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539 lines
17 KiB
ReStructuredText
Executable File
539 lines
17 KiB
ReStructuredText
Executable File
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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The Boost Parameter Library Python Binding Documentation
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+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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|(logo)|__
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.. |(logo)| image:: ../../../../boost.png
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:alt: Boost
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__ ../../../../index.htm
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:Authors: Daniel Wallin
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:Contact: dalwan01@student.umu.se
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:organization: `Boost Consulting`_
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:date: $Date$
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:copyright: Copyright David Abrahams, Daniel Wallin
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2005. Distributed under the Boost Software License,
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Version 1.0. (See accompanying file LICENSE_1_0.txt
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or copy at http://www.boost.org/LICENSE_1_0.txt)
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.. _`Boost Consulting`: http://www.boost-consulting.com
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.. role:: class
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:class: class
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.. role:: concept
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:class: concept
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.. role:: function
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:class: function
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.. |KeywordsSpec| replace:: :concept:`KeywordsSpec`
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.. contents::
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:depth: 1
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Introduction
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------------
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``boost/parameter/python.hpp`` introduces a group of def_visitors_ that can
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be used to easily expose Boost.Parameter-enabled member functions to Python with
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Boost.Python. It also provides a function template ``def()`` that can be used
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to expose Boost.Parameter-enabled free functions.
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.. Need a link from "def_visitors" to the place in Python docs
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.. where that's defined.
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.. _def_visitors: ../../../python/doc/v2/def_visitor.html
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To bind a Boost.Parameter enabled function the keyword tags must be specified.
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Additionally, because Boost.Parameter enabled functions are templates, the
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desired function signature must be specified.
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The keyword tags are specified using the syntax described in |KeywordsSpec| below,
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and the signature as an MPL sequence of parameter types. Additional data may be
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needed in the signature sequence by specific binding utilities. For example,
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``function`` requires the return type to be part of the signature sequence.
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Tutorial
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--------
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In this section we will outline the steps needed to bind a simple Boost.Parameter
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enabled member function to Python. Knowledge of the Boost.Parameter macros are
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required to understand this section.
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The class and member function we are interested in binding looks like this::
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// First the keywords
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BOOST_PARAMETER_KEYWORD(tag, title)
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BOOST_PARAMETER_KEYWORD(tag, width)
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BOOST_PARAMETER_KEYWORD(tag, height)
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class window
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{
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public:
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BOOST_PARAMETER_MEMBER_FUNCTION((void), open, tag,
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(required (title, (std::string)))
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(optional (width, (unsigned), 400)
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(height, (unsigned), 400))
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);
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};
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It defines a member function ``open()`` with one required parameter and two
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optional ones. To bind this member function to Python we use the binding
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utility ``function``. ``function`` is a ``def_visitor`` which we'll instantiate and
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pass to ``boost::python::class_::def()``.
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::
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BOOST_PYTHON_MODULE(my_module)
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{
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using namespace boost::python;
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namespace py = boost::parameter::python;
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class_<window>("window")
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.def("open",
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py::function<
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mpl::vector3<tag::title, tag::width*, tag::height*>
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, mpl::vector4<void, std::string, unsigned, unsigned>
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>()
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);
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}
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concept |KeywordsSpec|
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----------------------
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A |KeywordsSpec| is an MPL sequence where each element is either:
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* A *required* keyword of the form ``K``
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* **or**, an *optional* keyword of the form ``K*``
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* **or**, a *special* keyword of the form ``K**``
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where ``K`` is a `keyword tag type`_.
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.. _keyword tag type: reference.html#terminology
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.. here you have to say, "where K is..."
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.. #
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The **arity range** of a |KeywordsSpec| is defined as:
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.. parsed-literal::
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[ mpl::size<S> - number of *special* keyword tags in ``S`` , mpl::size<S> ]
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For example, the **arity range** of ``mpl::vector2<x,y>`` is [2,2], the **arity range** of
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``mpl::vector2<x,y*>`` is [2,2] and the **arity range** of ``mpl::vector2<x,y**>`` is [1,2].
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.. It makes no sense to say that the "range" of something is x,
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.. where x is just a number. A range goes from x to y. I don't
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.. know what this is supposed to mean. I also don't know what the
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.. comma in the definition means, or why I don't see it in the
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.. results above. I'd have guessed that the arity range of
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.. vector2<x,y**> was [1,2] ... which makes some sense.
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.. #
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*special* keyword tags
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---------------------------------
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Sometimes it is desirable to have a default value for a parameter that differ
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in type from the parameter. This technique is useful for doing simple tag-dispatching
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based on the presence of a parameter. An example_ of this is given in the Boost.Parameter
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docs. The example uses a different technique, but could also have been written like this:
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.. parsed-literal::
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template <class ArgumentPack>
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void dfs_dispatch(ArgumentPack& args, mpl::false\_)
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{
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*…compute and use default color map…*
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}
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template <class ArgumentPack, class ColorMap>
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void dfs_dispatch(ArgumentPack& args, ColorMap colormap)
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{
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*…use colormap…*
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}
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template <class ArgumentPack>
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void depth_first_search(ArgumentPack& args)
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{
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core::dfs_dispatch(args, args[color | mpl::false_());
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}
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.. _example: index.html#dispatching-based-on-the-presence-of-a-default
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In the above example the type of the default for ``color`` is ``mpl::false_``, a
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type that is distinct from any color map that the user might supply.
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When binding the case outlined above, the default type for ``color`` will not
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be convertible to the parameter type. Therefore we need to tag the ``color``
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keyword as a *special* keyword. By doing this we tell the binding functions
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that it needs to generate two overloads, one with the ``color`` parameter
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present and one without. If we would have had two *special* keywords, four
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overloads would need to be generated. The number of generated overloads is
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equal to ``2^N``, where ``N`` is the number of *special* keywords.
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.. The entire section below was rewritten.
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.. If the default type for an argument is not convertible to the argument type, as
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specified to the binding functions below, that argument must be specified as a
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*special* argument.
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.. This whole thing comes out of order. You need to explain that
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.. to bind pararameter-endabled functions to python you need to
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.. specify an parameter type (not an argument type), and that an
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.. optional argument has to have a default value _and_ type.
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.. I think. If that's not the right explanation, you need to say
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.. something that sets up similar context.
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In the example below the default type for argument ``y`` is ``char const[5]``, but
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the argument type is ``int``. Therefore ``y`` must be specified as a *special*
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argument in the |KeywordsSpec|.
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.. The example below doesn't make any sense to me. Where does char
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.. const[5] come from? Why would I choose a different argument
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.. type from a default type. What is the effect on the resulting
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.. Python interface?
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Doing this will generate ``2^N`` overloads, where ``N`` is the number of *special* parameters.
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In this case two overloads will be generated, one with ``y`` included and one without.
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Having many *special* keywords will result in lots of overloads, and stress the
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compiler.
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.. using "this" without an antecedent above. What are we "doing?"
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.. Don't you mean 2^N?
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Note that this makes the *arity range* ``[1,2]``, so we'll need two forwarding overloads.
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.. parsed-literal::
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BOOST_PARAMETER_FUNCTION((void), f, tag,
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(required (x, \*))
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(optional (y, \*))
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)
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{
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std::cout << args[x] << args[y | "none"] << "\n";
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}
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struct f_fwd
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{
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template <class A0, class A1>
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void operator()(boost::type<void>, A0 const& a0)
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{
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f(a0);
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}
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template <class A0, class A1>
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void operator()(boost::type<void>, A0 const& a0, A1 const& a1)
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{
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f(a0, a1);
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def("f",
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function<
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fwd
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, mpl::vector2<tag::x, **tag::y\*\***>
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, mpl::vector3<void, int, int>
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>()
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);
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}
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.. You don't explain what those boost::type<void> things are all
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.. about.
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.. Weren't we going to generate the f_fwd struct ourselves?
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.. I don't think this code has been tested. I see the identifier
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.. "fwd" above, which surely must be wrong.
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------------------------------------------------------------------------------
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class template ``init``
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-----------------------
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Defines a named parameter enabled constructor.
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.. parsed-literal::
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template <class Keywords, class Signature>
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struct init : python::def_visitor<init<Keywords, Signature> >
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{
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template <class Class>
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void def(Class& class\_);
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};
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``init`` requirements
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~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword ,
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in the order dictated by ``Keywords``.
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* ``Class`` must support these expressions:
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======================================================= ==================== ==============================================
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Expression Return type Requirements
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======================================================= ==================== ==============================================
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``Class(a0, ..., aN)`` \- ``a0``..\ ``aN`` are tagged arguments.
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======================================================= ==================== ==============================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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.. parsed-literal::
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struct base { /\* ... \*/ };
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class X : base
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{
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public:
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BOOST_PARAMETER_CONSTRUCTOR(X, (base),
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(required (x, \*))
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(optional (y, \*))
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)
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def(
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init<
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector2<int, int>
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>()
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);
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}
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------------------------------------------------------------------------------
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class template ``call``
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-----------------------
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Defines a ``__call__`` operator, mapped to ``operator()`` in C++.
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.. parsed-literal::
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template <class Keywords, class Signature>
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struct call : python::def_visitor<call<Keywords, Signature> >
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{
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template <class Class>
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void def(Class& class\_);
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};
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``call`` requirements
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~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword parameters,
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in the order dictated by ``Keywords``, and the return type prepended.
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* ``Class`` must support these expressions, where ``c`` is an instance of ``Class``:
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======================================================= ==================== ==============================================
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Expression Return type Requirements
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======================================================= ==================== ==============================================
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``c(a0, ..., aN)`` Convertible to ``R`` ``a0``..\ ``aN`` are tagged arguments.
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======================================================= ==================== ==============================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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.. parsed-literal::
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typedef parameter::parameters<
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parameter::required<tag::x>
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, parameter::optional<tag::y>
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> call_parameters;
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class X
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{
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public:
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template <class Args>
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int call_impl(Args const& args)
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{
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/\* ... \*/
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}
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template <class A0>
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int operator()(A0 const& a0)
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{
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return call_impl(call_parameters()(a0));
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}
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template <class A0, class A1>
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int operator()(A0 const& a0, A1 const& a1)
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{
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return call_impl(call_parameters()(a0,a1));
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def("f",
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call<
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector3<int, int, int>
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>()
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);
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}
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------------------------------------------------------------------------------
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class template ``function``
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---------------------------
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Defines a named parameter enabled member function.
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.. parsed-literal::
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template <class Fwd, class Keywords, class Signature>
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struct function : python::def_visitor<function<Fwd, Keywords, Signature> >
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{
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template <class Class, class Options>
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void def(Class& class\_, char const* name, Options const& options);
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};
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``function`` requirements
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~~~~~~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword parameters,
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in the order dictated by ``Keywords``, and the return type prepended.
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* An instance of ``Fwd`` must support this expression:
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======================================================= ==================== ==============================================
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Expression Return type Requirements
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======================================================= ==================== ==============================================
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``fwd(boost::type<R>(), self, a0, ..., aN)`` Convertible to ``R`` ``self`` is a reference to the object on which
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the function should be invoked. ``a0``..\ ``aN``
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are tagged arguments.
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======================================================= ==================== ==============================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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This example exports a member function ``f(int x, int y = ..)`` to Python.
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The |KeywordsSpec| ``mpl::vector2<tag::x, tag::y*>`` has an **arity range**
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of [2,2], so we only need one forwarding overload.
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.. parsed-literal::
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class X
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{
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public:
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BOOST_PARAMETER_MEMBER_FUNCTION((void), f, tag,
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(required (x, \*))
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(optional (y, \*))
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)
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{
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/\* .. \*/
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}
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};
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struct f_fwd
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{
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template <class A0, class A1>
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void operator()(boost::type<void>, X& self, A0 const& a0, A1 const& a1)
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{
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self.f(a0, a1);
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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class_<X>("X")
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.def("f",
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function<
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fwd
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector3<void, int, int>
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>()
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);
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}
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------------------------------------------------------------------------------
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function template ``def``
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-------------------------
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Defines a named parameter enabled free function in the current Python scope.
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.. parsed-literal::
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template <class Fwd, class Keywords, class Signature>
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void def(char const* name);
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``def`` requirements
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~~~~~~~~~~~~~~~~~~~~
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* ``Keywords`` is a model of |KeywordsSpec|.
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* ``Signature`` is an MPL sequence with the types of the keyword parameters,
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in the order dictated by ``Keywords``, and the return type prepended.
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* An instance of ``Fwd`` must support this expression:
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======================================================= ==================== ==============================================
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Expression Return type Requirements
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======================================================= ==================== ==============================================
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``fwd(boost::type<R>(), a0, ..., aN)`` Convertible to ``R`` ``a0``..\ ``aN`` are tagged arguments.
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======================================================= ==================== ==============================================
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For every ``N`` in ``[U,V]``, where ``[U,V]`` is the **arity range** of ``Keywords``.
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Example
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~~~~~~~
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This example exports a function ``f(int x, int y = ..)`` to Python.
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The |KeywordsSpec| ``mpl::vector2<tag::x, tag::y*>`` has an **arity range**
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of [2,2], so we only need one forwarding overload.
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.. parsed-literal::
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BOOST_PARAMETER_FUNCTION((void), f, tag,
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(required (x, \*))
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(optional (y, \*))
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)
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{
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/\* .. \*/
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}
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struct f_fwd
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{
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template <class A0, class A1>
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void operator()(boost::type<void>, A0 const& a0, A1 const& a1)
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{
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f(a0, a1);
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}
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};
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BOOST_PYTHON_MODULE(..)
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{
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def<
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fwd
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, mpl::vector2<tag::x, tag::y\*>
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, mpl::vector3<void, int, int>
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>("f");
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}
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Portability
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-----------
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The Boost.Parameter Python binding library requires *partial template specialization*.
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