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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../../../boost.css">
<title>Boost.Python - CallPolicies Concept</title>
</head>
<body link="#0000ff" vlink="#800080">
<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="../../../../c++boost.gif" border="0"></a></h3>
</td>
<td valign="top">
<h1 align="center">Boost.Python</h1>
<h2 align="center">CallPolicies Concept</h2>
</td>
</tr>
</table>
<hr>
<dl class="page-index">
<dt><a href="#introduction">Introduction</a></dt>
<dt><a href="#concept-requirements">Concept Requirements</a></dt>
<dl class="page-index">
<dt><a href="#CallPolicies-concept">CallPolicies Concept</a></dt>
</dl>
</dl>
<h2><a name="introduction"></a>Introduction</h2>
<p>Models of the CallPolicies concept are used to specialize the
behavior of Python callable objects generated by Boost.Python to
wrapped C++ objects like function and member function
pointers, providing three behaviors:
<ol>
<li> <code>precall</code> - Python argument tuple management before
the wrapped object is invoked
<li> <code>result_converter</code> - C++ return value handling
<li> <code>postcall</code> - Python argument tuple and result
management after the wrapped object is invoked
</ol>
<h2><a name="concept-requirements"></a>Concept Requirements</h2>
<h3><a name="CallPolicies-concept"></a>CallPolicies Concept</h3>
<p>In the table below, <code><b>x</b></code> denotes an object whose
type <code><b>P</b></code> is a model of CallPolicies,
<code><b>a</b></code> denotes a <code>PyObject*</code> pointing to
a Python argument tuple object, and <code><b>r</b></code> denotes a
<code>PyObject*</code> referring to a &quot;preliminary&quot; result
object.
<table summary="CallPolicies expressions" border="1" cellpadding="5">
<tr>
<td><b>Expression</b></td>
<td><b>Type</b></td>
<td><b>Result/Semantics</b></td>
</tr>
<tr>
<td valign="top"><code>x.precall(a)</code></td>
<td>convertible to <code>bool</code>
<td>returns <code>false</code> and <code><a
href="http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>()&nbsp;!=&nbsp;0</code>
upon failure, <code>true</code> otherwise.
<tr>
<td valign="top"><code>P::result_converter</code></td>
<td>A model of <a href="ResultConverter.html#ResultConverterGenerator-concept">ResultConverterGenerator</a>.
<td>An MPL unarymetafunction object used produce the
&quot;preliminary&quot result object.
<tr>
<td valign="top"><code>x.postcall(a, r)</code></td>
<td>convertible to <code>PyObject*</code>
<td>0 <code>0</code> and <code><a
href="http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>()&nbsp;!=&nbsp;0</code>
upon failure. Must &quot;conserve references&quot; even in the
event of an exception. In other words, if <code>r</code> is not
returned, its reference count must be decremented; if another
existing object is returned, its reference count must be
incremented.
</table>
Models of CallPolicies are required to be <a
href="../../../utility/CopyConstructible.html">CopyConstructible</a>.
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
19 May, 2002
<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
</p>
<p><i>&copy; Copyright <a href="../../../../people/dave_abrahams.htm">Dave
Abrahams</a> 2002. All Rights Reserved.</i>
<p>Permission to copy, use, modify, sell
and distribute this software is granted provided this copyright notice appears
in all copies. This software is provided "as is" without express or implied
warranty, and with no claim as to its suitability for any purpose.
</body>
</html>