<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <meta name="generator" content="HTML Tidy, see www.w3.org"> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" type="text/css" href="../boost.css"> <title>Boost.Python - <boost/python/ptr.hpp></title> <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 valign="top"> <h1 align="center"><a href="../index.html">Boost.Python</a></h1> <h2 align="center">Header <boost/python/ptr.hpp></h2> </table> <hr> <h2>Contents</h2> <dl class="page-index"> <dt><a href="#introduction">Introduction</a> <dt><a href="#functions">Functions</a> <dd> <dl class="page-index"> <dt><a href="#ptr-spec">ptr</a> </dl> <dt><a href="#classes">Classes</a> <dd> <dl class="page-index"> <dt><a href="#pointer_wrapper-spec">Class template <code>pointer_wrapper</code></a> <dd> <dl class="page-index"> <dt><a href="#pointer_wrapper-spec-synopsis">Class template <code>pointer_wrapper</code> synopsis</a> <dt><a href="#pointer_wrapper-spec-types">Class <code>pointer_wrapper</code> types</a> <dt><a href="#pointer_wrapper-spec-ctors">Class <code>pointer_wrapper</code> constructors and destructor</a> <dt><a href="#pointer_wrapper-spec-observers">Class <code>pointer_wrapper</code> observer functions</a> </dl> </dl> <dt><a href="#metafunctions">Metafunctions</a> <dd> <dl class="page-index"> <dt><a href="#is_pointer_wrapper-spec">Class template <code>is_pointer_wrapper</code></a> <dd> <dl class="page-index"> <dt><a href="#is_pointer_wrapper-spec-synopsis">Class template <code>is_pointer_wrapper</code> synopsis</a> </dl> <dt><a href="#unwrap_pointer-spec">Class template <code>unwrap_pointer</code></a> <dd> <dl class="page-index"> <dt><a href="#unwrap_pointer-spec-synopsis">Class template <code>unwrap_pointer</code> synopsis</a> </dl> </dl> <dt><a href="#examples">Example(s)</a> </dl> <hr> <h2><a name="introduction"></a>Introduction</h2> <p><code><boost/python/ptr.hpp></code> defines the <code>ptr()</code> function template, which allows users to specify how to convert C++ pointer values to python in the context of implementing overridable virtual functions, invoking Python callable objects, or explicitly converting C++ objects to Python. Normally, when passing pointers to Python callbacks, the pointee is copied to ensure that the Python object never holds a dangling reference. To specify that the new Python object should merely contain a copy of a pointer <code>p</code>, the user can pass <code><a href="#ptr-spec">ptr</a>(p)</code> instead of passing <code>p</code> directly. This interface is meant to mirror the use of <a href="../../../bind/ref.html"><code>boost::ref()</code></a>, which can be similarly used to prevent copying of referents. <p><code>ptr(p)</code> returns an instance of <code><a href="#pointer_wrapper-spec">pointer_wrapper<></a></code>, which can be detected using the <code><a href="#is_pointer_wrapper-spec">is_pointer_wrapper<></a></code> metafunction; <code><a href="#unwrap_pointer-spec">unwrap_pointer<></a></code> is a metafunction which extracts the original pointer type from a <code>pointer_wrapper<></code>. These classes can be thought of as implementation details. <h2><a name="functions"></a>Functions</h2> <pre> <a name="ptr-spec">template <class T></a> pointer_wrapper<T> ptr(T x); </pre> <dl class="ptr-semantics"> <dt><b>Requires:</b> <code>T</code> is a pointer type. <dt><b>Returns:</b> <code><a href="#pointer_wrapper-spec">pointer_wrapper</a><T>(x)</code> <dt><b>Throws:</b> nothing. </dl> <h2><a name="classes"></a>Classes</h2> <h3><a name="pointer_wrapper-spec"></a>Class template <code>pointer_wrapper</code></h3> <p>A "type envelope" which is returned by <a href="#ptr-spec">ptr()</a>, used to indicate reference semantics for pointers passed to Python callbacks. <h4><a name="pointer_wrapper-spec-synopsis"></a>Class <code>pointer_wrapper</code> synopsis</h4> <pre> namespace boost { namespace python { template<class Ptr> class pointer_wrapper { public: typedef Ptr type; explicit pointer_wrapper(Ptr x); operator Ptr() const; Ptr get() const; }; }} </pre> <h4><a name="pointer_wrapper-spec-types"></a>Class template <code>pointer_wrapper</code> types</h4> <pre> typedef Ptr type; </pre> The type of the pointer being wrapped. <h4><a name="pointer_wrapper-spec-ctors"></a>Class template <code>pointer_wrapper</code> constructors and destructor</h4> <pre> explicit pointer_wrapper(Ptr x); </pre> <dl class="function-semantics"> <dt><b>Requires:</b> <code>Ptr</code> is a pointer type. <dt><b>Effects:</b> Stores <code>x</code> in a the <code>pointer_wrapper<></code>. <dt><b>Throws:</b> nothing. </dl> <h4><a name="pointer_wrapper-spec-observers"></a>Class template <code>pointer_wrapper</code> observer functions</h4> <pre> operator Ptr() const; Ptr get() const; </pre> <dl class="function-semantics"> <dt><b>Returns:</b> a copy of the stored pointer. <dt><b>Rationale:</b> <code>pointer_wrapper</code> is intended to be a stand-in for the actual pointer type, but sometimes it's better to have an explicit way to retrieve the pointer. </dl> <h2><a name="metafunctions"></a>Metafunctions</h2> <h3><a name="is_pointer_wrapper-spec"></a>Class template <code>is_pointer_wrapper</code></h3> <p>A unary metafunction whose <code>value</code> is true iff its argument is a <code>pointer_wrapper<></code>. <h4><a name="is_pointer_wrapper-spec-synopsis"></a>Class template <code>is_pointer_wrapper</code> synopsis</h4> <pre> namespace boost { namespace python { template<class T> class is_pointer_wrapper { static <i>unspecified</i> value = ...; }; }} </pre> <dl class="metafunction-semantics"> <dt><b>Returns:</b> <code>true</code> iff <code>T</code> is a specialization of <code>pointer_wrapper<></code>. <dt><code>value</code> is an integral constant convertible to bool of unspecified type </dl> <h3><a name="unwrap_pointer-spec"></a>Class template <code>unwrap_pointer</code></h3> A unary metafunction which extracts the wrapped pointer type from a specialization of <code>pointer_wrapper<></code>. <h4><a name="unwrap_pointer-spec-synopsis"></a>Class template <code>unwrap_pointer</code> synopsis</h4> <pre> namespace boost { namespace python { template<class T> class unwrap_pointer { typedef <i>unspecified</i> type; }; }} </pre> <dl class="metafunction-semantics"> <dt><b>Returns:</b> <code>T::type</code> if <code>T</code> is a specialization of <code>pointer_wrapper<></code>, <code>T</code> otherwise </dl> <h2><a name="examples"></a>Example(s)</h2> This example illustrates the use of <code>ptr()</code> to prevent an object from being copied: <pre> #include <boost/python/call.hpp> #include <boost/python/ptr.hpp> class expensive_to_copy { ... }; void pass_as_arg(expensive_to_copy* x, PyObject* f) { // call the Python function f, passing a Python object built around // which refers to *x by-pointer. // // *** Note: ensuring that *x outlives the argument to f() is *** // *** up to the user! Failure to do so could result in a crash! *** boost::python::call<void>(f, ptr(x)); } ... </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><i>© Copyright <a href="http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002. </i> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</p>