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181 lines
5.6 KiB
HTML
181 lines
5.6 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<html>
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<head>
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<meta name="generator" content=
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"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python -
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<boost/python/reference_existing_object.hpp></title>
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</head>
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<body>
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277"
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alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">Header
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<boost/python/reference_existing_object.hpp></h2>
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</td>
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</tr>
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</table>
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<hr>
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<h2>Contents</h2>
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<dl class="page-index">
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<dt><a href="#classes">Classes</a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#reference_existing_object-spec">Class
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<code>reference_existing_object</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#reference_existing_object-spec-synopsis">Class
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<code>reference_existing_object</code> synopsis</a></dt>
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<dt><a href=
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"#reference_existing_object-spec-metafunctions">Class
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<code>reference_existing_object</code> metafunctions</a></dt>
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</dl>
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</dd>
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</dl>
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</dd>
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<dt><a href="#examples">Example</a></dt>
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</dl>
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<hr>
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="reference_existing_object-spec"></a>Class
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<code>reference_existing_object</code></h3>
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<p><code>reference_existing_object</code> is a model of <a href=
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"ResultConverter.html#ResultConverterGenerator-concept">ResultConverterGenerator</a>
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which can be used to wrap C++ functions which return a reference or
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pointer to a C++ object. When the wrapped function is called, the value
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referenced by its return value is not copied. A new Python object is
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created which contains a pointer to the referent, and no attempt is made
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to ensure that the lifetime of the referent is at least as long as that
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of the corresponding Python object. Thus, it can be <font color=
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"#ff0000"><b>highly dangerous</b></font> to use
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<code>reference_existing_object</code> without additional lifetime
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management from such models of <a href=
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"CallPolicies.html">CallPolicies</a> as <a href=
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"with_custodian_and_ward.html#with_custodian_and_ward-spec">with_custodian_and_ward</a>.
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This class is used in the implementation of <a href=
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"return_internal_reference.html#return_internal_reference-spec">return_internal_reference</a>.</p>
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<h4><a name="reference_existing_object-spec-synopsis"></a>Class
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<code>reference_existing_object</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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struct reference_existing_object
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{
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template <class T> struct apply;
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};
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}}
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</pre>
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<h4><a name="reference_existing_object-spec-metafunctions"></a>Class
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<code>reference_existing_object</code> metafunctions</h4>
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<pre>
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template <class T> struct apply
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</pre>
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<dl class="metafunction-semantics">
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<dt><b>Requires:</b> <code>T</code> is <code>U&</code> or
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<code>U*</code>for some <code>U</code>.</dt>
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<dt><b>Returns:</b> <code>typedef <a href=
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"to_python_indirect.html#to_python_indirect-spec">to_python_indirect</a><T,V>
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type</code>, where <code>V</code> is a class whose
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static <code>execute</code> function constructs an instance
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holder containing an <i>unowned</i>
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<code>U*</code> pointing to the referent of the wrapped function's
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return value.</dt>
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</dl>
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<h2><a name="examples"></a>Example</h2>
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<p>In C++:</p>
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<pre>
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#include <boost/python/module.hpp>
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#include <boost/python/class.hpp>
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#include <boost/python/reference_existing_object.hpp>
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#include <boost/python/return_value_policy.hpp>
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#include <utility>
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// classes to wrap
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struct Singleton
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{
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Singleton() : x(0) {}
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int exchange(int n) // set x and return the old value
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{
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std::swap(n, x);
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return n;
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}
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int x;
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};
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Singleton& get_it()
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{
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static Singleton just_one;
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return just_one;
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}
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// Wrapper code
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using namespace boost::python;
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BOOST_PYTHON_MODULE(singleton)
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{
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def("get_it", get_it,
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return_value_policy<reference_existing_object>());
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class_<Singleton>("Singleton")
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.def("exchange", &Singleton::exchange)
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;
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}
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</pre>
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In Python:
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<pre>
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>>> import singleton
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>>> s1 = singleton.get_it()
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>>> s2 = singleton.get_it()
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>>> id(s1) == id(s2) # s1 and s2 are not the same object
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0
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>>> s1.exchange(42) # but they reference the same C++ Singleton
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0
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>>> s2.exchange(99)
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42
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</pre>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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13 November, 2002
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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</p>
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<p><i>© Copyright <a href=
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"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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</body>
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</html>
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