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215 lines
5.9 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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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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<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 - <boost/python/has_back_reference.hpp></title>
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<style type="text/css">
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p.c3 {font-style: italic}
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h2.c2 {text-align: center}
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h1.c1 {text-align: center}
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</style>
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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="../../../../c++boost.gif" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 class="c1">Boost.Python</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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traits class template <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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static <i>unspecified</i> value = false;
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};
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}}
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</pre>
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<p>A "<a href="../../../../more/generic_programming.html#traits">traits
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class</a>" which is inspected by Boost.Python to determine how wrapped
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classes can be constructed.</p>
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<dl class="traits-semantics">
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<dt><code>value</code> is an integral constant convertible to bool of
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unspecified type.</dt>
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<dt>Specializations may substitute a value convertible to
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<code>true</code> for <code>value</code> iff for each invocation of
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<code>class_<WrappedClass>::def(init<</code><i>type-sequence...</i><code>
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>())</code>, there exists a corresponding constructor
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<code>WrappedClass::WrappedClass(PyObject*, </code><i>type-sequence...</i>
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<code>)</code>. If such a specialization exists, the
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<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.</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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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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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 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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{
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enum { value = 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 x;
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};
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boost::shared_ptr<Y> Y_self(boost::shared_ptr<Y> self) const { 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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29 September, 2002
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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. All Rights
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Reserved.</p>
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</body>
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</html>
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