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226 lines
6.5 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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<meta name="generator" content=
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"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - <boost/python/has_back_reference.hpp></title>
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</head>
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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 class="c1"><a href="../index.html">Boost.Python</a></h1>
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<h2 class="c2">Header
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<boost/python/has_back_reference.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="#introduction">Introduction</a></dt>
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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="#has_back_reference-spec">Class template
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<code>has_back_reference</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#has_back_reference-spec-synopsis">Class template
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<code>has_back_reference</code> synopsis</a></dt>
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</dl>
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</dd>
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<dt><a href="#examples">Example(s)</a></dt>
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</dl>
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</dd>
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</dl>
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<hr>
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<h2><a name="introduction"></a>Introduction</h2>
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<p><code><boost/python/has_back_reference.hpp></code> defines the
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predicate metafunction <code>has_back_reference<></code>, which can
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be specialized by the user to indicate that a wrapped class instance
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holds a <code>PyObject*</code> corresponding to a Python object.</p>
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="has_back_reference-spec"></a>Class template
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<code>has_back_reference</code></h3>
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<p>A unary metafunction whose <code>value</code> is true iff its argument
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is a <code>pointer_wrapper<></code>.</p>
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<h4><a name="has_back_reference-spec-synopsis"></a>Class template
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<code>has_back_reference</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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template<class WrappedClass> class has_back_reference
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{
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typedef mpl::false_ type;
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};
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}}
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</pre>
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<p>A "<a href="../../../mpl/doc/refmanual/metafunction.html">
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metafunction</a>" that is inspected by Boost.Python to determine how
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wrapped classes can be constructed.</p>
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<dl class="traits-semantics">
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<dt><code>type::value</code> is an integral constant convertible to bool
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of unspecified type.</dt>
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<dt>Specializations may substitute a <code>true</code>-valued integral constant wrapper for
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<code>type</code> iff for each invocation of
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<code>class_<WrappedClass>::def(init<</code>
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<i>type-sequence...</i><code>>())</code> and the implicitly wrapped
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copy constructor (unless it is <a href="class.html#class_-spec">
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noncopyable</a>), there exists a corresponding constructor
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<code>WrappedClass::WrappedClass(PyObject*, </code>
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<i>type-sequence...</i><code>)</code>. If such a specialization exists,
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the <code>WrappedClass</code> constructors will be called with a "back
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reference" pointer to the corresponding Python object whenever they are
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invoked from Python. The easiest way to provide this nested <code>
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type
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</code>
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is to
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derive the specialization from <code>mpl::true_</code>.
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</dt>
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</dl>
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<h2><a name="examples"></a>Example</h2>
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<h3>C++ module definition</h3>
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<pre>
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#include <boost/python/class.hpp>
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#include <boost/python/module.hpp>
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#include <boost/python/has_back_reference.hpp>
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#include <boost/python/handle.hpp>
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#include <boost/shared_ptr.hpp>
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using namespace boost::python;
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using boost::shared_ptr;
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struct X
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{
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X(PyObject* self) : m_self(self), m_x(0) {}
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X(PyObject* self, int x) : m_self(self), m_x(x) {}
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X(PyObject* self, X const& other) : m_self(self), m_x(other.m_x) {}
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handle<> self() { return handle<>(borrowed(m_self)); }
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int get() { return m_x; }
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void set(int x) { m_x = x; }
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PyObject* m_self;
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int m_x;
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};
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// specialize has_back_reference for X
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namespace boost { namespace python
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{
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template <>
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struct has_back_reference<X>
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: mpl::true_
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{};
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}}
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struct Y
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{
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Y() : m_x(0) {}
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Y(int x) : m_x(x) {}
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int get() { return m_x; }
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void set(int x) { m_x = x; }
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int m_x;
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};
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shared_ptr<Y>
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Y_self(shared_ptr<Y> self) { return self; }
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BOOST_PYTHON_MODULE(back_references)
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{
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class_<X>("X")
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.def(init<int>())
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.def("self", &X::self)
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.def("get", &X::get)
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.def("set", &X::set)
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;
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class_<Y, shared_ptr<Y> >("Y")
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.def(init<int>())
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.def("get", &Y::get)
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.def("set", &Y::set)
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.def("self", Y_self)
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;
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}
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</pre>
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The following Python session illustrates that <code>x.self()</code>
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returns the same Python object on which it is invoked, while
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<code>y.self()</code> must create a new Python object which refers to the
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same Y instance.
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<h3>Python code</h3>
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<pre>
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>>> from back_references import *
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>>> x = X(1)
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>>> x2 = x.self()
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>>> x2 is x
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<b>1</b>
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>>> (x.get(), x2.get())
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(1, 1)
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>>> x.set(10)
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>>> (x.get(), x2.get())
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(10, 10)
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>>>
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>>>
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>>> y = Y(2)
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>>> y2 = y.self()
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>>> y2 is y
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<b>0</b>
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>>> (y.get(), y2.get())
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(2, 2)
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>>> y.set(20)
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>>> (y.get(), y2.get())
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(20, 20)
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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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18 July, 2004
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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</p>
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<p class="c3">© Copyright <a href=
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"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002
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.</p>
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</body>
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</html>
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