mirror of
https://github.com/boostorg/python.git
synced 2026-01-21 05:02:17 +00:00
162 lines
5.1 KiB
HTML
162 lines
5.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
|
|
<meta name="generator" content="HTML Tidy, 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">
|
|
|
|
<title>Boost.Python -
|
|
<boost/python/reference_existing_object.hpp></title>
|
|
|
|
<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 valign="top">
|
|
<h1 align="center">Boost.Python</h1>
|
|
|
|
<h2 align="center">Header
|
|
<boost/python/reference_existing_object.hpp></h2>
|
|
</table>
|
|
<hr>
|
|
|
|
<h2>Contents</h2>
|
|
|
|
<dl class="page-index">
|
|
<dt><a href="#classes">Classes</a>
|
|
|
|
<dd>
|
|
<dl class="page-index">
|
|
<dt><a href="#reference_existing_object-spec">Class
|
|
<code>reference_existing_object</code></a>
|
|
|
|
<dd>
|
|
<dl class="page-index">
|
|
<dt><a href="#reference_existing_object-spec-synopsis">Class
|
|
<code>reference_existing_object</code> synopsis</a>
|
|
|
|
<dt><a href="#reference_existing_object-spec-metafunctions">Class
|
|
<code>reference_existing_object</code> metafunctions</a>
|
|
</dl>
|
|
</dl>
|
|
|
|
<dt><a href="#examples">Example</a>
|
|
</dl>
|
|
<hr>
|
|
|
|
<h2><a name="classes"></a>Classes</h2>
|
|
|
|
<h3><a name="reference_existing_object-spec"></a>Class
|
|
<code>reference_existing_object</code></h3>
|
|
|
|
<p><code>reference_existing_object</code> is a model of <a href=
|
|
"ResultConverter.html#ResultConverterGenerator-concept">ResultConverterGenerator</a> which can be
|
|
used to wrap C++ functions which return a reference or pointer to a C++
|
|
object. When the wrapped function is called, the value referenced by its
|
|
return value is not copied. A new Python object is created which contains a
|
|
pointer to the referent, and no attempt is made to ensure that the lifetime
|
|
of the referent is at least as long as that of the corresponding Python
|
|
object. Thus, it can be <font color="#ff0000"><b>highly
|
|
dangerous</b></font> to use <code>reference_existing_object</code> without
|
|
additional lifetime management from such models of <a href=
|
|
"CallPolicies.html">CallPolicies</a> as <a href=
|
|
"with_custodian_and_ward.html#with_custodian_and_ward-spec">with_custodian_and_ward</a>.
|
|
This class is used in the implementation of <a href=
|
|
"return_internal_reference.html#return_internal_reference-spec">return_internal_reference</a>.
|
|
|
|
<h4><a name="reference_existing_object-spec-synopsis"></a>Class
|
|
<code>reference_existing_object</code> synopsis</h4>
|
|
<pre>
|
|
namespace boost { namespace python
|
|
{
|
|
struct reference_existing_object
|
|
{
|
|
template <class T> struct apply;
|
|
};
|
|
}}
|
|
</pre>
|
|
|
|
<h4><a name="reference_existing_object-spec-metafunctions"></a>Class
|
|
<code>reference_existing_object</code> metafunctions</h4>
|
|
<pre>
|
|
template <class T> struct apply
|
|
</pre>
|
|
|
|
<dl class="metafunction-semantics">
|
|
<dt><b>Requires:</b> <code>T</code> is <code>U&</code> or
|
|
<code>U*</code>for some <code>U</code>.
|
|
|
|
<dt><b>Returns:</b> <code>typedef <a href=
|
|
"to_python_indirect.html#to_python_indirect-spec">to_python_indirect</a><T,V>
|
|
type</code>, where <code>V</code> is a <a href=
|
|
"to_python_indirect.html#HolderObjectGenerator">HolderObjectGenerator</a>
|
|
which constructs an instance holder containing an <i>unowned</i>
|
|
<code>U*</code> pointing to the referent of the wrapped function's return
|
|
value.
|
|
</dl>
|
|
|
|
<h2><a name="examples"></a>Example</h2>
|
|
|
|
<p>In C++:
|
|
<pre>
|
|
#include <boost/python/module.hpp>
|
|
#include <boost/python/class.hpp>
|
|
#include <boost/python/reference_existing_object.hpp>
|
|
#include <boost/python/return_value_policy.hpp>
|
|
#include <utility>
|
|
|
|
// classes to wrap
|
|
struct Singleton
|
|
{
|
|
Singleton() : x(0) {}
|
|
|
|
int exchange(int n) // set x and return the old value
|
|
{
|
|
std::swap(n, x);
|
|
return n;
|
|
}
|
|
|
|
int x;
|
|
};
|
|
|
|
Singleton& get_it()
|
|
{
|
|
static Singleton just_one;
|
|
return just_one;
|
|
}
|
|
|
|
// Wrapper code
|
|
using namespace boost::python;
|
|
BOOST_PYTHON_MODULE_INIT(singleton)
|
|
{
|
|
module("singleton")
|
|
.def("get_it", get_it)
|
|
.add(
|
|
class_<Singleton>()
|
|
.def("exchange", &Singleton::exchange)
|
|
);
|
|
}
|
|
</pre>
|
|
In Python:
|
|
<pre>
|
|
>>> import singleton
|
|
>>> s1 = singleton.get_it()
|
|
>>> s2 = singleton.get_it()
|
|
>>> id(s1) == id(s2) # s1 and s2 are not the same object
|
|
0
|
|
>>> s1.exchange(42) # but they reference the same C++ Singleton
|
|
0
|
|
>>> s2.exchange(99)
|
|
42
|
|
</pre>
|
|
|
|
<p>Revised
|
|
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
|
|
14 February 2002 <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
|
|
|
|
|
|
<p><i>© Copyright <a href="../../../../people/dave_abrahams.htm">Dave
|
|
Abrahams</a> 2002. All Rights Reserved.</i>
|
|
|