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<title>Boost.Python -
<boost/python/reference_existing_object.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 align="center"><a href="../index.html">Boost.Python</a></h1>
<h2 align="center">Header
<boost/python/reference_existing_object.hpp></h2>
</td>
</tr>
</table>
<hr>
<h2>Contents</h2>
<dl class="page-index">
<dt><a href="#classes">Classes</a></dt>
<dd>
<dl class="page-index">
<dt><a href="#reference_existing_object-spec">Class
<code>reference_existing_object</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#reference_existing_object-spec-synopsis">Class
<code>reference_existing_object</code> synopsis</a></dt>
<dt><a href=
"#reference_existing_object-spec-metafunctions">Class
<code>reference_existing_object</code> metafunctions</a></dt>
</dl>
</dd>
</dl>
</dd>
<dt><a href="#examples">Example</a></dt>
</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>.</p>
<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>
<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 class whose
static <code>execute</code> function constructs an instance
holder containing an <i>unowned</i>
<code>U*</code> pointing to the referent of the wrapped function's
return value.</dt>
</dl>
<h2><a name="examples"></a>Example</h2>
<p>In C++:</p>
<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(singleton)
{
def("get_it", get_it,
return_value_policy<reference_existing_object>());
class_<Singleton>("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 -->
13 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>© Copyright <a href=
"../../../../people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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