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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<link rel="stylesheet" type="text/css" href="../boost.css">
<title>Boost.Python -
<boost/python/to_python_converter.hpp></title>
</head>
<body>
<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
"header">
<tr>
<td valign="top" width="300">
<h3><a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
<h2 align="center">Header
<boost/python/to_python_converter.hpp></h2>
</td>
</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="#to_python_converter-spec">Class Template
<code>to_python_converter</code></a></dt>
<dd>
<dl class="page-index">
<dt><a href="#to_python_converter-spec-synopsis">Class Template
<code>to_python_converter</code> synopsis</a></dt>
<dt><a href="#to_python_converter-spec-ctors">Class Template
<code>to_python_converter</code> constructor</a></dt>
</dl>
</dd>
</dl>
</dd>
<dt><a href="#examples">Example</a></dt>
</dl>
<hr>
<h2><a name="introduction"></a>Introduction</h2>
<code>to_python_converter</code> registers a conversion from objects of a
given C++ type into a Python object.
<h2><a name="classes"></a>Classes</h2>
<h3><a name="to_python_converter-spec"></a>Class template
<code>to_python_converter</code></h3>
<code>to_python_converter</code> adds a wrapper around a static member
function of its second template parameter, handling low-level details
such as insertion into the converter registry.
<table border="1" summary="to_python_converter template parameters">
<caption>
<b><code>to_python_converter</code> template parameters</b><br>
In the table below, <b><code>x</code></b> denotes an object of type
<code>T</code>
</caption>
<tr>
<th>Parameter</th>
<th>Requirements</th>
<th>Description</th>
</tr>
<tr>
<td><code>T</code></td>
<td>
</td>
<td>The C++ type of the source object in the conversion</td>
</tr>
<tr>
<td><code>Conversion</code></td>
<td>
<code>PyObject* p = Conversion::convert(x)</code>,<br>
if <code>p == 0</code>, <code><a href=
"http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>() != 0</code>.</td>
<td>A class type whose static member function <code>convert</code>
does the real work of the conversion.</td>
</tr>
</table>
<h4><a name="to_python_converter-spec-synopsis"></a>Class template
<code>to_python_converter</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
template <class T, class Conversion>
struct to_python_converter
{
to_python_converter();
};
}}
</pre>
<h4><a name="to_python_converter-spec-ctors"></a>Class template
<code>to_python_converter</code> constructor</h4>
<pre>
to_python_converter();
</pre>
<dl class="function-semantics">
<dt><b>Effects:</b> Registers a to_python converter which uses
<code>Conversion::convert()</code> to do its work.</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 placed the corresponding
declarations in <code>"noddy.h"</code>. Because
<code>noddy_NoddyObject</code> is the ultimate trivial extension type,
the example is a bit contrived: it wraps a function for which all
information is contained in the <i>type</i> of its return value.
<h3>C++ module definition</h3>
<pre>
#include <boost/python/reference.hpp>
#include <boost/python/module.hpp>
#include "noddy.h"
struct tag {};
tag make_tag() { return tag(); }
using namespace boost::python;
struct tag_to_noddy
{
static PyObject* convert(tag const& x)
{
return PyObject_New(noddy_NoddyObject, &noddy_NoddyType);
}
};
BOOST_PYTHON_MODULE(to_python_converter)
{
def("make_tag", make_tag);
to_python_converter<tag, tag_to_noddy>();
}
</pre>
<h3>Python code</h3>
<pre>
>>> import to_python_converter
>>> def always_none():
... return None
...
>>> def choose_function(x):
... if (x % 2 != 0):
... return to_python_converter.make_tag
... else:
... return always_none
...
>>> a = [ choose_function(x) for x in range(5) ]
>>> b = [ f() for f in a ]
>>> type(b[0])
<type 'NoneType'>
>>> type(b[1])
<type 'Noddy'>
>>> type(b[2])
<type 'NoneType'>
>>> type(b[3])
<type 'Noddy'>
</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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