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<title>Boost.Python - <boost/python/has_back_reference.hpp></title>
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<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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<h1 class="c1"><a href="../index.html">Boost.Python</a></h1>
<h2 class="c2">Header
<boost/python/has_back_reference.hpp></h2>
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</tr>
</table>
<hr>
<h2>Contents</h2>
<dl class="page-index">
<dt><a href="#introduction">Introduction</a></dt>
<dt><a href="#classes">Classes</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#has_back_reference-spec">Class template
<code>has_back_reference</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#has_back_reference-spec-synopsis">Class template
<code>has_back_reference</code> synopsis</a></dt>
</dl>
</dd>
<dt><a href="#examples">Example(s)</a></dt>
</dl>
</dd>
</dl>
<hr>
<h2><a name="introduction"></a>Introduction</h2>
<p><code><boost/python/has_back_reference.hpp></code> defines the
predicate metafunction <code>has_back_reference<></code>, which can
be specialized by the user to indicate that a wrapped class instance
holds a <code>PyObject*</code> corresponding to a Python object.</p>
<h2><a name="classes"></a>Classes</h2>
<h3><a name="has_back_reference-spec"></a>Class template
<code>has_back_reference</code></h3>
<p>A unary metafunction whose <code>value</code> is true iff its argument
is a <code>pointer_wrapper<></code>.</p>
<h4><a name="has_back_reference-spec-synopsis"></a>Class template
<code>has_back_reference</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template<class WrappedClass> class has_back_reference
{
typedef mpl::false_ type;
};
}}
</pre>
<p>A "<a href="../../../mpl/doc/refmanual/metafunction.html">
metafunction</a>" that is inspected by Boost.Python to determine how
wrapped classes can be constructed.</p>
<dl class="traits-semantics">
<dt><code>type::value</code> is an integral constant convertible to bool
of unspecified type.</dt>
<dt>Specializations may substitute a <code>true</code>-valued integral constant wrapper for
<code>type</code> iff for each invocation of
<code>class_<WrappedClass>::def(init<</code>
<i>type-sequence...</i><code>>())</code> and the implicitly wrapped
copy constructor (unless it is <a href="class.html#class_-spec">
noncopyable</a>), there exists a corresponding constructor
<code>WrappedClass::WrappedClass(PyObject*, </code>
<i>type-sequence...</i><code>)</code>. If such a specialization exists,
the <code>WrappedClass</code> constructors will be called with a "back
reference" pointer to the corresponding Python object whenever they are
invoked from Python. The easiest way to provide this nested <code>
type
</code>
is to
derive the specialization from <code>mpl::true_</code>.
</dt>
</dl>
<h2><a name="examples"></a>Example</h2>
<h3>C++ module definition</h3>
<pre>
#include <boost/python/class.hpp>
#include <boost/python/module.hpp>
#include <boost/python/has_back_reference.hpp>
#include <boost/python/handle.hpp>
#include <boost/shared_ptr.hpp>
using namespace boost::python;
using boost::shared_ptr;
struct X
{
X(PyObject* self) : m_self(self), m_x(0) {}
X(PyObject* self, int x) : m_self(self), m_x(x) {}
X(PyObject* self, X const& other) : m_self(self), m_x(other.m_x) {}
handle<> self() { return handle<>(borrowed(m_self)); }
int get() { return m_x; }
void set(int x) { m_x = x; }
PyObject* m_self;
int m_x;
};
// specialize has_back_reference for X
namespace boost { namespace python
{
template <>
struct has_back_reference<X>
: mpl::true_
{};
}}
struct Y
{
Y() : m_x(0) {}
Y(int x) : m_x(x) {}
int get() { return m_x; }
void set(int x) { m_x = x; }
int m_x;
};
shared_ptr<Y>
Y_self(shared_ptr<Y> self) { return self; }
BOOST_PYTHON_MODULE(back_references)
{
class_<X>("X")
.def(init<int>())
.def("self", &X::self)
.def("get", &X::get)
.def("set", &X::set)
;
class_<Y, shared_ptr<Y> >("Y")
.def(init<int>())
.def("get", &Y::get)
.def("set", &Y::set)
.def("self", Y_self)
;
}
</pre>
The following Python session illustrates that <code>x.self()</code>
returns the same Python object on which it is invoked, while
<code>y.self()</code> must create a new Python object which refers to the
same Y instance.
<h3>Python code</h3>
<pre>
>>> from back_references import *
>>> x = X(1)
>>> x2 = x.self()
>>> x2 is x
<b>1</b>
>>> (x.get(), x2.get())
(1, 1)
>>> x.set(10)
>>> (x.get(), x2.get())
(10, 10)
>>>
>>>
>>> y = Y(2)
>>> y2 = y.self()
>>> y2 is y
<b>0</b>
>>> (y.get(), y2.get())
(2, 2)
>>> y.set(20)
>>> (y.get(), y2.get())
(20, 20)
</pre>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
18 July, 2004
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p class="c3">© Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002
.</p>
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