int </span><span class=identifier>f2</span><span class=special>(</span><span class=keyword>int </span><span class=identifier>x</span><span class=special>, </span><span class=keyword>double </span><span class=identifier>y</span><span class=special>) </span><span class=special>{ </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>x</span><span class=special>,</span><span class=identifier>y</span><span class=special>); </span><span class=special>}
</span><span class=comment>/*...*/
// in module init
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=identifier>f</span><span class=special>); </span><span class=comment>// all arguments
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=identifier>f2</span><span class=special>); </span><span class=comment>// two arguments
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;f&quot;</span><span class=special>, </span><span class=identifier>f1</span><span class=special>); </span><span class=comment>// one argument
</span></pre></code>
<p>
When you want to wrap functions (or member functions) that either:</p>
<ul><li>have default arguments, or</li><li>are overloaded with a common sequence of initial arguments</li></ul><p>
Boost.Python now has a way to make it easier.</p>
<p>
For instance, given a function:</p>
<code><pre>
<span class=keyword>int </span><span class=identifier>foo</span><span class=special>(</span><span class=keyword>int </span><span class=identifier>a</span><span class=special>, </span><span class=keyword>char </span><span class=identifier>b </span><span class=special>= </span><span class=number>1</span><span class=special>, </span><span class=keyword>unsigned </span><span class=identifier>c </span><span class=special>= </span><span class=number>2</span><span class=special>, </span><span class=keyword>double </span><span class=identifier>d </span><span class=special>= </span><span class=number>3</span><span class=special>);
</span></pre></code>
<p>
The macro invocation:</p>
<code><pre>
<span class=identifier>BOOST_PYTHON_FUNCTION_OVERLOADS</span><span class=special>(</span><span class=identifier>foo_overloads</span><span class=special>, </span><span class=identifier>foo</span><span class=special>, </span><span class=number>1</span><span class=special>, </span><span class=number>4</span><span class=special>)
</span></pre></code>
<p>
Will automatically create the thin wrappers for us. This macro will create
a class <tt>foo_overloads</tt> that can be passed on to <tt>def(...)</tt>. The third
and fourth macro argument are the minimum arguments and maximum arguments,
respectively. In our <tt>foo</tt> function the minimum number of arguments is 1
and the maximum number of arguments is 4. The <tt>def(...)</tt> function will
automatically add all the foo variants for us:</p>
<code><pre>
<span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;foo&quot;</span><span class=special>, </span><span class=identifier>foo</span><span class=special>, </span><span class=identifier>foo_overloads</span><span class=special>());
</span></pre></code>
<p>
A similar facility is provided for class constructors, again, with
default arguments or a sequence of overloads. Remember init&lt;...&gt;? For example,
given a class X with a constructor:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>X
</span><span class=special>{
</span><span class=identifier>X</span><span class=special>(</span><span class=keyword>int </span><span class=identifier>a</span><span class=special>, </span><span class=keyword>char </span><span class=identifier>b </span><span class=special>= </span><span class=literal>'D'</span><span class=special>, </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>c </span><span class=special>= </span><span class=string>&quot;constructor&quot;</span><span class=special>, </span><span class=keyword>double </span><span class=identifier>d </span><span class=special>= </span><span class=number>0.0</span><span class=special>);
</span><span class=comment>/*...*/
</span><span class=special>}
</span></pre></code>
<p>
You can easily add this constructor to Boost.Python in one shot:</p>
<code><pre>
<span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>init</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>, </span><span class=identifier>optional</span><span class=special>&lt;</span><span class=keyword>char</span><span class=special>, </span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string</span><span class=special>, </span><span class=keyword>double</span><span class=special>&gt; </span><span class=special>&gt;())
</span></pre></code>
<p>
Notice the use of <tt>init&lt;...&gt;</tt> and <tt>optional&lt;...&gt;</tt> to signify the default
(optional arguments).</p>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Derived Object types</b></font>
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<p>
Boost.Python comes with a set of derived <tt>object</tt> types corresponding to
that of Python's:</p>
<ul><li>list</li><li>dict</li><li>tuple</li><li>str</li><li>long_</li><li>enum</li></ul><p>
These derived <tt>object</tt> types act like real Python types. For instance:</p>
<code><pre>
<span class=identifier>str</span><span class=special>(</span><span class=number>1</span><span class=special>) </span><span class=special>==&gt; </span><span class=string>&quot;1&quot;
</span></pre></code>
<p>
Wherever appropriate, a particular derived <tt>object</tt> has corresponding
Python type's methods. For instance, <tt>dict</tt> has a <tt>keys()</tt> method:</p>
<code><pre>
<span class=identifier>d</span><span class=special>.</span><span class=identifier>keys</span><span class=special>()
</span></pre></code>
<p>
<tt>make_tuple</tt> is provided for declaring <i>tuple literals</i>. Example:</p>
<code><pre>
<span class=identifier>make_tuple</span><span class=special>(</span><span class=number>123</span><span class=special>, </span><span class=literal>'D'</span><span class=special>, </span><span class=string>&quot;Hello, World&quot;</span><span class=special>, </span><span class=number>0.0</span><span class=special>);
</span></pre></code>
<p>
In C++, when Boost.Python <tt>object</tt>s are used as arguments to functions,
subtype matching is required. For example, when a function <tt>f</tt>, as
declared below, is wrapped, it will only accept instances of Python's
<tt>str</tt> type and subtypes.</p>
<code><pre>
<span class=keyword>void </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>str </span><span class=identifier>name</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>object </span><span class=identifier>n2 </span><span class=special>= </span><span class=identifier>name</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;upper&quot;</span><span class=special>)(); </span><span class=comment>// NAME = name.upper()
</span><span class=identifier>str </span><span class=identifier>NAME </span><span class=special>= </span><span class=identifier>name</span><span class=special>.</span><span class=identifier>upper</span><span class=special>(); </span><span class=comment>// better
</span><span class=identifier>object </span><span class=identifier>msg </span><span class=special>= </span><span class=string>&quot;%s is bigger than %s&quot; </span><span class=special>% </span><span class=identifier>make_tuple</span><span class=special>(</span><span class=identifier>NAME</span><span class=special>,</span><span class=identifier>name</span><span class=special>);
</span><span class=special>}
</span></pre></code>
<p>
In finer detail:</p>
<code><pre>
<span class=identifier>str </span><span class=identifier>NAME </span><span class=special>= </span><span class=identifier>name</span><span class=special>.</span><span class=identifier>upper</span><span class=special>();
</span></pre></code>
<p>
Illustrates that we provide versions of the str type's methods as C++
member functions.</p>
<code><pre>
<span class=identifier>object </span><span class=identifier>msg </span><span class=special>= </span><span class=string>&quot;%s is bigger than %s&quot; </span><span class=special>% </span><span class=identifier>make_tuple</span><span class=special>(</span><span class=identifier>NAME</span><span class=special>,</span><span class=identifier>name</span><span class=special>);
</span></pre></code>
<p>
Demonstrates that you can write the C++ equivalent of <tt>&quot;format&quot; % x,y,z</tt>
in Python, which is useful since there's no easy way to do that in std C++.</p>
<p>
<img src="theme/alert.gif"></img> <b>Beware</b> the common pitfall of forgetting that the constructors
of most of Python's mutable types make copies, just as in Python.</p>
<p>
Python:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>d </span><span class=special>= </span><span class=identifier>dict</span><span class=special>(</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>__dict__</span><span class=special>) </span>#<span class=identifier>copies </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>__dict__
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>d</span><span class=special>[</span><span class=literal>'whatever'</span><span class=special>] </span>#<span class=identifier>modifies </span><span class=identifier>the </span><span class=identifier>copy
</span></pre></code>
<p>
C++:</p>
<code><pre>
<span class=identifier>dict </span><span class=identifier>d</span><span class=special>(</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;__dict__&quot;</span><span class=special>)); </span>#<span class=identifier>copies </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>__dict__
</span><span class=identifier>d</span><span class=special>[</span><span class=literal>'whatever'</span><span class=special>] </span><span class=special>= </span><span class=number>3</span><span class=special>; </span>#<span class=identifier>modifies </span><span class=identifier>the </span><span class=identifier>copy
</span></pre></code>
<a name="class__lt_t_gt__as_objects"></a><h2>class_&lt;T&gt; as objects</h2><p>
Due to the dynamic nature of Boost.Python objects, any <tt>class_&lt;T&gt;</tt> may
also be one of these types! The following code snippet wraps the class
(type) object.</p>
<p>
We can use this to create wrapped instances. Example:</p>
<code><pre>
<span class=identifier>object </span><span class=identifier>vec345 </span><span class=special>= </span><span class=special>(
</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Vec2</span><span class=special>&gt;(</span><span class=string>&quot;Vec2&quot;</span><span class=special>, </span><span class=identifier>init</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>, </span><span class=keyword>double</span><span class=special>&gt;())
</span><span class=special>.</span><span class=identifier>def_readonly</span><span class=special>(</span><span class=string>&quot;length&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Point</span><span class=special>::</span><span class=identifier>length</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def_readonly</span><span class=special>(</span><span class=string>&quot;angle&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>Point</span><span class=special>::</span><span class=identifier>angle</span><span class=special>)
</span><span class=special>)(</span><span class=number>3.0</span><span class=special>, </span><span class=number>4.0</span><span class=special>);
</span><span class=identifier>assert</span><span class=special>(</span><span class=identifier>vec345</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;length&quot;</span><span class=special>) </span><span class=special>== </span><span class=number>5.0</span><span class=special>);
</span></pre></code>
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<title>Enums</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Enums</b></font>
</td>
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<p>
Boost.Python has a nifty facility to capture and wrap C++ enums. While
Python has no <tt>enum</tt> type, we'll often want to expose our C++ enums to
Python as an <tt>int</tt>. Boost.Python's enum facility makes this easy while
taking care of the proper conversions from Python's dynamic typing to C++'s
strong static typing (in C++, ints cannot be implicitly converted to
enums). To illustrate, given a C++ enum:</p>
<code><pre>
<span class=keyword>enum </span><span class=identifier>choice </span><span class=special>{ </span><span class=identifier>red</span><span class=special>, </span><span class=identifier>blue </span><span class=special>};
</span></pre></code>
<p>
the construct:</p>
<code><pre>
<span class=identifier>enum_</span><span class=special>&lt;</span><span class=identifier>choice</span><span class=special>&gt;(</span><span class=string>&quot;choice&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>value</span><span class=special>(</span><span class=string>&quot;red&quot;</span><span class=special>, </span><span class=identifier>red</span><span class=special>)
</span><span class=special>.</span><span class=identifier>value</span><span class=special>(</span><span class=string>&quot;blue&quot;</span><span class=special>, </span><span class=identifier>blue</span><span class=special>)
</span><span class=special>;
</span></pre></code>
<p>
can be used to expose to Python. The new enum type is created in the
current <tt>scope()</tt>, which is usually the current module. The snippet above
creates a Python class derived from Python's <tt>int</tt> type which is
associated with the C++ type passed as its first parameter.</p>
<table width="80%" border="0" align="center">
<tr>
<td class="note_box">
<img src="theme/lens.gif"></img> <b>what is a scope?</b><br><br> The scope is a class that has an
associated global Python object which controls the Python namespace in
which new extension classes and wrapped functions will be defined as
attributes. Details can be found <a href="../../v2/scope.html">
here</a>. </td>
</tr>
</table>
<p>
You can access those values in Python as</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>my_module</span><span class=special>.</span><span class=identifier>choice</span><span class=special>.</span><span class=identifier>red
</span><span class=identifier>my_module</span><span class=special>.</span><span class=identifier>choice</span><span class=special>.</span><span class=identifier>red
</span></pre></code>
<p>
where my_module is the module where the enum is declared. You can also
create a new scope around a class:</p>
<code><pre>
<span class=identifier>scope </span><span class=identifier>in_X</span><span class=special>(</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>X</span><span class=special>&gt;(</span><span class=string>&quot;X&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>( </span><span class=special>... </span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>( </span><span class=special>... </span><span class=special>)
</span><span class=special>);
</span><span class=comment>// Expose X::nested as X.nested
</span><span class=identifier>enum_</span><span class=special>&lt;</span><span class=identifier>X</span><span class=special>::</span><span class=identifier>nested</span><span class=special>&gt;(</span><span class=string>&quot;nested&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>value</span><span class=special>(</span><span class=string>&quot;red&quot;</span><span class=special>, </span><span class=identifier>red</span><span class=special>)
</span><span class=special>.</span><span class=identifier>value</span><span class=special>(</span><span class=string>&quot;blue&quot;</span><span class=special>, </span><span class=identifier>blue</span><span class=special>)
</span><span class=special>;
</span></pre></code>
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<p>
All C++ exceptions must be caught at the boundary with Python code. This
boundary is the point where C++ meets Python. Boost.Python provides a
default exception handler that translates selected standard exceptions,
then gives up:</p>
<code><pre>
<span class=identifier>raise </span><span class=identifier>RuntimeError</span><span class=special>, </span><span class=literal>'unidentifiable C++ Exception'
</span></pre></code>
<p>
Users may provide custom translation. Here's an example:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>PodBayDoorException</span><span class=special>;
</span><span class=keyword>void </span><span class=identifier>translator</span><span class=special>(</span><span class=identifier>PodBayDoorException</span><span class=special>&amp; </span><span class=identifier>x</span><span class=special>) </span><span class=special>{
</span><span class=identifier>PyErr_SetString</span><span class=special>(</span><span class=identifier>PyExc_UserWarning</span><span class=special>, </span><span class=string>&quot;I'm sorry Dave...&quot;</span><span class=special>);
</span><span class=special>}
</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>kubrick</span><span class=special>) </span><span class=special>{
</span><span class=identifier>register_exception_translator</span><span class=special>&lt;
</span><span class=identifier>PodBayDoorException</span><span class=special>&gt;(</span><span class=identifier>translator</span><span class=special>);
</span><span class=special>...
</span></pre></code>
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<title>Exposing Classes</title>
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<p>
Now let's expose a C++ class to Python.</p>
<p>
Consider a C++ class/struct that we want to expose to Python:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>World
</span><span class=special>{
</span><span class=keyword>void </span><span class=identifier>set</span><span class=special>(</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>) </span><span class=special>{ </span><span class=keyword>this</span><span class=special>-&gt;</span><span class=identifier>msg </span><span class=special>= </span><span class=identifier>msg</span><span class=special>; </span><span class=special>}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>greet</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=identifier>msg</span><span class=special>; </span><span class=special>}
</span><span class=identifier>std</span><span class=special>::</span><span class=identifier>string </span><span class=identifier>msg</span><span class=special>;
</span><span class=special>};
</span></pre></code>
<p>
We can expose this to Python by writing a corresponding Boost.Python
C++ Wrapper:</p>
<code><pre>
<span class=preprocessor>#include </span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>python</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;
</span><span class=keyword>using </span><span class=keyword>namespace </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>python</span><span class=special>;
</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>hello</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>World</span><span class=special>&gt;(</span><span class=string>&quot;World&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;greet&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>greet</span><span class=special>)
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;set&quot;</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>World</span><span class=special>::</span><span class=identifier>set</span><span class=special>)
</span><span class=special>;
</span><span class=special>}
</span></pre></code>
<p>
Here, we wrote a C++ class wrapper that exposes the member functions
<tt>greet</tt> and <tt>set</tt>. Now, after building our module as a shared library, we
may use our class <tt>World</tt> in Python. Here's a sample Python session:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>import </span><span class=identifier>hello
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>planet </span><span class=special>= </span><span class=identifier>hello</span><span class=special>.</span><span class=identifier>World</span><span class=special>()
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>planet</span><span class=special>.</span><span class=identifier>set</span><span class=special>(</span><span class=literal>'howdy'</span><span class=special>)
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>planet</span><span class=special>.</span><span class=identifier>greet</span><span class=special>()
</span><span class=literal>'howdy'
</span></pre></code>
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<td width="85%">
<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Extracting C++ objects</b></font>
</td>
</tr>
</table>
<br>
<table border="0">
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</tr>
</table>
<p>
At some point, we will need to get C++ values out of object instances. This
can be achieved with the <tt>extract&lt;T&gt;</tt> function. Consider the following:</p>
<code><pre>
<span class=keyword>double </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>o</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;length&quot;</span><span class=special>); </span><span class=comment>// compile error
</span></pre></code>
<p>
In the code above, we got a compiler error because Boost.Python
<tt>object</tt> can't be implicitly converted to <tt>double</tt>s. Instead, what
we wanted to do above can be achieved by writing:</p>
<code><pre>
<span class=keyword>double </span><span class=identifier>l </span><span class=special>= </span><span class=identifier>extract</span><span class=special>&lt;</span><span class=keyword>double</span><span class=special>&gt;(</span><span class=identifier>o</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;length&quot;</span><span class=special>));
</span><span class=identifier>Vec2</span><span class=special>&amp; </span><span class=identifier>v </span><span class=special>= </span><span class=identifier>extract</span><span class=special>&lt;</span><span class=identifier>Vec2</span><span class=special>&amp;&gt;(</span><span class=identifier>o</span><span class=special>);
</span><span class=identifier>assert</span><span class=special>(</span><span class=identifier>l </span><span class=special>== </span><span class=identifier>v</span><span class=special>.</span><span class=identifier>length</span><span class=special>());
</span></pre></code>
<p>
The first line attempts to extract the &quot;length&quot; attribute of the
Boost.Python <tt>object</tt> <tt>o</tt>. The second line attempts to <i>extract</i> the
<tt>Vec2</tt> object from held by the Boost.Python <tt>object</tt> <tt>o</tt>.</p>
<p>
Take note that we said &quot;attempt to&quot; above. What if the Boost.Python
<tt>object</tt> <tt>o</tt> does not really hold a <tt>Vec2</tt> type? This is certainly
a possibility considering the dynamic nature of Python <tt>object</tt>s. To
be on the safe side, if the C++ type can't be extracted, an
appropriate exception is thrown. To avoid an exception, we need to
test for extractibility:</p>
<code><pre>
<span class=identifier>extract</span><span class=special>&lt;</span><span class=identifier>Vec2</span><span class=special>&amp;&gt; </span><span class=identifier>x</span><span class=special>(</span><span class=identifier>o</span><span class=special>);
</span><span class=keyword>if </span><span class=special>(</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>check</span><span class=special>()) </span><span class=special>{
</span><span class=identifier>Vec2</span><span class=special>&amp; </span><span class=identifier>v </span><span class=special>= </span><span class=identifier>x</span><span class=special>(); </span><span class=special>...
</span></pre></code>
<p>
<img src="theme/bulb.gif"></img> The astute reader might have noticed that the <tt>extract&lt;T&gt;</tt>
facility in fact solves the mutable copying problem:</p>
<code><pre>
<span class=identifier>dict </span><span class=identifier>d </span><span class=special>= </span><span class=identifier>extract</span><span class=special>&lt;</span><span class=identifier>dict</span><span class=special>&gt;(</span><span class=identifier>x</span><span class=special>.</span><span class=identifier>attr</span><span class=special>(</span><span class=string>&quot;__dict__&quot;</span><span class=special>));
</span><span class=identifier>d</span><span class=special>[</span><span class=literal>'whatever'</span><span class=special>] </span><span class=special>= </span><span class=number>3</span><span class=special>; </span>#<span class=identifier>modifies </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>__dict__ </span><span class=special>!
</span></pre></code>
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
no claim as to its suitability for any purpose. </font> </p>
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<html>
<head>
<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
<title>Functions</title>
<link rel="stylesheet" href="theme/style.css" type="text/css">
<link rel="prev" href="class_operators_special_functions.html">
<link rel="next" href="call_policies.html">
</head>
<body>
<table width="100%" height="48" border="0" cellspacing="2">
<tr>
<td><img src="theme/c%2B%2Bboost.gif">
</td>
<td width="85%">
<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Functions</b></font>
</td>
</tr>
</table>
<br>
<table border="0">
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<p>
In this chapter, we'll look at Boost.Python powered functions in closer
detail. We shall see some facilities to make exposing C++ functions to
Python safe from potential pifalls such as dangling pointers and
references. We shall also see facilities that will make it even easier for
us to expose C++ functions that take advantage of C++ features such as
overloading and default arguments.</p>
<blockquote><p><i>Read on...</i></p></blockquote><p>
But before you do, you might want to fire up Python 2.2 or later and type
<tt>&gt;&gt;&gt; import this</tt>.</p>
<code><pre>
&gt;&gt;&gt; import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
</pre></code><table border="0">
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
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<div><h2 class="title">
<a name="python"></a>Chapter 1. python 1.0</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Joel</span> <span class="surname">de Guzman</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">David</span> <span class="surname">Abrahams</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2002-2005 Joel
de Guzman, David Abrahams</p></div>
<div><div class="legalnotice">
<a name="id2632684"></a><p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
http://www.boost.org/LICENSE_1_0.txt </a>)
</p>
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<p><b>Table of Contents</b></p>
<dl>
<dt><span class="section"><a href="index.html#python.quickstart">QuickStart</a></span></dt>
<dt><span class="section"><a href="python/hello.html"> Building Hello World</a></span></dt>
<dt><span class="section"><a href="python/exposing.html"> Exposing Classes</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="python/exposing.html#python.constructors">Constructors</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.class_data_members">Class Data Members</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.class_properties">Class Properties</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.inheritance">Inheritance</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.class_virtual_functions">Class Virtual Functions</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.virtual_functions_with_default_implementations">Virtual Functions with Default Implementations</a></span></dt>
<dt><span class="section"><a href="python/exposing.html#python.class_operators_special_functions">Class Operators/Special Functions</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="python/functions.html">Functions</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="python/functions.html#python.call_policies">Call Policies</a></span></dt>
<dt><span class="section"><a href="python/functions.html#python.overloading">Overloading</a></span></dt>
<dt><span class="section"><a href="python/functions.html#python.default_arguments">Default Arguments</a></span></dt>
<dt><span class="section"><a href="python/functions.html#python.auto_overloading">Auto-Overloading</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="python/object.html"> Object Interface</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="python/object.html#python.basic_interface">Basic Interface</a></span></dt>
<dt><span class="section"><a href="python/object.html#python.derived_object_types">Derived Object types</a></span></dt>
<dt><span class="section"><a href="python/object.html#python.extracting_c___objects">Extracting C++ objects</a></span></dt>
<dt><span class="section"><a href="python/object.html#python.enums">Enums</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="python/embedding.html">Embedding</a></span></dt>
<dd><dl><dt><span class="section"><a href="python/embedding.html#python.using_the_interpreter">Using the interpreter</a></span></dt></dl></dd>
<dt><span class="section"><a href="python/iterators.html">Iterators</a></span></dt>
<dt><span class="section"><a href="python/exception.html"> Exception Translation</a></span></dt>
<dt><span class="section"><a href="python/techniques.html"> General Techniques</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="python/techniques.html#python.creating_packages">Creating Packages</a></span></dt>
<dt><span class="section"><a href="python/techniques.html#python.extending_wrapped_objects_in_python">Extending Wrapped Objects in Python</a></span></dt>
<dt><span class="section"><a href="python/techniques.html#python.reducing_compiling_time">Reducing Compiling Time</a></span></dt>
</dl></dd>
</dl>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.quickstart"></a>QuickStart</h2></div></div></div>
<p>
The Boost Python Library is a framework for interfacing Python and C++. It
allows you to quickly and seamlessly expose C++ classes functions and objects
to Python, and vice-versa, using no special tools -- just your C++ compiler.
It is designed to wrap C++ interfaces non-intrusively, so that you should not
have to change the C++ code at all in order to wrap it, making Boost.Python
ideal for exposing 3rd-party libraries to Python. The library's use of advanced
metaprogramming techniques simplifies its syntax for users, so that wrapping
code takes on the look of a kind of declarative interface definition language
(IDL).
</p>
<a name="quickstart.hello_world"></a><h3>
<a name="id2595112"></a>
Hello World
</h3>
<p>
Following C/C++ tradition, let's start with the "hello, world". A
C++ Function:
</p>
<pre class="programlisting">
<span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">greet</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="string">"hello, world"</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
can be exposed to Python by writing a Boost.Python wrapper:
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">;</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">hello</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"greet"</span><span class="special">,</span> <span class="identifier">greet</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
That's it. We're done. We can now build this as a shared library. The resulting
DLL is now visible to Python. Here's a sample Python session:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">hello</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">print</span> <span class="identifier">hello</span><span class="special">.</span><span class="identifier">greet</span><span class="special">()</span>
<span class="identifier">hello</span><span class="special">,</span> <span class="identifier">world</span>
</pre>
<p>
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
<span class="emphasis"><em><span class="bold"><strong>Next stop... Building your Hello World
module from start to finish...</strong></span></em></span>
</p>
<p>
</p>
</blockquote></div>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><small><p>Last revised: May 18, 2007 at 15:46:01 GMT</p></small></td>
<td align="right"><small></small></td>
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<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Embedding</title>
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</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.embedding"></a>Embedding</h2></div></div></div>
<div class="toc"><dl><dt><span class="section"><a href="embedding.html#python.using_the_interpreter">Using the interpreter</a></span></dt></dl></div>
<p>
By now you should know how to use Boost.Python to call your C++ code from Python.
However, sometimes you may need to do the reverse: call Python code from the
C++-side. This requires you to <span class="emphasis"><em>embed</em></span> the Python interpreter
into your C++ program.
</p>
<p>
Currently, Boost.Python does not directly support everything you'll need when
embedding. Therefore you'll need to use the <a href="http://www.python.org/doc/current/api/api.html" target="_top">Python/C
API</a> to fill in the gaps. However, Boost.Python already makes embedding
a lot easier and, in a future version, it may become unnecessary to touch the
Python/C API at all. So stay tuned... <span class="inlinemediaobject"><img src="../images/smiley.png" alt="smiley"></span>
</p>
<a name="embedding.building_embedded_programs"></a><h3>
<a name="id2654982"></a>
Building embedded programs
</h3>
<p>
To be able to embed python into your programs, you have to link to both Boost.Python's
as well as Python's own runtime library.
</p>
<p>
Boost.Python's library comes in two variants. Both are located in Boost's
<code class="literal">/libs/python/build/bin-stage</code> subdirectory. On Windows, the
variants are called <code class="literal">boost_python.lib</code> (for release builds)
and <code class="literal">boost_python_debug.lib</code> (for debugging). If you can't
find the libraries, you probably haven't built Boost.Python yet. See <a href="../../../../building.html" target="_top">Building and Testing</a> on how to do this.
</p>
<p>
Python's library can be found in the <code class="literal">/libs</code> subdirectory
of your Python directory. On Windows it is called pythonXY.lib where X.Y is
your major Python version number.
</p>
<p>
Additionally, Python's <code class="literal">/include</code> subdirectory has to be added
to your include path.
</p>
<p>
In a Jamfile, all the above boils down to:
</p>
<pre class="programlisting">projectroot c:\projects\embedded_program ; # location of the program
# bring in the rules for python
SEARCH on python.jam = $(BOOST_BUILD_PATH) ;
include python.jam ;
exe embedded_program # name of the executable
: #sources
embedded_program.cpp
: # requirements
&lt;find-library&gt;boost_python &lt;library-path&gt;c:\boost\libs\python
$(PYTHON_PROPERTIES)
&lt;library-path&gt;$(PYTHON_LIB_PATH)
&lt;find-library&gt;$(PYTHON_EMBEDDED_LIBRARY) ;
</pre>
<a name="embedding.getting_started"></a><h3>
<a name="id2655076"></a>
Getting started
</h3>
<p>
Being able to build is nice, but there is nothing to build yet. Embedding the
Python interpreter into one of your C++ programs requires these 4 steps:
</p>
<div class="orderedlist"><ol type="1">
<li>
#include <code class="literal">&lt;boost/python.hpp&gt;</code><br><br>
</li>
<li>
Call <a href="http://www.python.org/doc/current/api/initialization.html#l2h-652" target="_top">Py_Initialize</a>()
to start the interpreter and create the <code class="literal"><span class="underline">_main</span>_</code>
module.<br><br>
</li>
<li>
Call other Python C API routines to use the interpreter.<br><br>
</li>
</ol></div>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> <span class="bold"><strong>Note that at this time you must
not call <a href="http://www.python.org/doc/current/api/initialization.html#l2h-656" target="_top">Py_Finalize</a>()
to stop the interpreter. This may be fixed in a future version of boost.python.</strong></span>
</p>
</div>
<p>
(Of course, there can be other C++ code between all of these steps.)
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
<span class="emphasis"><em><span class="bold"><strong>Now that we can embed the interpreter in
our programs, lets see how to put it to use...</strong></span></em></span>
</p>
<p>
</p>
</blockquote></div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.using_the_interpreter"></a>Using the interpreter</h3></div></div></div>
<p>
As you probably already know, objects in Python are reference-counted. Naturally,
the <code class="literal">PyObject</code>s of the Python/C API are also reference-counted.
There is a difference however. While the reference-counting is fully automatic
in Python, the Python<span class="emphasis"><em>C API requires you to do it [@http:</em></span>/www.python.org/doc/current/api/refcounts.html
by hand]. This is messy and especially hard to get right in the presence
of C++ exceptions. Fortunately Boost.Python provides the <a href="../../../../v2/handle.html" target="_top">handle</a>
and <a href="../../../../v2/object.html" target="_top">object</a> class templates to
automate the process.
</p>
<a name="using_the_interpreter.running_python_code"></a><h3>
<a name="id2655255"></a>
Running Python code
</h3>
<p>
Boost.python provides three related functions to run Python code from C++.
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">eval</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">expression</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
<span class="identifier">object</span> <span class="identifier">exec</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">code</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
<span class="identifier">object</span> <span class="identifier">exec_file</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">filename</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">globals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">(),</span> <span class="identifier">object</span> <span class="identifier">locals</span> <span class="special">=</span> <span class="identifier">object</span><span class="special">())</span>
</pre>
<p>
eval evaluates the given expression and returns the resulting value. exec
executes the given code (typically a set of statements) returning the result,
and exec_file executes the code contained in the given file.
</p>
<p>
The <code class="literal">globals</code> and <code class="literal">locals</code> parameters are
Python dictionaries containing the globals and locals of the context in which
to run the code. For most intents and purposes you can use the namespace
dictionary of the <code class="literal"><span class="underline">_main</span>_</code>
module for both parameters.
</p>
<p>
Boost.python provides a function to import a module:
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">import</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">name</span><span class="special">)</span>
</pre>
<p>
import imports a python module (potentially loading it into the running process
first), and returns it.
</p>
<p>
Let's import the <code class="literal"><span class="underline">_main</span>_</code>
module and run some Python code in its namespace:
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">main_module</span> <span class="special">=</span> <span class="identifier">import</span><span class="special">(</span><span class="string">"__main__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">main_namespace</span> <span class="special">=</span> <span class="identifier">main_module</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">ignored</span> <span class="special">=</span> <span class="identifier">exec</span><span class="special">(</span><span class="string">"hello = file('hello.txt', 'w')\n"</span>
<span class="string">"hello.write('Hello world!')\n"</span>
<span class="string">"hello.close()"</span><span class="special">,</span>
<span class="identifier">main_namespace</span><span class="special">);</span>
</pre>
<p>
This should create a file called 'hello.txt' in the current directory containing
a phrase that is well-known in programming circles.
</p>
<a name="using_the_interpreter.manipulating_python_objects"></a><h3>
<a name="id2655783"></a>
Manipulating Python objects
</h3>
<p>
Often we'd like to have a class to manipulate Python objects. But we have
already seen such a class above, and in the <a href="object.html" target="_top">previous
section</a>: the aptly named <code class="literal">object</code> class and its
derivatives. We've already seen that they can be constructed from a <code class="literal">handle</code>.
The following examples should further illustrate this fact:
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">main_module</span> <span class="special">=</span> <span class="identifier">import</span><span class="special">(</span><span class="string">"__main__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">main_namespace</span> <span class="special">=</span> <span class="identifier">main_module</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">ignored</span> <span class="special">=</span> <span class="identifier">exec</span><span class="special">(</span><span class="string">"result = 5 ** 2"</span><span class="special">,</span> <span class="identifier">main_namespace</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">five_squared</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">main_namespace</span><span class="special">[</span><span class="string">"result"</span><span class="special">]);</span>
</pre>
<p>
Here we create a dictionary object for the <code class="literal"><span class="underline">_main</span>_</code>
module's namespace. Then we assign 5 squared to the result variable and read
this variable from the dictionary. Another way to achieve the same result
is to use eval instead, which returns the result directly:
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">eval</span><span class="special">(</span><span class="string">"5 ** 2"</span><span class="special">);</span>
<span class="keyword">int</span> <span class="identifier">five_squared</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">result</span><span class="special">);</span>
</pre>
<a name="using_the_interpreter.exception_handling"></a><h3>
<a name="id2656116"></a>
Exception handling
</h3>
<p>
If an exception occurs in the evaluation of the python expression, <a href="../../../../v2/errors.html#error_already_set-spec" target="_top">error_already_set</a>
is thrown:
</p>
<pre class="programlisting">
<span class="keyword">try</span>
<span class="special">{</span>
<span class="identifier">object</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">eval</span><span class="special">(</span><span class="string">"5/0"</span><span class="special">);</span>
<span class="comment">// execution will never get here:
</span> <span class="keyword">int</span> <span class="identifier">five_divided_by_zero</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">result</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="identifier">error_already_set</span> <span class="keyword">const</span> <span class="special">&amp;)</span>
<span class="special">{</span>
<span class="comment">// handle the exception in some way
</span><span class="special">}</span>
</pre>
<p>
The <code class="literal">error_already_set</code> exception class doesn't carry any
information in itself. To find out more about the Python exception that occurred,
you need to use the <a href="http://www.python.org/doc/api/exceptionHandling.html" target="_top">exception
handling functions</a> of the Python<span class="emphasis"><em>C API in your catch-statement.
This can be as simple as calling [@http:</em></span>/www.python.org/doc/api/exceptionHandling.html#l2h-70
PyErr_Print()] to print the exception's traceback to the console, or comparing
the type of the exception with those of the <a href="http://www.python.org/doc/api/standardExceptions.html" target="_top">standard
exceptions</a>:
</p>
<pre class="programlisting">
<span class="keyword">catch</span><span class="special">(</span><span class="identifier">error_already_set</span> <span class="keyword">const</span> <span class="special">&amp;)</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">PyErr_ExceptionMatches</span><span class="special">(</span><span class="identifier">PyExc_ZeroDivisionError</span><span class="special">))</span>
<span class="special">{</span>
<span class="comment">// handle ZeroDivisionError specially
</span> <span class="special">}</span>
<span class="keyword">else</span>
<span class="special">{</span>
<span class="comment">// print all other errors to stderr
</span> <span class="identifier">PyErr_Print</span><span class="special">();</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
(To retrieve even more information from the exception you can use some of
the other exception handling functions listed <a href="http://www.python.org/doc/api/exceptionHandling.html" target="_top">here</a>.)
</p>
</div>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.exception"></a> Exception Translation</h2></div></div></div>
<p>
All C++ exceptions must be caught at the boundary with Python code. This boundary
is the point where C++ meets Python. Boost.Python provides a default exception
handler that translates selected standard exceptions, then gives up:
</p>
<pre class="programlisting">
<span class="keyword">raise</span> <span class="identifier">RuntimeError</span><span class="special">,</span> <span class="string">'unidentifiable C++ Exception'</span>
</pre>
<p>
Users may provide custom translation. Here's an example:
</p>
<pre class="programlisting">
<span class="identifier">struct</span> <span class="identifier">PodBayDoorException</span><span class="special">;</span>
<span class="identifier">void</span> <span class="identifier">translator</span><span class="special">(</span><span class="identifier">PodBayDoorException</span> <span class="identifier">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">PyErr_SetString</span><span class="special">(</span><span class="identifier">PyExc_UserWarning</span><span class="special">,</span> <span class="string">"I'm sorry Dave..."</span><span class="special">);</span>
<span class="special">}</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">kubrick</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">register_exception_translator</span><span class="special">&lt;</span>
<span class="identifier">PodBayDoorException</span><span class="special">&gt;(</span><span class="identifier">translator</span><span class="special">);</span>
<span class="special">...</span>
</pre>
</div>
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</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.exposing"></a> Exposing Classes</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="exposing.html#python.constructors">Constructors</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.class_data_members">Class Data Members</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.class_properties">Class Properties</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.inheritance">Inheritance</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.class_virtual_functions">Class Virtual Functions</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.virtual_functions_with_default_implementations">Virtual Functions with Default Implementations</a></span></dt>
<dt><span class="section"><a href="exposing.html#python.class_operators_special_functions">Class Operators/Special Functions</a></span></dt>
</dl></div>
<p>
Now let's expose a C++ class to Python.
</p>
<p>
Consider a C++ class/struct that we want to expose to Python:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">World</span>
<span class="special">{</span>
<span class="keyword">void</span> <span class="identifier">set</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">msg</span> <span class="special">=</span> <span class="identifier">msg</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">greet</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">msg</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
We can expose this to Python by writing a corresponding Boost.Python C++ Wrapper:
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">;</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">hello</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">World</span><span class="special">&gt;(</span><span class="string">"World"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"greet"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">greet</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"set"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">set</span><span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Here, we wrote a C++ class wrapper that exposes the member functions <code class="literal">greet</code>
and <code class="literal">set</code>. Now, after building our module as a shared library,
we may use our class <code class="literal">World</code> in Python. Here's a sample Python
session:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">hello</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">planet</span> <span class="special">=</span> <span class="identifier">hello</span><span class="special">.</span><span class="identifier">World</span><span class="special">()</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">planet</span><span class="special">.</span><span class="identifier">set</span><span class="special">(</span><span class="string">'howdy'</span><span class="special">)</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">planet</span><span class="special">.</span><span class="identifier">greet</span><span class="special">()</span>
<span class="string">'howdy'</span>
</pre>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.constructors"></a>Constructors</h3></div></div></div>
<p>
Our previous example didn't have any explicit constructors. Since <code class="literal">World</code>
is declared as a plain struct, it has an implicit default constructor. Boost.Python
exposes the default constructor by default, which is why we were able to
write
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">planet</span> <span class="special">=</span> <span class="identifier">hello</span><span class="special">.</span><span class="identifier">World</span><span class="special">()</span>
</pre>
<p>
We may wish to wrap a class with a non-default constructor. Let us build
on our previous example:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">World</span>
<span class="special">{</span>
<span class="identifier">World</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span><span class="special">):</span> <span class="identifier">msg</span><span class="special">(</span><span class="identifier">msg</span><span class="special">)</span> <span class="special">{}</span> <span class="comment">// added constructor
</span> <span class="keyword">void</span> <span class="identifier">set</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">msg</span> <span class="special">=</span> <span class="identifier">msg</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">greet</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">msg</span><span class="special">;</span> <span class="special">}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">msg</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
This time <code class="literal">World</code> has no default constructor; our previous
wrapping code would fail to compile when the library tried to expose it.
We have to tell <code class="literal">class_&lt;World&gt;</code> about the constructor
we want to expose instead.
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">;</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">hello</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">World</span><span class="special">&gt;(</span><span class="string">"World"</span><span class="special">,</span> <span class="identifier">init</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"greet"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">greet</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"set"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">set</span><span class="special">)</span>
<span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
<code class="literal">init&lt;std::string&gt;()</code> exposes the constructor taking
in a <code class="literal">std::string</code> (in Python, constructors are spelled
"<code class="literal">"<span class="underline">_init</span>_"</code>").
</p>
<p>
We can expose additional constructors by passing more <code class="literal">init&lt;...&gt;</code>s
to the <code class="literal">def()</code> member function. Say for example we have
another World constructor taking in two doubles:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">World</span><span class="special">&gt;(</span><span class="string">"World"</span><span class="special">,</span> <span class="identifier">init</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">init</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"greet"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">greet</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"set"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">World</span><span class="special">::</span><span class="identifier">set</span><span class="special">)</span>
<span class="special">;</span>
</pre>
<p>
On the other hand, if we do not wish to expose any constructors at all, we
may use <code class="literal">no_init</code> instead:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Abstract</span><span class="special">&gt;(</span><span class="string">"Abstract"</span><span class="special">,</span> <span class="identifier">no_init</span><span class="special">)</span>
</pre>
<p>
This actually adds an <code class="literal"><span class="underline">_init</span>_</code>
method which always raises a Python RuntimeError exception.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.class_data_members"></a>Class Data Members</h3></div></div></div>
<p>
Data members may also be exposed to Python so that they can be accessed as
attributes of the corresponding Python class. Each data member that we wish
to be exposed may be regarded as <span class="bold"><strong>read-only</strong></span>
or <span class="bold"><strong>read-write</strong></span>. Consider this class <code class="literal">Var</code>:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Var</span>
<span class="special">{</span>
<span class="identifier">Var</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">name</span><span class="special">(</span><span class="identifier">name</span><span class="special">),</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">{}</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">const</span> <span class="identifier">name</span><span class="special">;</span>
<span class="keyword">float</span> <span class="identifier">value</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
Our C++ <code class="literal">Var</code> class and its data members can be exposed
to Python:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Var</span><span class="special">&gt;(</span><span class="string">"Var"</span><span class="special">,</span> <span class="identifier">init</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def_readonly</span><span class="special">(</span><span class="string">"name"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Var</span><span class="special">::</span><span class="identifier">name</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def_readwrite</span><span class="special">(</span><span class="string">"value"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Var</span><span class="special">::</span><span class="identifier">value</span><span class="special">);</span>
</pre>
<p>
Then, in Python, assuming we have placed our Var class inside the namespace
hello as we did before:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">hello</span><span class="special">.</span><span class="identifier">Var</span><span class="special">(</span><span class="string">'pi'</span><span class="special">)</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span> <span class="special">=</span> <span class="number">3.14</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">print</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span><span class="special">,</span> <span class="string">'is around'</span><span class="special">,</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span>
<span class="identifier">pi</span> <span class="keyword">is</span> <span class="identifier">around</span> <span class="number">3.14</span>
</pre>
<p>
Note that <code class="literal">name</code> is exposed as <span class="bold"><strong>read-only</strong></span>
while <code class="literal">value</code> is exposed as <span class="bold"><strong>read-write</strong></span>.
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">name</span> <span class="special">=</span> <span class="string">'e'</span> <span class="comment"># can't change name
</span><span class="identifier">Traceback</span> <span class="special">(</span><span class="identifier">most</span> <span class="identifier">recent</span> <span class="identifier">call</span> <span class="identifier">last</span><span class="special">):</span>
<span class="identifier">File</span> <span class="string">"&lt;stdin&gt;"</span><span class="special">,</span> <span class="identifier">line</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">in</span> #
<span class="identifier">AttributeError</span><span class="special">:</span> <span class="identifier">can</span>#<span class="identifier">t</span> <span class="identifier">set</span> <span class="identifier">attribute</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.class_properties"></a>Class Properties</h3></div></div></div>
<p>
In C++, classes with public data members are usually frowned upon. Well designed
classes that take advantage of encapsulation hide the class' data members.
The only way to access the class' data is through access (getter/setter)
functions. Access functions expose class properties. Here's an example:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Num</span>
<span class="special">{</span>
<span class="identifier">Num</span><span class="special">();</span>
<span class="keyword">float</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">set</span><span class="special">(</span><span class="keyword">float</span> <span class="identifier">value</span><span class="special">);</span>
<span class="special">...</span>
<span class="special">};</span>
</pre>
<p>
However, in Python attribute access is fine; it doesn't neccessarily break
encapsulation to let users handle attributes directly, because the attributes
can just be a different syntax for a method call. Wrapping our <code class="literal">Num</code>
class using Boost.Python:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Num</span><span class="special">&gt;(</span><span class="string">"Num"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">add_property</span><span class="special">(</span><span class="string">"rovalue"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">get</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">add_property</span><span class="special">(</span><span class="string">"value"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">get</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">set</span><span class="special">);</span>
</pre>
<p>
And at last, in Python:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">Num</span><span class="special">()</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span> <span class="special">=</span> <span class="number">3.14</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span><span class="special">,</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">rovalue</span>
<span class="special">(</span><span class="number">3.14</span><span class="special">,</span> <span class="number">3.14</span><span class="special">)</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">rovalue</span> <span class="special">=</span> <span class="number">2.17</span> <span class="comment"># error!
</span></pre>
<p>
Take note that the class property <code class="literal">rovalue</code> is exposed as
<span class="bold"><strong>read-only</strong></span> since the <code class="literal">rovalue</code>
setter member function is not passed in:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">add_property</span><span class="special">(</span><span class="string">"rovalue"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">get</span><span class="special">)</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.inheritance"></a>Inheritance</h3></div></div></div>
<p>
In the previous examples, we dealt with classes that are not polymorphic.
This is not often the case. Much of the time, we will be wrapping polymorphic
classes and class hierarchies related by inheritance. We will often have
to write Boost.Python wrappers for classes that are derived from abstract
base classes.
</p>
<p>
Consider this trivial inheritance structure:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Base</span> <span class="special">{</span> <span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">();</span> <span class="special">};</span>
<span class="keyword">struct</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="identifier">Base</span> <span class="special">{};</span>
</pre>
<p>
And a set of C++ functions operating on <code class="literal">Base</code> and <code class="literal">Derived</code>
object instances:
</p>
<pre class="programlisting">
<span class="keyword">void</span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">Base</span><span class="special">*);</span>
<span class="keyword">void</span> <span class="identifier">d</span><span class="special">(</span><span class="identifier">Derived</span><span class="special">*);</span>
<span class="identifier">Base</span><span class="special">*</span> <span class="identifier">factory</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">new</span> <span class="identifier">Derived</span><span class="special">;</span> <span class="special">}</span>
</pre>
<p>
We've seen how we can wrap the base class <code class="literal">Base</code>:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&gt;(</span><span class="string">"Base"</span><span class="special">)</span>
<span class="comment">/*...*/</span>
<span class="special">;</span>
</pre>
<p>
Now we can inform Boost.Python of the inheritance relationship between <code class="literal">Derived</code>
and its base class <code class="literal">Base</code>. Thus:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Derived</span><span class="special">,</span> <span class="identifier">bases</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&gt;</span> <span class="special">&gt;(</span><span class="string">"Derived"</span><span class="special">)</span>
<span class="comment">/*...*/</span>
<span class="special">;</span>
</pre>
<p>
Doing so, we get some things for free:
</p>
<div class="orderedlist"><ol type="1">
<li>
Derived automatically inherits all of Base's Python methods (wrapped C++
member functions)
</li>
<li>
<span class="bold"><strong>If</strong></span> Base is polymorphic, <code class="literal">Derived</code>
objects which have been passed to Python via a pointer or reference to
<code class="literal">Base</code> can be passed where a pointer or reference to
<code class="literal">Derived</code> is expected.
</li>
</ol></div>
<p>
Now, we shall expose the C++ free functions <code class="literal">b</code> and <code class="literal">d</code>
and <code class="literal">factory</code>:
</p>
<pre class="programlisting">
<span class="identifier">def</span><span class="special">(</span><span class="string">"b"</span><span class="special">,</span> <span class="identifier">b</span><span class="special">);</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"d"</span><span class="special">,</span> <span class="identifier">d</span><span class="special">);</span>
<span class="identifier">def</span><span class="special">(</span><span class="string">"factory"</span><span class="special">,</span> <span class="identifier">factory</span><span class="special">);</span>
</pre>
<p>
Note that free function <code class="literal">factory</code> is being used to generate
new instances of class <code class="literal">Derived</code>. In such cases, we use
<code class="literal">return_value_policy&lt;manage_new_object&gt;</code> to instruct
Python to adopt the pointer to <code class="literal">Base</code> and hold the instance
in a new Python <code class="literal">Base</code> object until the the Python object
is destroyed. We shall see more of Boost.Python <a href="functions.html#python.call_policies" title="Call Policies">call
policies</a> later.
</p>
<pre class="programlisting">
<span class="comment">// Tell Python to take ownership of factory's result
</span><span class="identifier">def</span><span class="special">(</span><span class="string">"factory"</span><span class="special">,</span> <span class="identifier">factory</span><span class="special">,</span>
<span class="identifier">return_value_policy</span><span class="special">&lt;</span><span class="identifier">manage_new_object</span><span class="special">&gt;());</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.class_virtual_functions"></a>Class Virtual Functions</h3></div></div></div>
<p>
In this section, we shall learn how to make functions behave polymorphically
through virtual functions. Continuing our example, let us add a virtual function
to our <code class="literal">Base</code> class:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">()</span> <span class="special">{}</span>
<span class="keyword">virtual</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
One of the goals of Boost.Python is to be minimally intrusive on an existing
C++ design. In principle, it should be possible to expose the interface for
a 3rd party library without changing it. It is not ideal to add anything
to our class <code class="computeroutput"><span class="identifier">Base</span></code>. Yet, when
you have a virtual function that's going to be overridden in Python and called
polymorphically <span class="bold"><strong>from C++</strong></span>, we'll need to
add some scaffoldings to make things work properly. What we'll do is write
a class wrapper that derives from <code class="computeroutput"><span class="identifier">Base</span></code>
that will unintrusively hook into the virtual functions so that a Python
override may be called:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">BaseWrap</span> <span class="special">:</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">wrapper</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">f</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">get_override</span><span class="special">(</span><span class="string">"f"</span><span class="special">)();</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Notice too that in addition to inheriting from <code class="computeroutput"><span class="identifier">Base</span></code>,
we also multiply- inherited <code class="computeroutput"><span class="identifier">wrapper</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&gt;</span></code> (See <a href="../../../../v2/wrapper.html" target="_top">Wrapper</a>).
The <code class="computeroutput"><span class="identifier">wrapper</span></code> template makes
the job of wrapping classes that are meant to overridden in Python, easier.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/alert.png" alt="alert"></span> <span class="bold"><strong>MSVC6/7 Workaround</strong></span><br>
<br> If you are using Microsoft Visual C++ 6 or 7, you have to write <code class="computeroutput"><span class="identifier">f</span></code> as:<br> <br> <code class="computeroutput"><span class="keyword">return</span>
<span class="identifier">call</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">get_override</span><span class="special">(</span><span class="string">"f"</span><span class="special">).</span><span class="identifier">ptr</span><span class="special">());</span></code>.
</p>
</div>
<p>
BaseWrap's overridden virtual member function <code class="computeroutput"><span class="identifier">f</span></code>
in effect calls the corresponding method of the Python object through <code class="computeroutput"><span class="identifier">get_override</span></code>.
</p>
<p>
Finally, exposing <code class="computeroutput"><span class="identifier">Base</span></code>:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">BaseWrap</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span><span class="special">&gt;(</span><span class="string">"Base"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">pure_virtual</span><span class="special">(&amp;</span><span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span><span class="special">))</span>
<span class="special">;</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">pure_virtual</span></code> signals Boost.Python
that the function <code class="computeroutput"><span class="identifier">f</span></code> is a
pure virtual function.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> <span class="bold"><strong>member function and methods</strong></span><br>
<br> Python, like many object oriented languages uses the term <span class="bold"><strong>methods</strong></span>. Methods correspond roughly to C++'s <span class="bold"><strong>member functions</strong></span>
</p>
</div>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.virtual_functions_with_default_implementations"></a>Virtual Functions with Default Implementations</h3></div></div></div>
<p>
We've seen in the previous section how classes with pure virtual functions
are wrapped using Boost.Python's <a href="../../../../v2/wrapper.html" target="_top">class
wrapper</a> facilities. If we wish to wrap <span class="bold"><strong>non</strong></span>-pure-virtual
functions instead, the mechanism is a bit different.
</p>
<p>
Recall that in the <a href="exposing.html#python.class_virtual_functions" title="Class Virtual Functions">previous
section</a>, we wrapped a class with a pure virtual function that we then
implemented in C++, or Python classes derived from it. Our base class:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">virtual</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
had a pure virtual function <code class="literal">f</code>. If, however, its member
function <code class="literal">f</code> was not declared as pure virtual:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Base</span>
<span class="special">{</span>
<span class="keyword">virtual</span> <span class="special">~</span><span class="identifier">Base</span><span class="special">()</span> <span class="special">{}</span>
<span class="keyword">virtual</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
We wrap it this way:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">BaseWrap</span> <span class="special">:</span> <span class="identifier">Base</span><span class="special">,</span> <span class="identifier">wrapper</span><span class="special">&lt;</span><span class="identifier">Base</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">f</span><span class="special">()</span>
<span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">override</span> <span class="identifier">f</span> <span class="special">=</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">get_override</span><span class="special">(</span><span class="string">"f"</span><span class="special">))</span>
<span class="keyword">return</span> <span class="identifier">f</span><span class="special">();</span> <span class="comment">// *note*
</span> <span class="keyword">return</span> <span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">default_f</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span><span class="special">();</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Notice how we implemented <code class="computeroutput"><span class="identifier">BaseWrap</span><span class="special">::</span><span class="identifier">f</span></code>. Now,
we have to check if there is an override for <code class="computeroutput"><span class="identifier">f</span></code>.
If none, then we call <code class="computeroutput"><span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span><span class="special">()</span></code>.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/alert.png" alt="alert"></span> <span class="bold"><strong>MSVC6/7 Workaround</strong></span><br>
<br> If you are using Microsoft Visual C++ 6 or 7, you have to rewrite
the line with the <code class="computeroutput"><span class="special">*</span><span class="identifier">note</span><span class="special">*</span></code> as:<br> <br> <code class="computeroutput"><span class="keyword">return</span>
<span class="identifier">call</span><span class="special">&lt;</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*&gt;(</span><span class="identifier">f</span><span class="special">.</span><span class="identifier">ptr</span><span class="special">());</span></code>.
</p>
</div>
<p>
Finally, exposing:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">BaseWrap</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span><span class="special">&gt;(</span><span class="string">"Base"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">BaseWrap</span><span class="special">::</span><span class="identifier">default_f</span><span class="special">)</span>
<span class="special">;</span>
</pre>
<p>
Take note that we expose both <code class="computeroutput"><span class="special">&amp;</span><span class="identifier">Base</span><span class="special">::</span><span class="identifier">f</span></code> and <code class="computeroutput"><span class="special">&amp;</span><span class="identifier">BaseWrap</span><span class="special">::</span><span class="identifier">default_f</span></code>. Boost.Python needs to keep track
of 1) the dispatch function <code class="literal">f</code> and 2) the forwarding function
to its default implementation <code class="literal">default_f</code>. There's a special
<code class="literal">def</code> function for this purpose.
</p>
<p>
In Python, the results would be as expected:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">base</span> <span class="special">=</span> <span class="identifier">Base</span><span class="special">()</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">class</span> <span class="identifier">Derived</span><span class="special">(</span><span class="identifier">Base</span><span class="special">):</span>
<span class="special">...</span> <span class="keyword">def</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="special">...</span> <span class="keyword">return</span> <span class="number">42</span>
<span class="special">...</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">derived</span> <span class="special">=</span> <span class="identifier">Derived</span><span class="special">()</span>
</pre>
<p>
Calling <code class="literal">base.f()</code>:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">base</span><span class="special">.</span><span class="identifier">f</span><span class="special">()</span>
<span class="number">0</span>
</pre>
<p>
Calling <code class="literal">derived.f()</code>:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">derived</span><span class="special">.</span><span class="identifier">f</span><span class="special">()</span>
<span class="number">42</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.class_operators_special_functions"></a>Class Operators/Special Functions</h3></div></div></div>
<a name="class_operators_special_functions.python_operators"></a><h3>
<a name="id2646169"></a>
Python Operators
</h3>
<p>
C is well known for the abundance of operators. C++ extends this to the extremes
by allowing operator overloading. Boost.Python takes advantage of this and
makes it easy to wrap C++ operator-powered classes.
</p>
<p>
Consider a file position class <code class="literal">FilePos</code> and a set of operators
that take on FilePos instances:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">class</span> <span class="identifier">FilePos</span> <span class="special">{</span> <span class="comment">/*...*/</span> <span class="special">};</span>
<span class="identifier">FilePos</span> <span class="keyword">operator</span><span class="special">+(</span><span class="identifier">FilePos</span><span class="special">,</span> <span class="keyword">int</span><span class="special">);</span>
<span class="identifier">FilePos</span> <span class="keyword">operator</span><span class="special">+(</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">FilePos</span><span class="special">);</span>
<span class="keyword">int</span> <span class="keyword">operator</span><span class="special">-(</span><span class="identifier">FilePos</span><span class="special">,</span> <span class="identifier">FilePos</span><span class="special">);</span>
<span class="identifier">FilePos</span> <span class="keyword">operator</span><span class="special">-(</span><span class="identifier">FilePos</span><span class="special">,</span> <span class="keyword">int</span><span class="special">);</span>
<span class="identifier">FilePos</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">+=(</span><span class="identifier">FilePos</span><span class="special">&amp;,</span> <span class="keyword">int</span><span class="special">);</span>
<span class="identifier">FilePos</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">-=(</span><span class="identifier">FilePos</span><span class="special">&amp;,</span> <span class="keyword">int</span><span class="special">);</span>
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">&lt;(</span><span class="identifier">FilePos</span><span class="special">,</span> <span class="identifier">FilePos</span><span class="special">);</span>
</pre>
<p>
The class and the various operators can be mapped to Python rather easily
and intuitively:
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">FilePos</span><span class="special">&gt;(</span><span class="string">"FilePos"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">+</span> <span class="keyword">int</span><span class="special">())</span> <span class="comment">// __add__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="keyword">int</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">self</span><span class="special">)</span> <span class="comment">// __radd__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">-</span> <span class="identifier">self</span><span class="special">)</span> <span class="comment">// __sub__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">-</span> <span class="keyword">int</span><span class="special">())</span> <span class="comment">// __sub__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">+=</span> <span class="keyword">int</span><span class="special">())</span> <span class="comment">// __iadd__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">-=</span> <span class="identifier">other</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">self</span> <span class="special">&lt;</span> <span class="identifier">self</span><span class="special">);</span> <span class="comment">// __lt__
</span></pre>
<p>
The code snippet above is very clear and needs almost no explanation at all.
It is virtually the same as the operators' signatures. Just take note that
<code class="literal">self</code> refers to FilePos object. Also, not every class
<code class="literal">T</code> that you might need to interact with in an operator
expression is (cheaply) default-constructible. You can use <code class="literal">other&lt;T&gt;()</code>
in place of an actual <code class="literal">T</code> instance when writing "self
expressions".
</p>
<a name="class_operators_special_functions.special_methods"></a><h3>
<a name="id2646853"></a>
Special Methods
</h3>
<p>
Python has a few more <span class="emphasis"><em>Special Methods</em></span>. Boost.Python
supports all of the standard special method names supported by real Python
class instances. A similar set of intuitive interfaces can also be used to
wrap C++ functions that correspond to these Python <span class="emphasis"><em>special functions</em></span>.
Example:
</p>
<pre class="programlisting">
<span class="keyword">class</span> <span class="identifier">Rational</span>
<span class="special">{</span> <span class="keyword">public</span><span class="special">:</span> <span class="keyword">operator</span> <span class="keyword">double</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span> <span class="special">};</span>
<span class="identifier">Rational</span> <span class="identifier">pow</span><span class="special">(</span><span class="identifier">Rational</span><span class="special">,</span> <span class="identifier">Rational</span><span class="special">);</span>
<span class="identifier">Rational</span> <span class="identifier">abs</span><span class="special">(</span><span class="identifier">Rational</span><span class="special">);</span>
<span class="identifier">ostream</span><span class="special">&amp;</span> <span class="keyword">operator</span><span class="special">&lt;&lt;(</span><span class="identifier">ostream</span><span class="special">&amp;,</span><span class="identifier">Rational</span><span class="special">);</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Rational</span><span class="special">&gt;(</span><span class="string">"Rational"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">float_</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="comment">// __float__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">pow</span><span class="special">(</span><span class="identifier">self</span><span class="special">,</span> <span class="identifier">other</span><span class="special">&lt;</span><span class="identifier">Rational</span><span class="special">&gt;))</span> <span class="comment">// __pow__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">abs</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="comment">// __abs__
</span> <span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">str</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="comment">// __str__
</span> <span class="special">;</span>
</pre>
<p>
Need we say more?
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> What is the business of <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;</span></code>? Well, the method <code class="computeroutput"><span class="identifier">str</span></code>
requires the <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;</span></code>
to do its work (i.e. <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;&lt;</span></code> is used by the method defined by
<code class="computeroutput"><span class="identifier">def</span><span class="special">(</span><span class="identifier">str</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span></code>.
</p>
</div>
</div>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.functions"></a>Functions</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="functions.html#python.call_policies">Call Policies</a></span></dt>
<dt><span class="section"><a href="functions.html#python.overloading">Overloading</a></span></dt>
<dt><span class="section"><a href="functions.html#python.default_arguments">Default Arguments</a></span></dt>
<dt><span class="section"><a href="functions.html#python.auto_overloading">Auto-Overloading</a></span></dt>
</dl></div>
<p>
In this chapter, we'll look at Boost.Python powered functions in closer detail.
We shall see some facilities to make exposing C++ functions to Python safe
from potential pifalls such as dangling pointers and references. We shall also
see facilities that will make it even easier for us to expose C++ functions
that take advantage of C++ features such as overloading and default arguments.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
<span class="emphasis"><em>Read on...</em></span>
</p>
<p>
</p>
</blockquote></div>
<p>
But before you do, you might want to fire up Python 2.2 or later and type
<code class="literal">&gt;&gt;&gt; import this</code>.
</p>
<pre class="programlisting">&gt;&gt;&gt; import this
The Zen of Python, by Tim Peters
Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than <span class="bold"><strong>right</strong></span> now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
</pre>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.call_policies"></a>Call Policies</h3></div></div></div>
<p>
In C++, we often deal with arguments and return types such as pointers and
references. Such primitive types are rather, ummmm, low level and they really
don't tell us much. At the very least, we don't know the owner of the pointer
or the referenced object. No wonder languages such as Java and Python never
deal with such low level entities. In C++, it's usually considered a good
practice to use smart pointers which exactly describe ownership semantics.
Still, even good C++ interfaces use raw references and pointers sometimes,
so Boost.Python must deal with them. To do this, it may need your help. Consider
the following C++ function:
</p>
<pre class="programlisting">
<span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">);</span>
</pre>
<p>
How should the library wrap this function? A naive approach builds a Python
X object around result reference. This strategy might or might not work out.
Here's an example where it didn't
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">)</span> <span class="preprocessor"># x</span> <span class="identifier">refers</span> <span class="identifier">to</span> <span class="identifier">some</span> <span class="identifier">C</span><span class="special">++</span> <span class="identifier">X</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">del</span> <span class="identifier">y</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">some_method</span><span class="special">()</span> <span class="preprocessor"># CRASH</span><span class="special">!</span>
</pre>
<p>
What's the problem?
</p>
<p>
Well, what if f() was implemented as shown below:
</p>
<pre class="programlisting">
<span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">y</span><span class="special">.</span><span class="identifier">z</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
The problem is that the lifetime of result X&amp; is tied to the lifetime
of y, because the f() returns a reference to a member of the y object. This
idiom is is not uncommon and perfectly acceptable in the context of C++.
However, Python users should not be able to crash the system just by using
our C++ interface. In this case deleting y will invalidate the reference
to X. We have a dangling reference.
</p>
<p>
Here's what's happening:
</p>
<div class="orderedlist"><ol type="1">
<li>
<code class="literal">f</code> is called passing in a reference to <code class="literal">y</code>
and a pointer to <code class="literal">z</code>
</li>
<li>
A reference to <code class="literal">y.x</code> is returned
</li>
<li>
<code class="literal">y</code> is deleted. <code class="literal">x</code> is a dangling reference
</li>
<li>
<code class="literal">x.some_method()</code> is called
</li>
<li><span class="bold"><strong>BOOM!</strong></span></li>
</ol></div>
<p>
We could copy result into a new object:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">).</span><span class="identifier">set</span><span class="special">(</span><span class="number">42</span><span class="special">)</span> <span class="comment"># Result disappears
</span><span class="special">&gt;&gt;&gt;</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="comment"># No crash, but still bad
</span><span class="number">3.14</span>
</pre>
<p>
This is not really our intent of our C++ interface. We've broken our promise
that the Python interface should reflect the C++ interface as closely as
possible.
</p>
<p>
Our problems do not end there. Suppose Y is implemented as follows:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">Y</span>
<span class="special">{</span>
<span class="identifier">X</span> <span class="identifier">x</span><span class="special">;</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">z_value</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">z</span><span class="special">-&gt;</span><span class="identifier">value</span><span class="special">();</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Notice that the data member <code class="literal">z</code> is held by class Y using
a raw pointer. Now we have a potential dangling pointer problem inside Y:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">y</span><span class="special">,</span> <span class="identifier">z</span><span class="special">)</span> <span class="preprocessor"># y</span> <span class="identifier">refers</span> <span class="identifier">to</span> <span class="identifier">z</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">del</span> <span class="identifier">z</span> <span class="preprocessor"># Kill</span> <span class="identifier">the</span> <span class="identifier">z</span> <span class="identifier">object</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">z_value</span><span class="special">()</span> <span class="preprocessor"># CRASH</span><span class="special">!</span>
</pre>
<p>
For reference, here's the implementation of <code class="literal">f</code> again:
</p>
<pre class="programlisting">
<span class="identifier">X</span><span class="special">&amp;</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Y</span><span class="special">&amp;</span> <span class="identifier">y</span><span class="special">,</span> <span class="identifier">Z</span><span class="special">*</span> <span class="identifier">z</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">y</span><span class="special">.</span><span class="identifier">z</span> <span class="special">=</span> <span class="identifier">z</span><span class="special">;</span>
<span class="keyword">return</span> <span class="identifier">y</span><span class="special">.</span><span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
Here's what's happening:
</p>
<div class="orderedlist"><ol type="1">
<li>
<code class="literal">f</code> is called passing in a reference to <code class="literal">y</code>
and a pointer to <code class="literal">z</code>
</li>
<li>
A pointer to <code class="literal">z</code> is held by <code class="literal">y</code>
</li>
<li>
A reference to <code class="literal">y.x</code> is returned
</li>
<li>
<code class="literal">z</code> is deleted. <code class="literal">y.z</code> is a dangling pointer
</li>
<li>
<code class="literal">y.z_value()</code> is called
</li>
<li>
<code class="literal">z-&gt;value()</code> is called
</li>
<li><span class="bold"><strong>BOOM!</strong></span></li>
</ol></div>
<a name="call_policies.call_policies"></a><h3>
<a name="id2648560"></a>
Call Policies
</h3>
<p>
Call Policies may be used in situations such as the example detailed above.
In our example, <code class="literal">return_internal_reference</code> and <code class="literal">with_custodian_and_ward</code>
are our friends:
</p>
<pre class="programlisting">
<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">,</span>
<span class="identifier">return_internal_reference</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span>
<span class="identifier">with_custodian_and_ward</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span> <span class="special">&gt;());</span>
</pre>
<p>
What are the <code class="literal">1</code> and <code class="literal">2</code> parameters, you
ask?
</p>
<pre class="programlisting">
<span class="identifier">return_internal_reference</span><span class="special">&lt;</span><span class="number">1</span>
</pre>
<p>
Informs Boost.Python that the first argument, in our case <code class="literal">Y&amp;
y</code>, is the owner of the returned reference: <code class="literal">X&amp;</code>.
The "<code class="literal">1</code>" simply specifies the first argument.
In short: "return an internal reference <code class="literal">X&amp;</code> owned
by the 1st argument <code class="literal">Y&amp; y</code>".
</p>
<pre class="programlisting">
<span class="identifier">with_custodian_and_ward</span><span class="special">&lt;</span><span class="number">1</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>
</pre>
<p>
Informs Boost.Python that the lifetime of the argument indicated by ward
(i.e. the 2nd argument: <code class="literal">Z* z</code>) is dependent on the lifetime
of the argument indicated by custodian (i.e. the 1st argument: <code class="literal">Y&amp;
y</code>).
</p>
<p>
It is also important to note that we have defined two policies above. Two
or more policies can be composed by chaining. Here's the general syntax:
</p>
<pre class="programlisting">
<span class="identifier">policy1</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...,</span>
<span class="identifier">policy2</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...,</span>
<span class="identifier">policy3</span><span class="special">&lt;</span><span class="identifier">args</span><span class="special">...&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;</span>
</pre>
<p>
Here is the list of predefined call policies. A complete reference detailing
these can be found <a href="../../../../v2/reference.html#models_of_call_policies" target="_top">here</a>.
</p>
<div class="itemizedlist"><ul type="disc">
<li>
<span class="bold"><strong>with_custodian_and_ward</strong></span><br> Ties lifetimes
of the arguments
</li>
<li>
<span class="bold"><strong>with_custodian_and_ward_postcall</strong></span><br>
Ties lifetimes of the arguments and results
</li>
<li>
<span class="bold"><strong>return_internal_reference</strong></span><br> Ties lifetime
of one argument to that of result
</li>
<li>
<span class="bold"><strong>return_value_policy&lt;T&gt; with T one of:</strong></span><br>
</li>
<li>
<span class="bold"><strong>reference_existing_object</strong></span><br> naive
(dangerous) approach
</li>
<li>
<span class="bold"><strong>copy_const_reference</strong></span><br> Boost.Python
v1 approach
</li>
<li>
<span class="bold"><strong>copy_non_const_reference</strong></span><br>
</li>
<li>
<span class="bold"><strong>manage_new_object</strong></span><br> Adopt a pointer
and hold the instance
</li>
</ul></div>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/smiley.png" alt="smiley"></span> <span class="bold"><strong>Remember the Zen, Luke:</strong></span><br>
<br> "Explicit is better than implicit"<br> "In the face
of ambiguity, refuse the temptation to guess"<br>
</p>
</div>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.overloading"></a>Overloading</h3></div></div></div>
<p>
The following illustrates a scheme for manually wrapping an overloaded member
functions. Of course, the same technique can be applied to wrapping overloaded
non-member functions.
</p>
<p>
We have here our C++ class:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">X</span>
<span class="special">{</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">bool</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span> <span class="special">+</span> <span class="identifier">c</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">};</span>
</pre>
<p>
Class X has 4 overloaded functions. We shall start by introducing some member
function pointer variables:
</p>
<pre class="programlisting">
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx1</span><span class="special">)(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx2</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx3</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">int</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx4</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
</pre>
<p>
With these in hand, we can proceed to define and wrap this for Python:
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx1</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx2</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx3</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx4</span><span class="special">)</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.default_arguments"></a>Default Arguments</h3></div></div></div>
<p>
Boost.Python wraps (member) function pointers. Unfortunately, C++ function
pointers carry no default argument info. Take a function <code class="literal">f</code>
with default arguments:
</p>
<pre class="programlisting">
<span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span> <span class="special">=</span> <span class="number">3.14</span><span class="special">,</span> <span class="keyword">char</span> <span class="keyword">const</span><span class="special">*</span> <span class="special">=</span> <span class="string">"hello"</span><span class="special">);</span>
</pre>
<p>
But the type of a pointer to the function <code class="literal">f</code> has no information
about its default arguments:
</p>
<pre class="programlisting">
<span class="keyword">int</span><span class="special">(*</span><span class="identifier">g</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">char</span> <span class="keyword">const</span><span class="special">*)</span> <span class="special">=</span> <span class="identifier">f</span><span class="special">;</span> <span class="comment">// defaults lost!
</span></pre>
<p>
When we pass this function pointer to the <code class="literal">def</code> function,
there is no way to retrieve the default arguments:
</p>
<pre class="programlisting">
<span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">);</span> <span class="comment">// defaults lost!
</span></pre>
<p>
Because of this, when wrapping C++ code, we had to resort to manual wrapping
as outlined in the <a href="functions.html#python.overloading" title="Overloading">previous section</a>,
or writing thin wrappers:
</p>
<pre class="programlisting">
<span class="comment">// write "thin wrappers"
</span><span class="keyword">int</span> <span class="identifier">f1</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span> <span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">f2</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span><span class="identifier">y</span><span class="special">);</span> <span class="special">}</span>
<span class="comment">/*...*/</span>
<span class="comment">// in module init
</span> <span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f</span><span class="special">);</span> <span class="comment">// all arguments
</span> <span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f2</span><span class="special">);</span> <span class="comment">// two arguments
</span> <span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">f1</span><span class="special">);</span> <span class="comment">// one argument
</span></pre>
<p>
When you want to wrap functions (or member functions) that either:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
have default arguments, or
</li>
<li>
are overloaded with a common sequence of initial arguments
</li>
</ul></div>
<a name="default_arguments.boost_python_function_overloads"></a><h3>
<a name="id2650414"></a>
BOOST_PYTHON_FUNCTION_OVERLOADS
</h3>
<p>
Boost.Python now has a way to make it easier. For instance, given a function:
</p>
<pre class="programlisting">
<span class="keyword">int</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="keyword">unsigned</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">2</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">3</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
The macro invocation:
</p>
<pre class="programlisting">
<span class="identifier">BOOST_PYTHON_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">foo_overloads</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">4</span><span class="special">)</span>
</pre>
<p>
will automatically create the thin wrappers for us. This macro will create
a class <code class="literal">foo_overloads</code> that can be passed on to <code class="literal">def(...)</code>.
The third and fourth macro argument are the minimum arguments and maximum
arguments, respectively. In our <code class="literal">foo</code> function the minimum
number of arguments is 1 and the maximum number of arguments is 4. The <code class="literal">def(...)</code>
function will automatically add all the foo variants for us:
</p>
<pre class="programlisting">
<span class="identifier">def</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="identifier">foo_overloads</span><span class="special">());</span>
</pre>
<a name="default_arguments.boost_python_member_function_overloads"></a><h3>
<a name="id2650701"></a>
BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS
</h3>
<p>
Objects here, objects there, objects here there everywhere. More frequently
than anything else, we need to expose member functions of our classes to
Python. Then again, we have the same inconveniences as before when default
arguments or overloads with a common sequence of initial arguments come into
play. Another macro is provided to make this a breeze.
</p>
<p>
Like <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>, <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
may be used to automatically create the thin wrappers for wrapping member
functions. Let's have an example:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">george</span>
<span class="special">{</span>
<span class="keyword">void</span>
<span class="identifier">wack_em</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span> <span class="special">=</span> <span class="char">'x'</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
The macro invocation:
</p>
<pre class="programlisting">
<span class="identifier">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">george_overloads</span><span class="special">,</span> <span class="identifier">wack_em</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
</pre>
<p>
will generate a set of thin wrappers for george's <code class="literal">wack_em</code>
member function accepting a minimum of 1 and a maximum of 3 arguments (i.e.
the third and fourth macro argument). The thin wrappers are all enclosed
in a class named <code class="literal">george_overloads</code> that can then be used
as an argument to <code class="literal">def(...)</code>:
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"wack_em"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">george</span><span class="special">::</span><span class="identifier">wack_em</span><span class="special">,</span> <span class="identifier">george_overloads</span><span class="special">());</span>
</pre>
<p>
See the <a href="../../../../v2/overloads.html#BOOST_PYTHON_FUNCTION_OVERLOADS-spec" target="_top">overloads
reference</a> for details.
</p>
<a name="default_arguments.init_and_optional"></a><h3>
<a name="id2651031"></a>
init and optional
</h3>
<p>
A similar facility is provided for class constructors, again, with default
arguments or a sequence of overloads. Remember <code class="literal">init&lt;...&gt;</code>?
For example, given a class X with a constructor:
</p>
<pre class="programlisting">
<span class="keyword">struct</span> <span class="identifier">X</span>
<span class="special">{</span>
<span class="identifier">X</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">b</span> <span class="special">=</span> <span class="char">'D'</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">c</span> <span class="special">=</span> <span class="string">"constructor"</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">);</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
You can easily add this constructor to Boost.Python in one shot:
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">init</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="special">&gt;())</span>
</pre>
<p>
Notice the use of <code class="literal">init&lt;...&gt;</code> and <code class="literal">optional&lt;...&gt;</code>
to signify the default (optional arguments).
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.auto_overloading"></a>Auto-Overloading</h3></div></div></div>
<p>
It was mentioned in passing in the previous section that <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>
and <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code> can also be
used for overloaded functions and member functions with a common sequence
of initial arguments. Here is an example:
</p>
<pre class="programlisting">
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">b</span><span class="special">,</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">/*...*/</span>
<span class="special">}</span>
</pre>
<p>
Like in the previous section, we can generate thin wrappers for these overloaded
functions in one-shot:
</p>
<pre class="programlisting">
<span class="identifier">BOOST_PYTHON_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">foo_overloads</span><span class="special">,</span> <span class="identifier">foo</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="number">3</span><span class="special">)</span>
</pre>
<p>
Then...
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"foo"</span><span class="special">,</span> <span class="special">(</span><span class="keyword">void</span><span class="special">(*)(</span><span class="keyword">bool</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">char</span><span class="special">))</span><span class="number">0</span><span class="special">,</span> <span class="identifier">foo_overloads</span><span class="special">());</span>
</pre>
<p>
Notice though that we have a situation now where we have a minimum of zero
(0) arguments and a maximum of 3 arguments.
</p>
<a name="auto_overloading.manual_wrapping"></a><h3>
<a name="id2651734"></a>
Manual Wrapping
</h3>
<p>
It is important to emphasize however that <span class="bold"><strong>the overloaded
functions must have a common sequence of initial arguments</strong></span>. Otherwise,
our scheme above will not work. If this is not the case, we have to wrap
our functions <a href="functions.html#python.overloading" title="Overloading">manually</a>.
</p>
<p>
Actually, we can mix and match manual wrapping of overloaded functions and
automatic wrapping through <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
and its sister, <code class="literal">BOOST_PYTHON_FUNCTION_OVERLOADS</code>. Following
up on our example presented in the section <a href="functions.html#python.overloading" title="Overloading">on
overloading</a>, since the first 4 overload functins have a common sequence
of initial arguments, we can use <code class="literal">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</code>
to automatically wrap the first three of the <code class="literal">def</code>s and
manually wrap just the last. Here's how we'll do this:
</p>
<pre class="programlisting">
<span class="identifier">BOOST_PYTHON_MEMBER_FUNCTION_OVERLOADS</span><span class="special">(</span><span class="identifier">xf_overloads</span><span class="special">,</span> <span class="identifier">f</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="number">4</span><span class="special">)</span>
</pre>
<p>
Create a member function pointers as above for both X::f overloads:
</p>
<pre class="programlisting">
<span class="keyword">bool</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx1</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">char</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
<span class="keyword">int</span> <span class="special">(</span><span class="identifier">X</span><span class="special">::*</span><span class="identifier">fx2</span><span class="special">)(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">)</span> <span class="special">=</span> <span class="special">&amp;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">f</span><span class="special">;</span>
</pre>
<p>
Then...
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx1</span><span class="special">,</span> <span class="identifier">xf_overloads</span><span class="special">());</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"f"</span><span class="special">,</span> <span class="identifier">fx2</span><span class="special">)</span>
</pre>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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de Guzman, David Abrahams</small></td>
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<title>Building Hello World</title>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.hello"></a> Building Hello World</h2></div></div></div>
<a name="hello.from_start_to_finish"></a><h3>
<a name="id2595436"></a>
From Start To Finish
</h3>
<p>
Now the first thing you'd want to do is to build the Hello World module and
try it for yourself in Python. In this section, we shall outline the steps
necessary to achieve that. We shall use the build tool that comes bundled with
every boost distribution: <span class="bold"><strong>bjam</strong></span>.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> <span class="bold"><strong>Building without bjam</strong></span><br>
<br> Besides bjam, there are of course other ways to get your module built.
What's written here should not be taken as "the one and only way".
There are of course other build tools apart from <code class="literal">bjam</code>.<br>
<br> Take note however that the preferred build tool for Boost.Python is
bjam. There are so many ways to set up the build incorrectly. Experience shows
that 90% of the "I can't build Boost.Python" problems come from people
who had to use a different tool.
</p>
</div>
<p>
We shall skip over the details. Our objective will be to simply create the
hello world module and run it in Python. For a complete reference to building
Boost.Python, check out: <a href="../../../../building.html" target="_top">building.html</a>.
After this brief <span class="emphasis"><em>bjam</em></span> tutorial, we should have built two
DLLs:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
boost_python.dll
</li>
<li>
hello.pyd
</li>
</ul></div>
<p>
if you are on Windows, and
</p>
<div class="itemizedlist"><ul type="disc">
<li>
libboost_python.so
</li>
<li>
hello.so
</li>
</ul></div>
<p>
if you are on Unix.
</p>
<p>
The tutorial example can be found in the directory: <code class="literal">libs/python/example/tutorial</code>.
There, you can find:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
hello.cpp
</li>
<li>
Jamfile
</li>
</ul></div>
<p>
The <code class="literal">hello.cpp</code> file is our C++ hello world example. The
<code class="literal">Jamfile</code> is a minimalist <span class="emphasis"><em>bjam</em></span> script
that builds the DLLs for us.
</p>
<p>
Before anything else, you should have the bjam executable in your boost directory
or somewhere in your path such that <code class="literal">bjam</code> can be executed
in the command line. Pre-built Boost.Jam executables are available for most
platforms. The complete list of Bjam executables can be found <a href="http://sourceforge.net/project/showfiles.php?group_id=7586" target="_top">here</a>.
</p>
<a name="hello.let_s_jam_"></a><h3>
<a name="id2595623"></a>
Let's Jam!
</h3>
<p>
<span class="inlinemediaobject"><img src="../images/jam.png" alt="jam"></span>
</p>
<p>
Here is our minimalist Jamfile:
</p>
<pre class="programlisting"># This is the top of our own project tree
project-root ;
import python ;
extension hello # Declare a Python extension called hello
: hello.cpp # source
# requirements and dependencies for Boost.Python extensions
&lt;template&gt;@boost/libs/python/build/extension
;
</pre>
<p>
First, we need to specify our location. You may place your project anywhere.
<code class="literal">project-root</code> allows you to do that.
</p>
<pre class="programlisting">project-root ;
</pre>
<p>
By doing so, you'll need a Jamrules file. Simply copy the one in the <a href="../../../../../example/tutorial/Jamrules" target="_top">example/tutorial directory</a>
and tweak the <code class="literal">path-global BOOST_ROOT</code> to where your boost
root directory is. The file has <a href="../../../../../example/tutorial/Jamrules" target="_top">detailed
instructions</a> you can follow.
</p>
<p>
Then we will import the definitions needed by Python modules:
</p>
<pre class="programlisting">import python ;
</pre>
<p>
Finally we declare our <code class="literal">hello</code> extension:
</p>
<pre class="programlisting">extension hello # Declare a Python extension called hello
: hello.cpp # source
# requirements and dependencies for Boost.Python extensions
&lt;template&gt;@boost/libs/python/build/extension
;
</pre>
<p>
The last part tells BJam that we are depending on the Boost Python Library.
</p>
<a name="hello.running_bjam"></a><h3>
<a name="id2595752"></a>
Running bjam
</h3>
<p>
<span class="emphasis"><em>bjam</em></span> is run using your operating system's command line
interpreter.
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
Start it up.
</p>
<p>
</p>
</blockquote></div>
<p>
Make sure that the environment is set so that we can invoke the C++ compiler.
With MSVC, that would mean running the <code class="literal">Vcvars32.bat</code> batch
file. For instance:
</p>
<pre class="programlisting">C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\Tools\vsvars32.bat
</pre>
<p>
Some environment variables will have to be setup for proper building of our
Python modules. Example:
</p>
<pre class="programlisting">set PYTHON_ROOT=c:/dev/tools/python
set PYTHON_VERSION=2.2
</pre>
<p>
The above assumes that the Python installation is in <code class="literal">c:/dev/tools/python</code>
and that we are using Python version 2.2. You'll have to tweak these appropriately.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/tip.png" alt="tip"></span> Be sure not to include a third number, e.g. <span class="bold"><strong>not</strong></span> "2.2.1", even if that's the version you
have.
</p>
</div>
<p>
Take note that you may also do that through the Jamrules file we put in our
project as detailed above. The file has <a href="../../../../../example/tutorial/Jamrules" target="_top">detailed
instructions</a> you can follow.
</p>
<p>
Now we are ready... Be sure to <code class="literal">cd</code> to <code class="literal">libs/python/example/tutorial</code>
where the tutorial <code class="literal">"hello.cpp"</code> and the <code class="literal">"Jamfile"</code>
is situated.
</p>
<p>
Finally:
</p>
<pre class="programlisting">
<span class="identifier">bjam</span> <span class="special">-</span><span class="identifier">sTOOLS</span><span class="special">=</span><span class="identifier">vc</span><span class="special">-</span><span class="number">7</span><span class="identifier">_1</span>
</pre>
<p>
We are again assuming that we are using Microsoft Visual C++ version 7.1. If
not, then you will have to specify the appropriate tool. See <a href="../../../../../../../tools/build/index.html" target="_top">Building
Boost Libraries</a> for further details.
</p>
<p>
It should be building now:
</p>
<pre class="programlisting">cd C:\dev\boost\libs\python\example\tutorial
bjam -sTOOLS=msvc
...patience...
...found 1703 targets...
...updating 40 targets...
</pre>
<p>
And so on... Finally:
</p>
<pre class="programlisting">Creating library bin\boost\libs\python\build\boost_python.dll\vc-7_1\debug\th
reading-multi\boost_python.lib and object bin\boost\libs\python\build\boost_pyth
on.dll\vc-7_1\debug\threading-multi\boost_python.exp
vc-C++ bin\tutorial\hello.pyd\vc-7_1\debug\threading-multi\hello.obj
hello.cpp
vc-Link bin\tutorial\hello.pyd\vc-7_1\debug\threading-multi\hello.pyd bin\tutori
al\hello.pyd\vc-7_1\debug\threading-multi\hello.lib
Creating library bin\tutorial\hello.pyd\vc-7_1\debug\threading-multi\hello.li
b and object bin\tutorial\hello.pyd\vc-7_1\debug\threading-multi\hello.exp
...updated 31 targets...
</pre>
<p>
If all is well, you should now have:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
boost_python.dll
</li>
<li>
hello.pyd
</li>
</ul></div>
<p>
if you are on Windows, and
</p>
<div class="itemizedlist"><ul type="disc">
<li>
libboost_python.so
</li>
<li>
hello.so
</li>
</ul></div>
<p>
if you are on Unix.
</p>
<p>
<code class="literal">boost_python.dll</code> and <code class="literal">hello.pyd</code> can be
found somewhere in your project's <code class="literal">bin</code> directory. After a
successful build, you make it possible for the system to find boost_python.dll
or libboost_python.so (usually done with LD_LIBRARY_PATH, DYLD_LIBRARY_PATH,
or some other variable on *nix and with PATH on Windows) and for Python to
find the hello module (Done with PYTHONPATH on all systems.)
</p>
<p>
You may now fire up Python and run our hello module:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">hello</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">print</span> <span class="identifier">hello</span><span class="special">.</span><span class="identifier">greet</span><span class="special">()</span>
<span class="identifier">hello</span><span class="special">,</span> <span class="identifier">world</span>
</pre>
<p>
</p>
<div class="blockquote"><blockquote class="blockquote">
<p>
</p>
<p>
<span class="bold"><strong>There you go... Have fun!</strong></span>
</p>
<p>
</p>
</blockquote></div>
</div>
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</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.iterators"></a>Iterators</h2></div></div></div>
<p>
In C++, and STL in particular, we see iterators everywhere. Python also has
iterators, but these are two very different beasts.
</p>
<p>
<span class="bold"><strong>C++ iterators:</strong></span>
</p>
<div class="itemizedlist"><ul type="disc">
<li>
C++ has 5 type categories (random-access, bidirectional, forward, input,
output)
</li>
<li>
There are 2 Operation categories: reposition, access
</li>
<li>
A pair of iterators is needed to represent a (first/last) range.
</li>
</ul></div>
<p>
<span class="bold"><strong>Python Iterators:</strong></span>
</p>
<div class="itemizedlist"><ul type="disc">
<li>
1 category (forward)
</li>
<li>
1 operation category (next())
</li>
<li>
Raises StopIteration exception at end
</li>
</ul></div>
<p>
The typical Python iteration protocol: <code class="literal"><span class="bold"><strong>for y
in x...</strong></span></code> is as follows:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">__iter__</span><span class="special">()</span> <span class="comment"># get iterator
</span><span class="keyword">try</span><span class="special">:</span>
<span class="keyword">while</span> <span class="number">1</span><span class="special">:</span>
<span class="identifier">y</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">()</span> <span class="comment"># get each item
</span> <span class="special">...</span> <span class="comment"># process y
</span><span class="keyword">except</span> <span class="identifier">StopIteration</span><span class="special">:</span> <span class="keyword">pass</span> <span class="comment"># iterator exhausted
</span></pre>
<p>
Boost.Python provides some mechanisms to make C++ iterators play along nicely
as Python iterators. What we need to do is to produce appropriate <code class="computeroutput"><span class="identifier">__iter__</span></code> function from C++ iterators that
is compatible with the Python iteration protocol. For example:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">get_iterator</span> <span class="special">=</span> <span class="identifier">iterator</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;();</span>
<span class="identifier">object</span> <span class="identifier">iter</span> <span class="special">=</span> <span class="identifier">get_iterator</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
<span class="identifier">object</span> <span class="identifier">first</span> <span class="special">=</span> <span class="identifier">iter</span><span class="special">.</span><span class="identifier">next</span><span class="special">();</span>
</pre>
<p>
Or for use in class_&lt;&gt;:
</p>
<pre class="programlisting">
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"__iter__"</span><span class="special">,</span> <span class="identifier">iterator</span><span class="special">&lt;</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;())</span>
</pre>
<p>
<span class="bold"><strong>range</strong></span>
</p>
<p>
We can create a Python savvy iterator using the range function:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
range(start, finish)
</li>
<li>
range&lt;Policies,Target&gt;(start, finish)
</li>
</ul></div>
<p>
Here, start/finish may be one of:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
member data pointers
</li>
<li>
member function pointers
</li>
<li>
adaptable function object (use Target parameter)
</li>
</ul></div>
<p>
<span class="bold"><strong>iterator</strong></span>
</p>
<div class="itemizedlist"><ul type="disc"><li>
iterator&lt;T, Policies&gt;()
</li></ul></div>
<p>
Given a container <code class="literal">T</code>, iterator is a shortcut that simply
calls <code class="literal">range</code> with &amp;T::begin, &amp;T::end.
</p>
<p>
Let's put this into action... Here's an example from some hypothetical bogon
Particle accelerator code:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">f</span> <span class="special">=</span> <span class="identifier">Field</span><span class="special">()</span>
<span class="keyword">for</span> <span class="identifier">x</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">pions</span><span class="special">:</span>
<span class="identifier">smash</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span>
<span class="keyword">for</span> <span class="identifier">y</span> <span class="keyword">in</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">bogons</span><span class="special">:</span>
<span class="identifier">count</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span>
</pre>
<p>
Now, our C++ Wrapper:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">F</span><span class="special">&gt;(</span><span class="string">"Field"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"pions"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_begin</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">p_end</span><span class="special">))</span>
<span class="special">.</span><span class="identifier">property</span><span class="special">(</span><span class="string">"bogons"</span><span class="special">,</span> <span class="identifier">range</span><span class="special">(&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_begin</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">F</span><span class="special">::</span><span class="identifier">b_end</span><span class="special">));</span>
</pre>
<p>
<span class="bold"><strong>stl_input_iterator</strong></span>
</p>
<p>
So far, we have seen how to expose C++ iterators and ranges to Python. Sometimes
we wish to go the other way, though: we'd like to pass a Python sequence to
an STL algorithm or use it to initialize an STL container. We need to make
a Python iterator look like an STL iterator. For that, we use <code class="computeroutput"><span class="identifier">stl_input_iterator</span><span class="special">&lt;&gt;</span></code>.
Consider how we might implement a function that exposes <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;::</span><span class="identifier">assign</span><span class="special">()</span></code> to Python:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">list_assign</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">l</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">o</span><span class="special">)</span> <span class="special">{</span>
<span class="comment">// Turn a Python sequence into an STL input range
</span> <span class="identifier">stl_input_iterator</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">begin</span><span class="special">(</span><span class="identifier">o</span><span class="special">),</span> <span class="identifier">end</span><span class="special">;</span>
<span class="identifier">l</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">// Part of the wrapper for list&lt;int&gt;
</span><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">list</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&gt;(</span><span class="string">"list_int"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="string">"assign"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">list_assign</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;)</span>
<span class="comment">// ...
</span> <span class="special">;</span>
</pre>
<p>
Now in Python, we can assign any integer sequence to <code class="computeroutput"><span class="identifier">list_int</span></code>
objects:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">x</span> <span class="special">=</span> <span class="identifier">list_int</span><span class="special">();</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">assign</span><span class="special">([</span><span class="number">1</span><span class="special">,</span><span class="number">2</span><span class="special">,</span><span class="number">3</span><span class="special">,</span><span class="number">4</span><span class="special">,</span><span class="number">5</span><span class="special">])</span>
</pre>
</div>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.object"></a> Object Interface</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="object.html#python.basic_interface">Basic Interface</a></span></dt>
<dt><span class="section"><a href="object.html#python.derived_object_types">Derived Object types</a></span></dt>
<dt><span class="section"><a href="object.html#python.extracting_c___objects">Extracting C++ objects</a></span></dt>
<dt><span class="section"><a href="object.html#python.enums">Enums</a></span></dt>
</dl></div>
<p>
Python is dynamically typed, unlike C++ which is statically typed. Python variables
may hold an integer, a float, list, dict, tuple, str, long etc., among other
things. In the viewpoint of Boost.Python and C++, these Pythonic variables
are just instances of class <code class="literal">object</code>. We shall see in this
chapter how to deal with Python objects.
</p>
<p>
As mentioned, one of the goals of Boost.Python is to provide a bidirectional
mapping between C++ and Python while maintaining the Python feel. Boost.Python
C++ <code class="literal">object</code>s are as close as possible to Python. This should
minimize the learning curve significantly.
</p>
<p>
<span class="inlinemediaobject"><img src="../images/python.png" alt="python"></span>
</p>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.basic_interface"></a>Basic Interface</h3></div></div></div>
<p>
Class <code class="literal">object</code> wraps <code class="literal">PyObject*</code>. All the
intricacies of dealing with <code class="literal">PyObject</code>s such as managing
reference counting are handled by the <code class="literal">object</code> class. C++
object interoperability is seamless. Boost.Python C++ <code class="literal">object</code>s
can in fact be explicitly constructed from any C++ object.
</p>
<p>
To illustrate, this Python code snippet:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">def</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">):</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="string">'foo'</span><span class="special">):</span>
<span class="identifier">x</span><span class="special">[</span><span class="number">3</span><span class="special">:</span><span class="number">7</span><span class="special">]</span> <span class="special">=</span> <span class="string">'bar'</span>
<span class="keyword">else</span><span class="special">:</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">items</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)</span>
<span class="keyword">return</span> <span class="identifier">x</span>
<span class="keyword">def</span> <span class="identifier">getfunc</span><span class="special">():</span>
<span class="keyword">return</span> <span class="identifier">f</span><span class="special">;</span>
</pre>
<p>
Can be rewritten in C++ using Boost.Python facilities this way:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">object</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">object</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">y</span> <span class="special">==</span> <span class="string">"foo"</span><span class="special">)</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">slice</span><span class="special">(</span><span class="number">3</span><span class="special">,</span><span class="number">7</span><span class="special">)</span> <span class="special">=</span> <span class="string">"bar"</span><span class="special">;</span>
<span class="keyword">else</span>
<span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"items"</span><span class="special">)</span> <span class="special">+=</span> <span class="identifier">y</span><span class="special">(</span><span class="number">3</span><span class="special">,</span> <span class="identifier">x</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">x</span><span class="special">;</span>
<span class="special">}</span>
<span class="identifier">object</span> <span class="identifier">getfunc</span><span class="special">()</span> <span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">f</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
Apart from cosmetic differences due to the fact that we are writing the code
in C++, the look and feel should be immediately apparent to the Python coder.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.derived_object_types"></a>Derived Object types</h3></div></div></div>
<p>
Boost.Python comes with a set of derived <code class="literal">object</code> types
corresponding to that of Python's:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
list
</li>
<li>
dict
</li>
<li>
tuple
</li>
<li>
str
</li>
<li>
long_
</li>
<li>
enum
</li>
</ul></div>
<p>
These derived <code class="literal">object</code> types act like real Python types.
For instance:
</p>
<pre class="programlisting">
<span class="identifier">str</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">==&gt;</span> <span class="string">"1"</span>
</pre>
<p>
Wherever appropriate, a particular derived <code class="literal">object</code> has
corresponding Python type's methods. For instance, <code class="literal">dict</code>
has a <code class="literal">keys()</code> method:
</p>
<pre class="programlisting">
<span class="identifier">d</span><span class="special">.</span><span class="identifier">keys</span><span class="special">()</span>
</pre>
<p>
<code class="literal">make_tuple</code> is provided for declaring <span class="emphasis"><em>tuple literals</em></span>.
Example:
</p>
<pre class="programlisting">
<span class="identifier">make_tuple</span><span class="special">(</span><span class="number">123</span><span class="special">,</span> <span class="char">'D'</span><span class="special">,</span> <span class="string">"Hello, World"</span><span class="special">,</span> <span class="number">0.0</span><span class="special">);</span>
</pre>
<p>
In C++, when Boost.Python <code class="literal">object</code>s are used as arguments
to functions, subtype matching is required. For example, when a function
<code class="literal">f</code>, as declared below, is wrapped, it will only accept
instances of Python's <code class="literal">str</code> type and subtypes.
</p>
<pre class="programlisting">
<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">str</span> <span class="identifier">name</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">object</span> <span class="identifier">n2</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"upper"</span><span class="special">)();</span> <span class="comment">// NAME = name.upper()
</span> <span class="identifier">str</span> <span class="identifier">NAME</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">upper</span><span class="special">();</span> <span class="comment">// better
</span> <span class="identifier">object</span> <span class="identifier">msg</span> <span class="special">=</span> <span class="string">"%s is bigger than %s"</span> <span class="special">%</span> <span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">NAME</span><span class="special">,</span><span class="identifier">name</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
In finer detail:
</p>
<pre class="programlisting">
<span class="identifier">str</span> <span class="identifier">NAME</span> <span class="special">=</span> <span class="identifier">name</span><span class="special">.</span><span class="identifier">upper</span><span class="special">();</span>
</pre>
<p>
Illustrates that we provide versions of the str type's methods as C++ member
functions.
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">msg</span> <span class="special">=</span> <span class="string">"%s is bigger than %s"</span> <span class="special">%</span> <span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">NAME</span><span class="special">,</span><span class="identifier">name</span><span class="special">);</span>
</pre>
<p>
Demonstrates that you can write the C++ equivalent of <code class="literal">"format"
% x,y,z</code> in Python, which is useful since there's no easy way to
do that in std C++.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/alert.png" alt="alert"></span> <span class="bold"><strong>Beware</strong></span> the common pitfall
of forgetting that the constructors of most of Python's mutable types make
copies, just as in Python.
</p>
</div>
<p>
Python:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">dict</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">__dict__</span><span class="special">)</span> <span class="comment"># copies x.__dict__
</span><span class="special">&gt;&gt;&gt;</span> <span class="identifier">d</span><span class="special">[</span><span class="string">'whatever'</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span> <span class="comment"># modifies the copy
</span></pre>
<p>
C++:
</p>
<pre class="programlisting">
<span class="identifier">dict</span> <span class="identifier">d</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">));</span> <span class="comment">// copies x.__dict__
</span><span class="identifier">d</span><span class="special">[</span><span class="char">'whatever'</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> <span class="comment">// modifies the copy
</span></pre>
<a name="derived_object_types.class__lt_t_gt__as_objects"></a><h3>
<a name="id2653534"></a>
class_&lt;T&gt; as objects
</h3>
<p>
Due to the dynamic nature of Boost.Python objects, any <code class="literal">class_&lt;T&gt;</code>
may also be one of these types! The following code snippet wraps the class
(type) object.
</p>
<p>
We can use this to create wrapped instances. Example:
</p>
<pre class="programlisting">
<span class="identifier">object</span> <span class="identifier">vec345</span> <span class="special">=</span> <span class="special">(</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">Vec2</span><span class="special">&gt;(</span><span class="string">"Vec2"</span><span class="special">,</span> <span class="identifier">init</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;())</span>
<span class="special">.</span><span class="identifier">def_readonly</span><span class="special">(</span><span class="string">"length"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Point</span><span class="special">::</span><span class="identifier">length</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def_readonly</span><span class="special">(</span><span class="string">"angle"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">Point</span><span class="special">::</span><span class="identifier">angle</span><span class="special">)</span>
<span class="special">)(</span><span class="number">3.0</span><span class="special">,</span> <span class="number">4.0</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">vec345</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">)</span> <span class="special">==</span> <span class="number">5.0</span><span class="special">);</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.extracting_c___objects"></a>Extracting C++ objects</h3></div></div></div>
<p>
At some point, we will need to get C++ values out of object instances. This
can be achieved with the <code class="literal">extract&lt;T&gt;</code> function. Consider
the following:
</p>
<pre class="programlisting">
<span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">);</span> <span class="comment">// compile error
</span></pre>
<p>
In the code above, we got a compiler error because Boost.Python <code class="literal">object</code>
can't be implicitly converted to <code class="literal">double</code>s. Instead, what
we wanted to do above can be achieved by writing:
</p>
<pre class="programlisting">
<span class="keyword">double</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="identifier">o</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"length"</span><span class="special">));</span>
<span class="identifier">Vec2</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="identifier">Vec2</span><span class="special">&amp;&gt;(</span><span class="identifier">o</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">l</span> <span class="special">==</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">length</span><span class="special">());</span>
</pre>
<p>
The first line attempts to extract the "length" attribute of the
Boost.Python <code class="literal">object</code>. The second line attempts to <span class="emphasis"><em>extract</em></span>
the <code class="literal">Vec2</code> object from held by the Boost.Python <code class="literal">object</code>.
</p>
<p>
Take note that we said "attempt to" above. What if the Boost.Python
<code class="literal">object</code> does not really hold a <code class="literal">Vec2</code>
type? This is certainly a possibility considering the dynamic nature of Python
<code class="literal">object</code>s. To be on the safe side, if the C++ type can't
be extracted, an appropriate exception is thrown. To avoid an exception,
we need to test for extractibility:
</p>
<pre class="programlisting">
<span class="identifier">extract</span><span class="special">&lt;</span><span class="identifier">Vec2</span><span class="special">&amp;&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">o</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">check</span><span class="special">())</span> <span class="special">{</span>
<span class="identifier">Vec2</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">x</span><span class="special">();</span> <span class="special">...</span>
</pre>
<p>
<span class="inlinemediaobject"><img src="../images/tip.png" alt="tip"></span> The astute reader might have noticed that the <code class="literal">extract&lt;T&gt;</code>
facility in fact solves the mutable copying problem:
</p>
<pre class="programlisting">
<span class="identifier">dict</span> <span class="identifier">d</span> <span class="special">=</span> <span class="identifier">extract</span><span class="special">&lt;</span><span class="identifier">dict</span><span class="special">&gt;(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"__dict__"</span><span class="special">));</span>
<span class="identifier">d</span><span class="special">[</span><span class="string">"whatever"</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> <span class="comment">// modifies x.__dict__ !
</span></pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.enums"></a>Enums</h3></div></div></div>
<p>
Boost.Python has a nifty facility to capture and wrap C++ enums. While Python
has no <code class="literal">enum</code> type, we'll often want to expose our C++ enums
to Python as an <code class="literal">int</code>. Boost.Python's enum facility makes
this easy while taking care of the proper conversions from Python's dynamic
typing to C++'s strong static typing (in C++, ints cannot be implicitly converted
to enums). To illustrate, given a C++ enum:
</p>
<pre class="programlisting">
<span class="keyword">enum</span> <span class="identifier">choice</span> <span class="special">{</span> <span class="identifier">red</span><span class="special">,</span> <span class="identifier">blue</span> <span class="special">};</span>
</pre>
<p>
the construct:
</p>
<pre class="programlisting">
<span class="identifier">enum_</span><span class="special">&lt;</span><span class="identifier">choice</span><span class="special">&gt;(</span><span class="string">"choice"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"red"</span><span class="special">,</span> <span class="identifier">red</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"blue"</span><span class="special">,</span> <span class="identifier">blue</span><span class="special">)</span>
<span class="special">;</span>
</pre>
<p>
can be used to expose to Python. The new enum type is created in the current
<code class="literal">scope()</code>, which is usually the current module. The snippet
above creates a Python class derived from Python's <code class="literal">int</code>
type which is associated with the C++ type passed as its first parameter.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> <span class="bold"><strong>what is a scope?</strong></span><br>
<br> The scope is a class that has an associated global Python object which
controls the Python namespace in which new extension classes and wrapped
functions will be defined as attributes. Details can be found <a href="../../../../v2/scope.html" target="_top">here</a>.
</p>
</div>
<p>
You can access those values in Python as
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">my_module</span><span class="special">.</span><span class="identifier">choice</span><span class="special">.</span><span class="identifier">red</span>
<span class="identifier">my_module</span><span class="special">.</span><span class="identifier">choice</span><span class="special">.</span><span class="identifier">red</span>
</pre>
<p>
where my_module is the module where the enum is declared. You can also create
a new scope around a class:
</p>
<p>
</p>
<pre class="programlisting">
<span class="identifier">scope</span> <span class="identifier">in_X</span> <span class="special">=</span> <span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;(</span><span class="string">"X"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span> <span class="special">...</span> <span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span> <span class="special">...</span> <span class="special">)</span>
<span class="special">;</span>
<span class="comment">// Expose X::nested as X.nested
</span><span class="identifier">enum_</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">::</span><span class="identifier">nested</span><span class="special">&gt;(</span><span class="string">"nested"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"red"</span><span class="special">,</span> <span class="identifier">red</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">value</span><span class="special">(</span><span class="string">"blue"</span><span class="special">,</span> <span class="identifier">blue</span><span class="special">)</span>
<span class="special">;</span>
</pre>
</div>
</div>
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</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="python.techniques"></a> General Techniques</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="techniques.html#python.creating_packages">Creating Packages</a></span></dt>
<dt><span class="section"><a href="techniques.html#python.extending_wrapped_objects_in_python">Extending Wrapped Objects in Python</a></span></dt>
<dt><span class="section"><a href="techniques.html#python.reducing_compiling_time">Reducing Compiling Time</a></span></dt>
</dl></div>
<p>
Here are presented some useful techniques that you can use while wrapping code
with Boost.Python.
</p>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.creating_packages"></a>Creating Packages</h3></div></div></div>
<p>
A Python package is a collection of modules that provide to the user a certain
functionality. If you're not familiar on how to create packages, a good introduction
to them is provided in the <a href="http://www.python.org/doc/current/tut/node8.html" target="_top">Python
Tutorial</a>.
</p>
<p>
But we are wrapping C++ code, using Boost.Python. How can we provide a nice
package interface to our users? To better explain some concepts, let's work
with an example.
</p>
<p>
We have a C++ library that works with sounds: reading and writing various
formats, applying filters to the sound data, etc. It is named (conveniently)
<code class="literal">sounds</code>. Our library already has a neat C++ namespace hierarchy,
like so:
</p>
<pre class="programlisting">
<span class="identifier">sounds</span><span class="special">::</span><span class="identifier">core</span>
<span class="identifier">sounds</span><span class="special">::</span><span class="identifier">io</span>
<span class="identifier">sounds</span><span class="special">::</span><span class="identifier">filters</span>
</pre>
<p>
We would like to present this same hierarchy to the Python user, allowing
him to write code like this:
</p>
<pre class="programlisting">
<span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span>
<span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span><span class="special">.</span><span class="identifier">echo</span><span class="special">(...)</span> <span class="comment"># echo is a C++ function
</span></pre>
<p>
The first step is to write the wrapping code. We have to export each module
separately with Boost.Python, like this:
</p>
<pre class="programlisting">
<span class="special">/*</span> <span class="identifier">file</span> <span class="identifier">core</span><span class="special">.</span><span class="identifier">cpp</span> <span class="special">*/</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">core</span><span class="special">)</span>
<span class="special">{</span>
<span class="special">/*</span> <span class="identifier">export</span> <span class="identifier">everything</span> <span class="keyword">in</span> <span class="identifier">the</span> <span class="identifier">sounds</span><span class="special">::</span><span class="identifier">core</span> <span class="identifier">namespace</span> <span class="special">*/</span>
<span class="special">...</span>
<span class="special">}</span>
<span class="special">/*</span> <span class="identifier">file</span> <span class="identifier">io</span><span class="special">.</span><span class="identifier">cpp</span> <span class="special">*/</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">io</span><span class="special">)</span>
<span class="special">{</span>
<span class="special">/*</span> <span class="identifier">export</span> <span class="identifier">everything</span> <span class="keyword">in</span> <span class="identifier">the</span> <span class="identifier">sounds</span><span class="special">::</span><span class="identifier">io</span> <span class="identifier">namespace</span> <span class="special">*/</span>
<span class="special">...</span>
<span class="special">}</span>
<span class="special">/*</span> <span class="identifier">file</span> <span class="identifier">filters</span><span class="special">.</span><span class="identifier">cpp</span> <span class="special">*/</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">filters</span><span class="special">)</span>
<span class="special">{</span>
<span class="special">/*</span> <span class="identifier">export</span> <span class="identifier">everything</span> <span class="keyword">in</span> <span class="identifier">the</span> <span class="identifier">sounds</span><span class="special">::</span><span class="identifier">filters</span> <span class="identifier">namespace</span> <span class="special">*/</span>
<span class="special">...</span>
<span class="special">}</span>
</pre>
<p>
Compiling these files will generate the following Python extensions: <code class="literal">core.pyd</code>,
<code class="literal">io.pyd</code> and <code class="literal">filters.pyd</code>.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> The extension <code class="literal">.pyd</code> is used for python
extension modules, which are just shared libraries. Using the default for
your system, like <code class="literal">.so</code> for Unix and <code class="literal">.dll</code>
for Windows, works just as well.
</p>
</div>
<p>
Now, we create this directory structure for our Python package:
</p>
<pre class="programlisting">sounds/
__init__.py
core.pyd
filters.pyd
io.pyd
</pre>
<p>
The file <code class="literal">__init__.py</code> is what tells Python that the directory
<code class="literal">sounds/</code> is actually a Python package. It can be a empty
file, but can also perform some magic, that will be shown later.
</p>
<p>
Now our package is ready. All the user has to do is put <code class="literal">sounds</code>
into his <a href="http://www.python.org/doc/current/tut/node8.html#SECTION008110000000000000000" target="_top">PYTHONPATH</a>
and fire up the interpreter:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">io</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">sound</span> <span class="special">=</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">io</span><span class="special">.</span><span class="identifier">open</span><span class="special">(</span><span class="string">'file.mp3'</span><span class="special">)</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">new_sound</span> <span class="special">=</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span><span class="special">.</span><span class="identifier">echo</span><span class="special">(</span><span class="identifier">sound</span><span class="special">,</span> <span class="number">1.0</span><span class="special">)</span>
</pre>
<p>
Nice heh?
</p>
<p>
This is the simplest way to create hierarchies of packages, but it is not
very flexible. What if we want to add a <span class="emphasis"><em>pure</em></span> Python
function to the filters package, for instance, one that applies 3 filters
in a sound object at once? Sure, you can do this in C++ and export it, but
why not do so in Python? You don't have to recompile the extension modules,
plus it will be easier to write it.
</p>
<p>
If we want this flexibility, we will have to complicate our package hierarchy
a little. First, we will have to change the name of the extension modules:
</p>
<p>
</p>
<pre class="programlisting">
<span class="comment">/* file core.cpp */</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">_core</span><span class="special">)</span>
<span class="special">{</span>
<span class="special">...</span>
<span class="comment">/* export everything in the sounds::core namespace */</span>
<span class="special">}</span>
</pre>
<p>
Note that we added an underscore to the module name. The filename will have
to be changed to <code class="literal">_core.pyd</code> as well, and we do the same
to the other extension modules. Now, we change our package hierarchy like
so:
</p>
<pre class="programlisting">sounds/
__init__.py
core/
__init__.py
<span class="underline">core.pyd
filters/
\</span>_init__.py
<span class="underline">filters.pyd
io/
\</span>_init__.py
_io.pyd
</pre>
<p>
Note that we created a directory for each extension module, and added a __init__.py
to each one. But if we leave it that way, the user will have to access the
functions in the core module with this syntax:
</p>
<p>
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">_core</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">core</span><span class="special">.</span><span class="identifier">_core</span><span class="special">.</span><span class="identifier">foo</span><span class="special">(...)</span>
</pre>
<p>
which is not what we want. But here enters the <code class="literal">__init__.py</code>
magic: everything that is brought to the <code class="literal">__init__.py</code> namespace
can be accessed directly by the user. So, all we have to do is bring the
entire namespace from <code class="literal">_core.pyd</code> to <code class="literal">core/__init__.py</code>.
So add this line of code to <code class="literal">sounds<span class="emphasis"><em>core</em></span>__init__.py</code>:
</p>
<pre class="programlisting">
<span class="keyword">from</span> <span class="identifier">_core</span> <span class="keyword">import</span> <span class="special">*</span>
</pre>
<p>
We do the same for the other packages. Now the user accesses the functions
and classes in the extension modules like before:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span><span class="special">.</span><span class="identifier">echo</span><span class="special">(...)</span>
</pre>
<p>
with the additional benefit that we can easily add pure Python functions
to any module, in a way that the user can't tell the difference between a
C++ function and a Python function. Let's add a <span class="emphasis"><em>pure</em></span>
Python function, <code class="literal">echo_noise</code>, to the <code class="literal">filters</code>
package. This function applies both the <code class="literal">echo</code> and <code class="literal">noise</code>
filters in sequence in the given <code class="literal">sound</code> object. We create
a file named <code class="literal">sounds/filters/echo_noise.py</code> and code our
function:
</p>
<pre class="programlisting">
<span class="keyword">import</span> <span class="identifier">_filters</span>
<span class="keyword">def</span> <span class="identifier">echo_noise</span><span class="special">(</span><span class="identifier">sound</span><span class="special">):</span>
<span class="identifier">s</span> <span class="special">=</span> <span class="identifier">_filters</span><span class="special">.</span><span class="identifier">echo</span><span class="special">(</span><span class="identifier">sound</span><span class="special">)</span>
<span class="identifier">s</span> <span class="special">=</span> <span class="identifier">_filters</span><span class="special">.</span><span class="identifier">noise</span><span class="special">(</span><span class="identifier">sound</span><span class="special">)</span>
<span class="keyword">return</span> <span class="identifier">s</span>
</pre>
<p>
Next, we add this line to <code class="literal">sounds<span class="emphasis"><em>filters</em></span>__init__.py</code>:
</p>
<pre class="programlisting">
<span class="keyword">from</span> <span class="identifier">echo_noise</span> <span class="keyword">import</span> <span class="identifier">echo_noise</span>
</pre>
<p>
And that's it. The user now accesses this function like any other function
from the <code class="literal">filters</code> package:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">import</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">sounds</span><span class="special">.</span><span class="identifier">filters</span><span class="special">.</span><span class="identifier">echo_noise</span><span class="special">(...)</span>
</pre>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.extending_wrapped_objects_in_python"></a>Extending Wrapped Objects in Python</h3></div></div></div>
<p>
Thanks to Python's flexibility, you can easily add new methods to a class,
even after it was already created:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">class</span> <span class="identifier">C</span><span class="special">(</span><span class="identifier">object</span><span class="special">):</span> <span class="keyword">pass</span>
<span class="special">&gt;&gt;&gt;</span>
<span class="special">&gt;&gt;&gt;</span> <span class="comment"># a regular function
</span><span class="special">&gt;&gt;&gt;</span> <span class="keyword">def</span> <span class="identifier">C_str</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span> <span class="keyword">return</span> <span class="string">'A C instance!'</span>
<span class="special">&gt;&gt;&gt;</span>
<span class="special">&gt;&gt;&gt;</span> <span class="comment"># now we turn it in a member function
</span><span class="special">&gt;&gt;&gt;</span> <span class="identifier">C</span><span class="special">.</span><span class="identifier">__str__</span> <span class="special">=</span> <span class="identifier">C_str</span>
<span class="special">&gt;&gt;&gt;</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">C</span><span class="special">()</span>
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">print</span> <span class="identifier">c</span>
<span class="identifier">A</span> <span class="identifier">C</span> <span class="identifier">instance</span><span class="special">!</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">C_str</span><span class="special">(</span><span class="identifier">c</span><span class="special">)</span>
<span class="identifier">A</span> <span class="identifier">C</span> <span class="identifier">instance</span><span class="special">!</span>
</pre>
<p>
Yes, Python rox. <span class="inlinemediaobject"><img src="../images/smiley.png" alt="smiley"></span>
</p>
<p>
We can do the same with classes that were wrapped with Boost.Python. Suppose
we have a class <code class="literal">point</code> in C++:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">class</span> <span class="identifier">point</span> <span class="special">{...};</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">_geom</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">&gt;(</span><span class="string">"point"</span><span class="special">)...;</span>
<span class="special">}</span>
</pre>
<p>
If we are using the technique from the previous session, <a href="techniques.html#python.creating_packages" title="Creating Packages">Creating
Packages</a>, we can code directly into <code class="literal">geom/__init__.py</code>:
</p>
<p>
</p>
<pre class="programlisting">
<span class="keyword">from</span> <span class="identifier">_geom</span> <span class="keyword">import</span> <span class="special">*</span>
<span class="comment"># a regular function
</span><span class="keyword">def</span> <span class="identifier">point_str</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="keyword">return</span> <span class="identifier">str</span><span class="special">((</span><span class="identifier">self</span><span class="special">.</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">self</span><span class="special">.</span><span class="identifier">y</span><span class="special">))</span>
<span class="comment"># now we turn it into a member function
</span><span class="identifier">point</span><span class="special">.</span><span class="identifier">__str__</span> <span class="special">=</span> <span class="identifier">point_str</span>
</pre>
<p>
<span class="bold"><strong>All</strong></span> point instances created from C++ will
also have this member function! This technique has several advantages:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
Cut down compile times to zero for these additional functions
</li>
<li>
Reduce the memory footprint to virtually zero
</li>
<li>
Minimize the need to recompile
</li>
<li>
Rapid prototyping (you can move the code to C++ if required without changing
the interface)
</li>
</ul></div>
<p>
You can even add a little syntactic sugar with the use of metaclasses. Let's
create a special metaclass that "injects" methods in other classes.
</p>
<pre class="programlisting">
<span class="comment"># The one Boost.Python uses for all wrapped classes.
</span><span class="comment"># You can use here any class exported by Boost instead of "point"
</span><span class="identifier">BoostPythonMetaclass</span> <span class="special">=</span> <span class="identifier">point</span><span class="special">.</span><span class="identifier">__class__</span>
<span class="keyword">class</span> <span class="identifier">injector</span><span class="special">(</span><span class="identifier">object</span><span class="special">):</span>
<span class="keyword">class</span> <span class="identifier">__metaclass__</span><span class="special">(</span><span class="identifier">BoostPythonMetaclass</span><span class="special">):</span>
<span class="keyword">def</span> <span class="identifier">__init__</span><span class="special">(</span><span class="identifier">self</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="identifier">bases</span><span class="special">,</span> <span class="identifier">dict</span><span class="special">):</span>
<span class="keyword">for</span> <span class="identifier">b</span> <span class="keyword">in</span> <span class="identifier">bases</span><span class="special">:</span>
<span class="keyword">if</span> <span class="identifier">type</span><span class="special">(</span><span class="identifier">b</span><span class="special">)</span> <span class="keyword">not</span> <span class="keyword">in</span> <span class="special">(</span><span class="identifier">self</span><span class="special">,</span> <span class="identifier">type</span><span class="special">):</span>
<span class="keyword">for</span> <span class="identifier">k</span><span class="special">,</span><span class="identifier">v</span> <span class="keyword">in</span> <span class="identifier">dict</span><span class="special">.</span><span class="identifier">items</span><span class="special">():</span>
<span class="identifier">setattr</span><span class="special">(</span><span class="identifier">b</span><span class="special">,</span><span class="identifier">k</span><span class="special">,</span><span class="identifier">v</span><span class="special">)</span>
<span class="keyword">return</span> <span class="identifier">type</span><span class="special">.</span><span class="identifier">__init__</span><span class="special">(</span><span class="identifier">self</span><span class="special">,</span> <span class="identifier">name</span><span class="special">,</span> <span class="identifier">bases</span><span class="special">,</span> <span class="identifier">dict</span><span class="special">)</span>
<span class="comment"># inject some methods in the point foo
</span><span class="keyword">class</span> <span class="identifier">more_point</span><span class="special">(</span><span class="identifier">injector</span><span class="special">,</span> <span class="identifier">point</span><span class="special">):</span>
<span class="keyword">def</span> <span class="identifier">__repr__</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="keyword">return</span> <span class="string">'Point(x=%s, y=%s)'</span> <span class="special">%</span> <span class="special">(</span><span class="identifier">self</span><span class="special">.</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">self</span><span class="special">.</span><span class="identifier">y</span><span class="special">)</span>
<span class="keyword">def</span> <span class="identifier">foo</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="keyword">print</span> <span class="string">'foo!'</span>
</pre>
<p>
Now let's see how it got:
</p>
<pre class="programlisting">
<span class="special">&gt;&gt;&gt;</span> <span class="keyword">print</span> <span class="identifier">point</span><span class="special">()</span>
<span class="identifier">Point</span><span class="special">(</span><span class="identifier">x</span><span class="special">=</span><span class="number">10</span><span class="special">,</span> <span class="identifier">y</span><span class="special">=</span><span class="number">10</span><span class="special">)</span>
<span class="special">&gt;&gt;&gt;</span> <span class="identifier">point</span><span class="special">().</span><span class="identifier">foo</span><span class="special">()</span>
<span class="identifier">foo</span><span class="special">!</span>
</pre>
<p>
Another useful idea is to replace constructors with factory functions:
</p>
<pre class="programlisting">
<span class="identifier">_point</span> <span class="special">=</span> <span class="identifier">point</span>
<span class="keyword">def</span> <span class="identifier">point</span><span class="special">(</span><span class="identifier">x</span><span class="special">=</span><span class="number">0</span><span class="special">,</span> <span class="identifier">y</span><span class="special">=</span><span class="number">0</span><span class="special">):</span>
<span class="keyword">return</span> <span class="identifier">_point</span><span class="special">(</span><span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">)</span>
</pre>
<p>
In this simple case there is not much gained, but for constructurs with many
overloads and/or arguments this is often a great simplification, again with
virtually zero memory footprint and zero compile-time overhead for the keyword
support.
</p>
</div>
<div class="section" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="python.reducing_compiling_time"></a>Reducing Compiling Time</h3></div></div></div>
<p>
If you have ever exported a lot of classes, you know that it takes quite
a good time to compile the Boost.Python wrappers. Plus the memory consumption
can easily become too high. If this is causing you problems, you can split
the class_ definitions in multiple files:
</p>
<p>
</p>
<pre class="programlisting">
<span class="comment">/* file point.cpp */</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">point</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">export_point</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">&gt;(</span><span class="string">"point"</span><span class="special">)...;</span>
<span class="special">}</span>
<span class="comment">/* file triangle.cpp */</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">triangle</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="keyword">void</span> <span class="identifier">export_triangle</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">triangle</span><span class="special">&gt;(</span><span class="string">"triangle"</span><span class="special">)...;</span>
<span class="special">}</span>
</pre>
<p>
Now you create a file <code class="literal">main.cpp</code>, which contains the <code class="literal">BOOST_PYTHON_MODULE</code>
macro, and call the various export functions inside it.
</p>
<pre class="programlisting">
<span class="keyword">void</span> <span class="identifier">export_point</span><span class="special">();</span>
<span class="keyword">void</span> <span class="identifier">export_triangle</span><span class="special">();</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">_geom</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">export_point</span><span class="special">();</span>
<span class="identifier">export_triangle</span><span class="special">();</span>
<span class="special">}</span>
</pre>
<p>
Compiling and linking together all this files produces the same result as
the usual approach:
</p>
<pre class="programlisting">
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">point</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">triangle</span><span class="special">.</span><span class="identifier">h</span><span class="special">&gt;</span>
<span class="identifier">BOOST_PYTHON_MODULE</span><span class="special">(</span><span class="identifier">_geom</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">point</span><span class="special">&gt;(</span><span class="string">"point"</span><span class="special">)...;</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">triangle</span><span class="special">&gt;(</span><span class="string">"triangle"</span><span class="special">)...;</span>
<span class="special">}</span>
</pre>
<p>
but the memory is kept under control.
</p>
<p>
This method is recommended too if you are developing the C++ library and
exporting it to Python at the same time: changes in a class will only demand
the compilation of a single cpp, instead of the entire wrapper code.
</p>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> If you're exporting your classes with <a href="../../../../../pyste/index.html" target="_top">Pyste</a>,
take a look at the <code class="literal">--multiple</code> option, that generates the
wrappers in various files as demonstrated here.
</p>
</div>
<div class="sidebar">
<p class="title"><b></b></p>
<p>
<span class="inlinemediaobject"><img src="../images/note.png" alt="note"></span> This method is useful too if you are getting the error
message <span class="emphasis"><em>"fatal error C1204:Compiler limit:internal structure
overflow"</em></span> when compiling a large source file, as explained
in the <a href="../../../../v2/faq.html#c1204" target="_top">FAQ</a>.
</p>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Inheritance</b></font>
</td>
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<p>
In the previous examples, we dealt with classes that are not polymorphic.
This is not often the case. Much of the time, we will be wrapping
polymorphic classes and class hierarchies related by inheritance. We will
often have to write Boost.Python wrappers for classes that are derived from
abstract base classes.</p>
<p>
Consider this trivial inheritance structure:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Base </span><span class=special>{ </span><span class=keyword>virtual </span><span class=special>~</span><span class=identifier>Base</span><span class=special>(); </span><span class=special>};
</span><span class=keyword>struct </span><span class=identifier>Derived </span><span class=special>: </span><span class=identifier>Base </span><span class=special>{};
</span></pre></code>
<p>
And a set of C++ functions operating on <tt>Base</tt> and <tt>Derived</tt> object
instances:</p>
<code><pre>
<span class=keyword>void </span><span class=identifier>b</span><span class=special>(</span><span class=identifier>Base</span><span class=special>*);
</span><span class=keyword>void </span><span class=identifier>d</span><span class=special>(</span><span class=identifier>Derived</span><span class=special>*);
</span><span class=identifier>Base</span><span class=special>* </span><span class=identifier>factory</span><span class=special>() </span><span class=special>{ </span><span class=keyword>return </span><span class=keyword>new </span><span class=identifier>Derived</span><span class=special>; </span><span class=special>}
</span></pre></code>
<p>
We've seen how we can wrap the base class <tt>Base</tt>:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Base</span><span class=special>&gt;(</span><span class=string>&quot;Base&quot;</span><span class=special>)
</span><span class=comment>/*...*/
</span><span class=special>;
</span></pre></code>
<p>
Now we can inform Boost.Python of the inheritance relationship between
<tt>Derived</tt> and its base class <tt>Base</tt>. Thus:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Derived</span><span class=special>, </span><span class=identifier>bases</span><span class=special>&lt;</span><span class=identifier>Base</span><span class=special>&gt; </span><span class=special>&gt;(</span><span class=string>&quot;Derived&quot;</span><span class=special>)
</span><span class=comment>/*...*/
</span><span class=special>;
</span></pre></code>
<p>
Doing so, we get some things for free:</p>
<ol><li>Derived automatically inherits all of Base's Python methods (wrapped C++ member functions)</li><li><b>If</b> Base is polymorphic, <tt>Derived</tt> objects which have been passed to Python via a pointer or reference to <tt>Base</tt> can be passed where a pointer or reference to <tt>Derived</tt> is expected.</li></ol><p>
Now, we shall expose the C++ free functions <tt>b</tt> and <tt>d</tt> and <tt>factory</tt>:</p>
<code><pre>
<span class=identifier>def</span><span class=special>(</span><span class=string>&quot;b&quot;</span><span class=special>, </span><span class=identifier>b</span><span class=special>);
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;d&quot;</span><span class=special>, </span><span class=identifier>d</span><span class=special>);
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;factory&quot;</span><span class=special>, </span><span class=identifier>factory</span><span class=special>);
</span></pre></code>
<p>
Note that free function <tt>factory</tt> is being used to generate new
instances of class <tt>Derived</tt>. In such cases, we use
<tt>return_value_policy&lt;manage_new_object&gt;</tt> to instruct Python to adopt
the pointer to <tt>Base</tt> and hold the instance in a new Python <tt>Base</tt>
object until the the Python object is destroyed. We shall see more of
Boost.Python <a href="call_policies.html">
call policies</a> later.</p>
<code><pre>
<span class=comment>// Tell Python to take ownership of factory's result
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;factory&quot;</span><span class=special>, </span><span class=identifier>factory</span><span class=special>,
</span><span class=identifier>return_value_policy</span><span class=special>&lt;</span><span class=identifier>manage_new_object</span><span class=special>&gt;());
</span></pre></code>
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<p>
In C++, and STL in particular, we see iterators everywhere. Python also has
iterators, but these are two very different beasts.</p>
<p>
<b>C++ iterators:</b></p>
<ul><li>C++ has 5 type categories (random-access, bidirectional, forward, input, output)</li><li>There are 2 Operation categories: reposition, access</li><li>A pair of iterators is needed to represent a (first/last) range.</li></ul><p>
<b>Python Iterators:</b></p>
<ul><li>1 category (forward)</li><li>1 operation category (next())</li><li>Raises StopIteration exception at end</li></ul><p>
The typical Python iteration protocol: <tt><b>for y in x...</b></tt> is as follows:</p>
<code><pre>
<span class=identifier>iter </span><span class=special>= </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>__iter__</span><span class=special>() </span>#<span class=identifier>get </span><span class=identifier>iterator
</span><span class=keyword>try</span><span class=special>:
</span><span class=keyword>while </span><span class=number>1</span><span class=special>:
</span><span class=identifier>y </span><span class=special>= </span><span class=identifier>iter</span><span class=special>.</span><span class=identifier>next</span><span class=special>() </span>#<span class=identifier>get </span><span class=identifier>each </span><span class=identifier>item
</span><span class=special>... </span>#<span class=identifier>process </span><span class=identifier>y
</span><span class=identifier>except </span><span class=identifier>StopIteration</span><span class=special>: </span><span class=identifier>pass </span>#<span class=identifier>iterator </span><span class=identifier>exhausted
</span></pre></code>
<p>
Boost.Python provides some mechanisms to make C++ iterators play along
nicely as Python iterators. What we need to do is to produce
appropriate __iter__ function from C++ iterators that is compatible
with the Python iteration protocol. For example:</p>
<code><pre>
<span class=identifier>object </span><span class=identifier>get_iterator </span><span class=special>= </span><span class=identifier>iterator</span><span class=special>&lt;</span><span class=identifier>vector</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt; </span><span class=special>&gt;();
</span><span class=identifier>object </span><span class=identifier>iter </span><span class=special>= </span><span class=identifier>get_iterator</span><span class=special>(</span><span class=identifier>v</span><span class=special>);
</span><span class=identifier>object </span><span class=identifier>first </span><span class=special>= </span><span class=identifier>iter</span><span class=special>.</span><span class=identifier>next</span><span class=special>();
</span></pre></code>
<p>
Or for use in class_&lt;&gt;:</p>
<code><pre>
<span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;__iter__&quot;</span><span class=special>, </span><span class=identifier>iterator</span><span class=special>&lt;</span><span class=identifier>vector</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt; </span><span class=special>&gt;())
</span></pre></code>
<p>
<b>range</b></p>
<p>
We can create a Python savvy iterator using the range function:</p>
<ul><li>range(start, finish)</li><li>range&lt;Policies,Target&gt;(start, finish)</li></ul><p>
Here, start/finish may be one of:</p>
<ul><li>member data pointers</li><li>member function pointers</li><li>adaptable function object (use Target parameter)</li></ul><p>
<b>iterator</b></p>
<ul><li>iterator&lt;T, Policies&gt;()</li></ul><p>
Given a container <tt>T</tt>, iterator is a shortcut that simply calls <tt>range</tt>
with &amp;T::begin, &amp;T::end.</p>
<p>
Let's put this into action... Here's an example from some hypothetical
bogon Particle accelerator code:</p>
<code><pre>
<span class=identifier>f </span><span class=special>= </span><span class=identifier>Field</span><span class=special>()
</span><span class=keyword>for </span><span class=identifier>x </span><span class=identifier>in </span><span class=identifier>f</span><span class=special>.</span><span class=identifier>pions</span><span class=special>:
</span><span class=identifier>smash</span><span class=special>(</span><span class=identifier>x</span><span class=special>)
</span><span class=keyword>for </span><span class=identifier>y </span><span class=identifier>in </span><span class=identifier>f</span><span class=special>.</span><span class=identifier>bogons</span><span class=special>:
</span><span class=identifier>count</span><span class=special>(</span><span class=identifier>y</span><span class=special>)
</span></pre></code>
<p>
Now, our C++ Wrapper:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>F</span><span class=special>&gt;(</span><span class=string>&quot;Field&quot;</span><span class=special>)
</span><span class=special>.</span><span class=identifier>property</span><span class=special>(</span><span class=string>&quot;pions&quot;</span><span class=special>, </span><span class=identifier>range</span><span class=special>(&amp;</span><span class=identifier>F</span><span class=special>::</span><span class=identifier>p_begin</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>F</span><span class=special>::</span><span class=identifier>p_end</span><span class=special>))
</span><span class=special>.</span><span class=identifier>property</span><span class=special>(</span><span class=string>&quot;bogons&quot;</span><span class=special>, </span><span class=identifier>range</span><span class=special>(&amp;</span><span class=identifier>F</span><span class=special>::</span><span class=identifier>b_begin</span><span class=special>, </span><span class=special>&amp;</span><span class=identifier>F</span><span class=special>::</span><span class=identifier>b_end</span><span class=special>));
</span></pre></code>
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<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
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<p>
Python is dynamically typed, unlike C++ which is statically typed. Python
variables may hold an integer, a float, list, dict, tuple, str, long etc.,
among other things. In the viewpoint of Boost.Python and C++, these
Pythonic variables are just instances of class <tt>object</tt>. We shall see in
this chapter how to deal with Python objects.</p>
<p>
As mentioned, one of the goals of Boost.Python is to provide a
bidirectional mapping between C++ and Python while maintaining the Python
feel. Boost.Python C++ <tt>object</tt>s are as close as possible to Python. This
should minimize the learning curve significantly.</p>
<p>
<img src="theme/python.png"></img></p>
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<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
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<head>
<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>QuickStart</b></font>
</td>
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<br>
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<p>
The Boost Python Library is a framework for interfacing Python and
C++. It allows you to quickly and seamlessly expose C++ classes
functions and objects to Python, and vice-versa, using no special
tools -- just your C++ compiler. It is designed to wrap C++ interfaces
non-intrusively, so that you should not have to change the C++ code at
all in order to wrap it, making Boost.Python ideal for exposing
3rd-party libraries to Python. The library's use of advanced
metaprogramming techniques simplifies its syntax for users, so that
wrapping code takes on the look of a kind of declarative interface
definition language (IDL).</p>
<a name="hello_world"></a><h2>Hello World</h2><p>
Following C/C++ tradition, let's start with the &quot;hello, world&quot;. A C++
Function:</p>
<code><pre>
<span class=keyword>char </span><span class=keyword>const</span><span class=special>* </span><span class=identifier>greet</span><span class=special>()
</span><span class=special>{
</span><span class=keyword>return </span><span class=string>&quot;hello, world&quot;</span><span class=special>;
</span><span class=special>}
</span></pre></code>
<p>
can be exposed to Python by writing a Boost.Python wrapper:</p>
<code><pre>
<span class=preprocessor>#include </span><span class=special>&lt;</span><span class=identifier>boost</span><span class=special>/</span><span class=identifier>python</span><span class=special>.</span><span class=identifier>hpp</span><span class=special>&gt;
</span><span class=keyword>using </span><span class=keyword>namespace </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>python</span><span class=special>;
</span><span class=identifier>BOOST_PYTHON_MODULE</span><span class=special>(</span><span class=identifier>hello</span><span class=special>)
</span><span class=special>{
</span><span class=identifier>def</span><span class=special>(</span><span class=string>&quot;greet&quot;</span><span class=special>, </span><span class=identifier>greet</span><span class=special>);
</span><span class=special>}
</span></pre></code>
<p>
That's it. We're done. We can now build this as a shared library. The
resulting DLL is now visible to Python. Here's a sample Python session:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>import </span><span class=identifier>hello
</span><span class=special>&gt;&gt;&gt; </span><span class=identifier>print </span><span class=identifier>hello</span><span class=special>.</span><span class=identifier>greet</span><span class=special>()
</span><span class=identifier>hello</span><span class=special>, </span><span class=identifier>world
</span></pre></code>
<blockquote><p><i><b>Next stop... Building your Hello World module from start to finish...</b></i></p></blockquote><table border="0">
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
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/*=============================================================================
Copyright (c) 2003 Bruno da Silva de Oliveira
Use, modification and distribution is subject to the Boost Software
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)
=============================================================================*/
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<p>© Copyright Beman Dawes, 2001</p>
<p>Distributed under the Boost Software License, Version 1.0. (See
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002 David Abrahams<br>Copyright &copy; 2002 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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@@ -13,10 +10,10 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">April 2002 Progress Report</h2>
</td>
</tr>
@@ -48,7 +45,7 @@ of work got done...
<h3><a name="arity">Arbitrary Arity Support</a></h3>
I began using the <a
href="../../../preprocessor/doc/index.html">Boost.Preprocessor</a>
href="../../../preprocessor/doc/index.htm">Boost.Preprocessor</a>
metaprogramming library to generate support for functions and member
functions of arbitrary arity, which was, to say the least, quite an
adventure. The feedback cycle resulting from my foray into
@@ -157,10 +154,10 @@ documentation).
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
3 May, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. </i></p>
2002. All Rights Reserved.</i></p>
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<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">CallPolicies Concept</h2>
</td>
@@ -116,7 +113,7 @@
"ResultConverter.html#ResultConverterGenerator-concept">ResultConverterGenerator</a>.</td>
<td>An MPL unary <a href=
"../../../mpl/doc/refmanual/metafunction-class.html">Metafunction
"../../../mpl/doc/paper/html/usage.html#metafunctions.classes">Metafunction
Class</a> used produce the "preliminary" result object.</td>
</tr>
@@ -139,12 +136,12 @@
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
19 May, 2002 <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
Reserved.</i></p>
<p>Permission to copy, use, modify, sell and distribute this software is
granted provided this copyright notice appears in all copies. This

View File

@@ -1,10 +1,7 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href=../../../../boost.css>
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<title>Boost.Python - Dereferenceable Concept</title>
</head>
<body link="#0000ff" vlink="#800080">
@@ -12,10 +9,10 @@
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">Dereferenceable Concept</h2>
</td>
</tr>
@@ -31,40 +28,35 @@
<h2><a name="introduction"></a>Introduction</h2>
<p>Instances of a Dereferenceable type can be used like a pointer to access an lvalue.
<p>Instances of a dereferenceable type can be used like a pointer to access an lvalue.
<h2><a name="concept-requirements"></a>Concept Requirements</h2>
<h3><a name="Dereferenceable-concept"></a>Dereferenceable Concept</h3>
<p>In the table below, <code><b>T</b></code> is a model of
Dereferenceable, and <code><b>x</b></code> denotes an object of
type <code>T</code>. In addition, all pointers are Dereferenceable.
<p>In the table below, <code><b>x</b></code> denotes an object whose
type is a model of Dereferenceable.
<table summary="Dereferenceable expressions" border="1" cellpadding="5">
<tr>
<td><b>Expression</b></td>
<td><b>Result</b></td>
<td><b>Operational Semantics</b></td>
<td><b>Requirements</b></td>
</tr>
<tr>
<td><code>get_pointer(x)</code></td>
<td>convertible to <code><a href="pointee.html#pointee-spec">pointee</a>&lt;T&gt;::type*</code>
<td><code>&amp;*x</code>, or a null pointer
<td valign="top"><code>*x</code></td>
<td>An lvalue
</tr>
<tr>
</table>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
18 December, 2003
10 May, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002-2003. </i>
Abrahams</a> 2002. All Rights Reserved.</i>
<p>Permission to copy, use, modify, sell
and distribute this software is granted provided this copyright notice appears

View File

@@ -1,10 +1,7 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href=../../../../boost.css>
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<title>Boost.Python - Extractor Concept</title>
</head>
<body link="#0000ff" vlink="#800080">
@@ -12,10 +9,10 @@
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">Extractor Concept</h2>
</td>
</tr>
@@ -82,11 +79,11 @@ are layout-compatible with PyObject.
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
22 May, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. </i>
Abrahams</a> 2002. All Rights Reserved.</i>
<p>Permission to copy, use, modify, sell
and distribute this software is granted provided this copyright notice appears

View File

@@ -1,10 +1,7 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href=../../../../boost.css>
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<title>Boost.Python - Holder Concept</title>
</head>
<body link="#0000ff" vlink="#800080">
@@ -12,10 +9,10 @@
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">HolderGenerator Concept</h2>
</td>
</tr>
@@ -60,11 +57,11 @@ type.
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
20 May, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. </i>
Abrahams</a> 2002. All Rights Reserved.</i>
<p>Permission to copy, use, modify, sell
and distribute this software is granted provided this copyright notice appears

View File

@@ -1,6 +1,3 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
@@ -13,10 +10,10 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">June 2002 Progress Report</h2>
</td>
</tr>
@@ -220,10 +217,10 @@ you'll just have to wait till next month (hopefully the beginning).
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
19 July, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. </i></p>
2002. All Rights Reserved.</i></p>
</body>
</html>

View File

@@ -1,6 +1,3 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
@@ -13,10 +10,10 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">March 2002 Progress Report</h2>
</td>
</tr>
@@ -228,10 +225,10 @@ worth doing anything about it.
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
1 April, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. </i></p>
2002. All Rights Reserved.</i></p>
</body>
</html>

View File

@@ -1,6 +1,3 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
@@ -13,10 +10,10 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">May 2002 Progress Report</h2>
</td>
</tr>
@@ -57,7 +54,7 @@ focused on reducing the support burden. In recent weeks, responding to
requests for support, espcially surrounding building the library, had
begun to impede progress on development. There was a major push to
release a stable 1.28.0 of Boost, including documentation of <a
href="../../../../tools/build/v1/build_system.htm">Boost.Build</a> and specific
href="../../../../tools/build/index.html">Boost.Build</a> and specific
<a href="../building.html">instructions</a> for building Boost.Python
v1. The documentation for Boost.Python v2 was also updated as
described <a href="#documentation">here</a>.
@@ -73,7 +70,7 @@ described <a href="#documentation">here</a>.
Martin Casado which uncovered the key mechanism required to allow
shared libraries to use functions from the Python executable. The
current solution used in Boost.Build relies on a <a
href="../../../../tools/build/v1/gen_aix_import_file.py">Python
href="../../../../tools/build/gen_aix_import_file.py">Python
Script</a> as part of the build process. This is not a problem for
Boost.Python, as Python will be available. However, the commands
issued by the script are so simple that a 100%-pure-Boost.Jam
@@ -87,7 +84,8 @@ described <a href="#documentation">here</a>.
Support for exposing C++ operators and functions as the corresponding
Python special methods was added. Thinking that the Boost.Python
v1 interface was a little too esoteric (especially the use of
<a href="../special.html#numeric">v1 interface</a> was a little too
esoteric (especially the use of
<code>left_operand&lt;...&gt;/right_operand&lt;...&gt;</code> for
asymmetric operands), I introduced a simple form of <a
href="http://osl.iu.edu/~tveldhui/papers/Expression-Templates/exprtmpl.html">expression
@@ -157,7 +155,7 @@ This forced the exposure of the <a
href="http://www.python.org/2.2/descrintro.html#property"><code>property</code></a>
interface used internally to implement the data member exposure
facility described in <a
href="Mar2002.html#data_members">March</a>. Properties are an
href="Mar2002#data_members">March</a>. Properties are an
incredibly useful idiom, so it's good to be able to provide them
at little new development cost.
@@ -214,7 +212,7 @@ Major updates were made to the following pages:
<blockquote>
<dl>
<dt><a href="call.html">call.html</a><dd> <dt>updated<dd>
<dt><a href="call.html">call.html</a><dd> <dt><a href="updated">updated</a><dd>
<dt><a href="class.html">class.html</a><dd>
<dt><a href="reference.html">reference.html</a><dd>
</dl>
@@ -302,10 +300,10 @@ to these issues will probably have to be formalized before long.
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
11 June, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. </i></p>
2002. All Rights Reserved.</i></p>
</body>
</html>

View File

@@ -1,8 +1,5 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta name="generator" content=
@@ -19,11 +16,11 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">ObjectWrapper and TypeWrapper Concepts</h2>
</td>
@@ -142,12 +139,17 @@ instances of the associated Python type will be considered a match.
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
13 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
30 Sept, 2002 <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
Reserved.</i></p>
<p>Permission to copy, use, modify, sell and distribute this software is
granted provided this copyright notice appears in all copies. This
software is provided "as is" without express or implied warranty, and
with no claim as to its suitability for any purpose.</p>
</body>
</html>

View File

@@ -1,10 +1,7 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href=../../../../boost.css>
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<title>Boost.Python - ResultConverter Concept</title>
</head>
<body link="#0000ff" vlink="#800080">
@@ -12,10 +9,10 @@
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
<h3><a href="../../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">ResultConverter Concept</h2>
</td>
</tr>
@@ -103,7 +100,7 @@ C++ function return type.
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. </i>
Abrahams</a> 2002. All Rights Reserved.</i>
<p>Permission to copy, use, modify, sell
and distribute this software is granted provided this copyright notice appears

View File

@@ -1,8 +1,5 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta name="generator" content=
@@ -19,11 +16,11 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">Acknowledgments</h2>
</td>
@@ -34,15 +31,6 @@
<p><a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a> is
the architect, designer, and implementor of <b>Boost.Python</b>.</p>
<p><a href="mailto:brett.calcott@paradise.net.nz">Brett Calcott</a>
contributed and maintains the Visual Studio project files and
documentation.</p>
<p><a href="mailto:Gottfried.Ganssauge-at-haufe.de">Gottfried
Ganßauge</a> supplied support for opaque pointer conversions,
complete with documentation and a regression test (and I didn't
even have to ask him for those)!
<p>Joel de Guzman implemented the <a href="overloads.html">default
argument support</a> and wrote the excellent <a href=
"../tutorial/index.html">tutorial documentation</a>.</p>
@@ -75,17 +63,6 @@
use the new preproccessor metaprogramming constructs and helping us to
work around buggy and slow C++ preprocessors.</p>
<p><a href="mailto:nicodemus-at-globalite.com.br">Bruno da Silva de
Oliveira</a> contributed the ingenious <a
href="../../pyste/index.html">Pyste</a> (&quot;Pie-Steh&quot;)
code generator.
<p><a href="mailto:nickm@sitius.com">Nikolay Mladenov</a> contributed
<code>staticmethod</code> support.</p>
<p>Martin Casado solved some sticky problems which allow us to build the
Boost.Python shared library for AIX's crazy dynamic linking model.</p>
<p><a href="mailto:achim@procoders.net">Achim Domma</a> contributed some
of the <a href="reference.html#object_wrappers">Object Wrappers</a> and
HTML templates for this documentation. Dave Hawkes contributed
@@ -94,6 +71,9 @@
definition syntax. Pearu Pearson wrote some of the test cases that are in
the current test suite.</p>
<p>Martin Casado solved some sticky problems which allow us to build the
Boost.Python shared library for AIX's crazy dynamic linking model.</p>
<p>The development of this version of Boost.Python was funded in part by
the <a href="http://www.llnl.gov/">Lawrence Livermore National
Laboratories</a> and by the <a href="http://cci.lbl.gov/">Computational
@@ -101,21 +81,8 @@
Laboratories.</p>
<p><a href="http://kogs-www.informatik.uni-hamburg.de/~koethe/">Ullrich
Koethe</a> had independently developed a similar system. When he
discovered Boost.Python v1, he generously contributed countless hours of
coding and much insight into improving it. He is responsible for an early
version of the support for function overloading and wrote the support for
reflecting C++ inheritance relationships. He has helped to improve
error-reporting from both Python and C++ (we hope to do as well in v2
again soon), and has designed the original support for exposing numeric
operators, including a way to avoid explicit coercion by means of
overloading.</p>
<p>The members of the boost mailing list and the Python community
supplied invaluable early feedback. In particular, Ron Clarke, Mark
Evans, Anton Gluck, Chuck Ingold, Prabhu Ramachandran, and Barry Scott
took the brave step of trying to use Boost.Python while it was still in
early stages of development.</p>
Koethe</a> provided the implementation of inheritance and special
method/operator support in the first version of Boost.Python.</p>
<p>The first version of Boost.Python would not have been possible without
the support of Dragon Systems, which supported its development and
@@ -124,12 +91,13 @@
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
26 November, 2002
08 October, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
Reserved.</i></p>
</body>
</html>

View File

@@ -1,12 +1,9 @@
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta name="generator" content=
"HTML Tidy for Cygwin (vers 1st April 2002), see www.w3.org">
"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../boost.css">
@@ -19,11 +16,11 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
alt="C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">Header &lt;boost/python/args.hpp&gt;</h2>
</td>
@@ -38,31 +35,7 @@
<dt><a href="#keyword-expression"><i>keyword-expressions</i></a></dt>
<dt><a href="#classes">Classes</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#arg-spec">class <code>arg</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#arg-synopsis">class <code>arg</code>
synopsis</a></dt>
<dt><a href="#arg-ctor">class <code>arg</code>
constructor</a></dt>
<dt><a href="#arg-operator">class <code>arg</code> template
<code>operator =</code></a></dt>
</dl>
</dd>
</dl>
</dd>
<dt><a href="#keyword-expression-operators"><i>Keyword-expression</i>
operator <code>,</code></a></dt>
<dt><a href="#functions">Functions (deprecated)</a></dt>
<dt><a href="#functions">Functions</a></dt>
<dd>
<dl class="page-index">
@@ -84,95 +57,27 @@
<p>A <b>keyword-expression</b> results in an object which holds a
sequence of <a href="definitions.html#ntbs">ntbs</a>es, and whose type
encodes the number of keywords specified. The <b>keyword-expression</b>
may contain default values for some or all of the keywords it holds</p>
encodes the number of keywords specified.</p>
<h2><a name="classes"></a>Classes</h2>
<h2><a name="functions"></a>Functions</h2>
<h3><a name="arg-spec"></a><code>class arg;</code></h3>
<p>The objects of class arg are keyword-expressions holding one keyword (
size one )</p>
<h4><a name="arg-synopsis"></a>Class <code>arg</code> synopsis</h4>
<h3><a name="args-spec"></a><code>args(</code>...<code>)</code></h3>
<pre>
namespace boost { namespace python
{
struct arg
{
template &lt;class T&gt;
arg &amp;perator = (T const &amp;value);
explicit arg (char const *name){elements[0].name = name;}
};
}}
</pre>
<h4><a name="arg-ctor"></a>Class <code>arg</code> constructor</h4>
<pre>
arg(char const* name);
<i>unspecified1</i> args(char const*);
<i>unspecified2</i> args(char const*, char const*);
.
.
.
<i>unspecifiedN</i> args(char const*, char const*, ... char const*);
</pre>
<dl class="function-semantics">
<dt><b>Requires:</b> The argument must be a <a href=
<dt><b>Requires:</b> Every argument must be a <a href=
"definitions.html#ntbs">ntbs</a>.</dt>
<dt><b>Effects:</b> Constructs an <code>arg</code> object holding a
keyword with name <code>name</code>.</dt>
</dl>
<h4><a name="arg-operator"></a>Class <code>arg</code> operator =</h4>
<pre>
template &lt;class T&gt; arg &amp;operator = (T const &amp;value);
</pre>
<dl class="function-semantics">
<dt><b>Requires:</b> The argument must convertible to python.</dt>
<dt><b>Effects:</b> Assigns default value for the keyword.</dt>
<dt><b>Returns:</b> Reference to <code>this</code>.</dt>
</dl>
<h2><a name="keyword-expression-operators"><i>Keyword-expression</i>
operator <code>,</code></a></h2>
<pre>
<i>keyword-expression</i> operator , (<i>keyword-expression</i>, const arg &amp;kw) const
<i>keyword-expression</i> operator , (<i>keyword-expression</i>, const char *name) const;
</pre>
<dl class="function-semantics">
<dt><b>Requires:</b> The argument <code>name</code> must be a <a href=
"definitions.html#ntbs">ntbs</a>.</dt>
<dt><b>Effects:</b> Extends the <i>keyword-expression</i> argument with
one more keyword.</dt>
<dt><b>Returns:</b> The extended <i>keyword-expression</i>.</dt>
</dl>
<h2><font color="#7F7F7F"><a name="functions"></a>Functions
(deprecated)</font></h2>
<h3><a name="args-spec"></a><code><font color=
"#7F7F7F">args</font>(</code>...<code>)</code></h3>
<pre>
<font color="#7F7F7F"> <i>unspecified1</i> args(char const*);
<i>unspecified2</i> args(char const*, char const*);
.
.
.
<i>unspecifiedN</i> args(char const*, char const*, ... char const*);
</font>
</pre>
<dl class="function-semantics">
<dt><font color="#7F7F7F"><b>Requires:</b> Every argument must be a <a
href="definitions.html#ntbs">ntbs</a>.</font></dt>
<dt><font color="#7F7F7F"><b>Returns:</b> an object representing a <a
href="#keyword-expression"><i>keyword-expression</i></a> encapsulating
the arguments passed.</font></dt>
<dt><b>Returns:</b> an object representing a <a href=
"#keyword-expression"><i>keyword-expression</i></a> encapsulating the
arguments passed.</dt>
</dl>
<h2><a name="examples"></a>Example</h2>
@@ -180,20 +85,19 @@ template &lt;class T&gt; arg &amp;operator = (T const &amp;value);
#include &lt;boost/python/def.hpp&gt;
using namespace boost::python;
int f(double x, double y, double z=0.0, double w=1.0);
int f(int x, int y, int z);
BOOST_PYTHON_MODULE(xxx)
{
def("f", f
, ( arg("x"), "y", arg("z")=0.0, arg("w")=1.0 )
);
def("f", f, args("x", "y", "z"));
}
</pre>
<p>Revised 01 August, 2003</p>
<p>Revised 05 November, 2001</p>
<p><i>&copy; Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002-2003.</i></p>
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002. All Rights
Reserved.</i></p>
</body>
</html>

32
doc/v2/bibliography.html Normal file
View File

@@ -0,0 +1,32 @@
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../boost.css">
<title>Boost.Python - Bibliography</title>
</head>
<body link="#0000ff" vlink="#800080">
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center">Boost.Python</h1>
<h2 align="center">Bibliography</h2>
</td>
</tr>
</table>
<hr>
{{bibliographical information}}
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
05 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. All Rights Reserved.</i></p>
</body>
</html>

View File

@@ -1,6 +1,3 @@
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
@@ -13,10 +10,10 @@
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
"C++ Boost" src="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h1 align="center">Boost.Python</h1>
<h2 align="center">Header &lt;call.hpp&gt;</h2>
</td>
</tr>
@@ -80,6 +77,6 @@ double apply2(PyObject* func, double x, double y)
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave Abrahams</a>
2002. </i></p>
2002. All Rights Reserved.</i></p>
</body>
</html>

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