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<title>Boost.Python - <boost/python/lvalue_from_python.hpp></title>
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<h3><a href="../../../../index.htm"><img height="86" width="277"
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h2 align="center">Header
<boost/python/lvalue_from_pytype.hpp></h2>
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<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="#lvalue_from_pytype-spec">Class Template
<code>lvalue_from_pytype</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#lvalue_from_pytype-spec-synopsis">Class Template
<code>lvalue_from_pytype</code> synopsis</a></dt>
<dt><a href="#lvalue_from_pytype-spec-ctors">Class Template
<code>lvalue_from_pytype</code> constructor</a></dt>
</dl>
</dd>
</dl>
<dl class="page-index">
<dt><a href="#extract_identity-spec">Class Template
<code>extract_identity</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#extract_identity-spec-synopsis">Class Template
<code>extract_identity</code> synopsis</a></dt>
<dt><a href="#extract_identity-spec-statics">Class Template
<code>extract_identity</code> static functions</a></dt>
</dl>
</dd>
<dt><a href="#extract_member-spec">Class Template
<code>extract_member</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#extract_member-spec-synopsis">Class Template
<code>extract_member</code> synopsis</a></dt>
<dt><a href="#extract_member-spec-statics">Class Template
<code>extract_member</code> static functions</a></dt>
</dl>
</dd>
</dl>
</dd>
<dt><a href="#examples">Example</a></dt>
</dl>
<hr>
<h2><a name="introduction"></a>Introduction</h2>
<code><boost/python/lvalue_from_pytype.hpp></code> supplies a
facility for extracting C++ objects from within Python instances of a
given type. This is typically useful for dealing with "traditional"
Python extension types.
<h2><a name="classes"></a>Classes</h2>
<h3><a name="lvalue_from_pytype-spec"></a>Class template
<code>lvalue_from_pytype</code></h3>
<p>Class template <code>lvalue_from_pytype</code> will register
from_python converters which, given an object of the given Python type,
can extract references and pointers to a particular C++ type. Its
template arguments are:</p>
<table border="1" summary="lvalue_from_pytype template parameters">
<caption>
<b><code>lvalue_from_pytype</code> Requirements</b><br>
In the table below, <b><code>x</code></b> denotes an object of type
<code>PythonObject&</code>
</caption>
<tr>
<th>Parameter</th>
<th>Requirements</th>
<th>Semantics</th>
</tr>
<tr>
<td><code>Extractor</code></td>
<td>a model of <a href=
"Extractor.html#Extractor-concept">Extractor</a> whose execute
function returns a reference type.</td>
<td>Extracts the lvalue from the Python object once its type has been
confirmed</td>
</tr>
<tr>
<td><code>python_type</code></td>
<td>A compile-time constant <code><a href=
"http://www.python.org/doc/2.2/ext/dnt-type-methods.html">PyTypeObject</a>*</code></td>
<td>The Python type of instances convertible by this converter.
Python subtypes are also convertible.</td>
</tr>
</table>
<h4><a name="lvalue_from_pytype-spec-synopsis"></a>Class template
<code>lvalue_from_pytype</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template <class Extractor, PyTypeObject const* python_type>
struct lvalue_from_pytype
{
lvalue_from_pytype();
};
}}
</pre>
<h4><a name="lvalue_from_pytype-spec-ctors"></a>Class template
<code>lvalue_from_pytype</code> constructor</h4>
<pre>
lvalue_from_pytype();
</pre>
<dl class="function-semantics">
<dt><b>Effects:</b> Registers converters which can convert Python
objects of the given type to lvalues of the type returned by
<code>Extractor::execute</code>.</dt>
</dl>
<h3><a name="extract_identity-spec"></a>Class template
<code>extract_identity</code></h3>
<p><code>extract_identity</code> is a model of <a href=
"Extractor.html#Extractor-concept">Extractor</a> which can be used in the
common case where the C++ type to be extracted is the same as the Python
object type.</p>
<h4><a name="extract_identity-spec-synopsis"></a>Class template
<code>extract_identity</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template <class InstanceType>
struct extract_identity
{
static InstanceType& execute(InstanceType& c);
};
}}
</pre>
<h4><a name="extract_identity-spec-statics"></a>Class template
<code>extract_identity</code> static functions</h4>
<pre>
InstanceType& execute(InstanceType& c);
</pre>
<dl class="function-semantics">
<dt><b>Returns:</b> <code>c</code></dt>
</dl>
<h3><a name="extract_member-spec"></a>Class template
<code>extract_member</code></h3>
<p><code>extract_member</code> is a model of <a href=
"Extractor.html#Extractor-concept">Extractor</a> which can be used in the
common case in the common case where the C++ type to be extracted is a
member of the Python object.</p>
<h4><a name="extract_member-spec-synopsis"></a>Class template
<code>extract_member</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template <class InstanceType, class MemberType, MemberType (InstanceType::*member)>
struct extract_member
{
static MemberType& execute(InstanceType& c);
};
}}
</pre>
<h4><a name="extract_member-spec-statics"></a>Class template
<code>extract_member</code> static functions</h4>
<pre>
static MemberType& execute(InstanceType& c);
</pre>
<dl class="function-semantics">
<dt><b>Returns:</b> <code>c.*member</code></dt>
</dl>
<h2><a name="examples"></a>Example</h2>
This example presumes that someone has implemented the standard <a href=
"http://www.python.org/doc/2.2/ext/dnt-basics.html">noddy example
module</a> from the Python documentation, and we want to build a module
which manipulates <code>Noddy</code>s. Since
<code>noddy_NoddyObject</code> is so simple that it carries no
interesting information, the example is a bit contrived: it assumes you
want to keep track of one particular object for some reason. This module
would have to be dynamically linked to the module which defines
<code>noddy_NoddyType</code>.
<h3>C++ module definition</h3>
<pre>
#include <boost/python/module.hpp>
#include <boost/python/handle.hpp>
#include <boost/python/borrowed.hpp>
#include <boost/python/lvalue_from_pytype.hpp>
// definition lifted from the Python docs
typedef struct {
PyObject_HEAD
} noddy_NoddyObject;
using namespace boost::python;
static handle<noddy_NoddyObject> cache;
bool is_cached(noddy_NoddyObject* x)
{
return x == cache.get();
}
void set_cache(noddy_NoddyObject* x)
{
cache = handle<noddy_NoddyObject>(borrowed(x));
}
BOOST_PYTHON_MODULE(noddy_cache)
{
def("is_cached", is_cached);
def("set_cache", set_cache);
// register Noddy lvalue converter
lvalue_from_pytype<extract_identity<noddy_NoddyObject>,&noddy_NoddyType>();
}
</pre>
<h3>Python code</h3>
<pre>
>>> import noddy
>>> n = noddy.new_noddy()
>>> import noddy_cache
>>> noddy_cache.is_cached(n)
0
>>> noddy_cache.set_cache(n)
>>> noddy_cache.is_cached(n)
1
>>> noddy_cache.is_cached(noddy.new_noddy())
0
</pre>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
20 November, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>© Copyright <a href=
"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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