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<title>boost/python/ptr.hpp</title>
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<a name="function_invocation_and_creation.boost_python_ptr_hpp"></a><a class="link" href="boost_python_ptr_hpp.html" title="boost/python/ptr.hpp">boost/python/ptr.hpp</a>
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<div class="toc"><dl class="toc">
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.functions">Functions</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code></a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_t">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
types</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_c">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
constructors and destructor</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_o">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
observer functions</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions">Metafunctions</a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.example">Example</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.introduction"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
<boost/python/ptr.hpp> defines the ptr() 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 p, the
user can pass ptr(p) instead of passing p directly. This interface is meant
to mirror the use of boost::ref(), which can be similarly used to prevent
copying of referents.
</p>
<p>
ptr(p) returns an instance of <a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper" title="Class template pointer_wrapper"><code class="computeroutput"><span class="identifier">pointer_wrapper</span><span class="special"><></span></code></a>,
which can be detected using the <a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_is_pointer_wrappe" title="Class template is_pointer_wrapper"><code class="computeroutput"><span class="identifier">is_pointer_wrapper</span><span class="special"><></span></code></a>
metafunction; <a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_unwrap_pointer" title="Class template unwrap_pointer"><code class="computeroutput"><span class="identifier">unwrap_pointer</span><span class="special"><></span></code></a>
is a metafunction which extracts the original pointer type from a <code class="computeroutput"><span class="identifier">pointer_wrapper</span><span class="special"><></span></code>.
These classes can be thought of as implementation details.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.functions"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.functions" title="Functions">Functions</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="identifier">pointer_wrapper</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">T</span> <span class="identifier">x</span><span class="special">);</span>
</pre>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Requires</span></dt>
<dd><p>
T is a pointer type.
</p></dd>
<dt><span class="term">Returns</span></dt>
<dd><p>
pointer_wrapper<T>(x)
</p></dd>
<dt><span class="term">Throws</span></dt>
<dd><p>
nothing.
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper" title="Class template pointer_wrapper">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code></a>
</h3></div></div></div>
<p>
A "type envelope" which is returned by <code class="computeroutput"><span class="identifier">ptr</span><span class="special">()</span></code>, used to indicate reference semantics
for pointers passed to Python callbacks.
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">python</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Ptr</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">pointer_wrapper</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">typedef</span> <span class="identifier">Ptr</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">explicit</span> <span class="identifier">pointer_wrapper</span><span class="special">(</span><span class="identifier">Ptr</span> <span class="identifier">x</span><span class="special">);</span>
<span class="keyword">operator</span> <span class="identifier">Ptr</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">Ptr</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}}</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_t"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_t" title="Class template pointer_wrapper types">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
types</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">Ptr</span> <span class="identifier">type</span><span class="special">;</span>
</pre>
<p>
The type of the pointer being wrapped.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_c"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_c" title="Class template pointer_wrapper constructors and destructor">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
constructors and destructor</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">explicit</span> <span class="identifier">pointer_wrapper</span><span class="special">(</span><span class="identifier">Ptr</span> <span class="identifier">x</span><span class="special">);</span>
</pre>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Requires</span></dt>
<dd><p>
<code class="computeroutput"><span class="identifier">Ptr</span></code> is a pointer
type
</p></dd>
<dt><span class="term">Effects</span></dt>
<dd><p>
Stores <code class="computeroutput"><span class="identifier">x</span></code> in a the
<code class="computeroutput"><span class="identifier">pointer_wrapper</span><span class="special"><></span></code>.
</p></dd>
<dt><span class="term">Throws</span></dt>
<dd><p>
nothing.
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_o"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.class_template_pointer_wrapper_o" title="Class template pointer_wrapper observer functions">Class
template <code class="computeroutput"><span class="identifier">pointer_wrapper</span></code>
observer functions</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">operator</span> <span class="identifier">Ptr</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">Ptr</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
</pre>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Returns</span></dt>
<dd><p>
a copy of the stored pointer.
</p></dd>
<dt><span class="term">Rationale</span></dt>
<dd><p>
pointer_wrapper 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.
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.metafunctions"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions" title="Metafunctions">Metafunctions</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_is_pointer_wrappe">Class
template <code class="computeroutput"><span class="identifier">is_pointer_wrapper</span></code></a></span></dt>
<dt><span class="section"><a href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_unwrap_pointer">Class
template <code class="computeroutput"><span class="identifier">unwrap_pointer</span></code></a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_is_pointer_wrappe"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_is_pointer_wrappe" title="Class template is_pointer_wrapper">Class
template <code class="computeroutput"><span class="identifier">is_pointer_wrapper</span></code></a>
</h4></div></div></div>
<p>
A unary metafunction whose value is true iff its argument is a pointer_wrapper<>.
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">python</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">is_pointer_wrapper</span>
<span class="special">{</span>
<span class="keyword">static</span> <span class="identifier">unspecified</span> <span class="identifier">value</span> <span class="special">=</span> <span class="special">...;</span>
<span class="special">};</span>
<span class="special">}}</span>
</pre>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Returns</span></dt>
<dd><p>
<code class="computeroutput"><span class="keyword">true</span></code> iff <code class="computeroutput"><span class="identifier">T</span></code> is a specialization of <code class="computeroutput"><span class="identifier">pointer_wrapper</span><span class="special"><></span></code>.
value is an integral constant convertible to bool of unspecified
type
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_unwrap_pointer"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.metafunctions.class_template_unwrap_pointer" title="Class template unwrap_pointer">Class
template <code class="computeroutput"><span class="identifier">unwrap_pointer</span></code></a>
</h4></div></div></div>
<p>
A unary metafunction which extracts the wrapped pointer type from a specialization
of pointer_wrapper<>.
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">python</span>
<span class="special">{</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">unwrap_pointer</span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">unspecified</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}}</span>
</pre>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Returns</span></dt>
<dd><p>
<code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">type</span></code> if <code class="computeroutput"><span class="identifier">T</span></code>
is a specialization of <code class="computeroutput"><span class="identifier">pointer_wrapper</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">T</span></code>
otherwise
</p></dd>
</dl>
</div>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="function_invocation_and_creation.boost_python_ptr_hpp.example"></a><a class="link" href="boost_python_ptr_hpp.html#function_invocation_and_creation.boost_python_ptr_hpp.example" title="Example">Example</a>
</h3></div></div></div>
<p>
This example illustrates the use of ptr() to prevent an object from being
copied:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">call</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">python</span><span class="special">/</span><span class="identifier">ptr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">expensive_to_copy</span>
<span class="special">{</span>
<span class="special">...</span>
<span class="special">};</span>
<span class="keyword">void</span> <span class="identifier">pass_as_arg</span><span class="special">(</span><span class="identifier">expensive_to_copy</span><span class="special">*</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">f</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// call the Python function f, passing a Python object built around</span>
<span class="comment">// which refers to *x by-pointer.</span>
<span class="comment">//</span>
<span class="comment">// *** Note: ensuring that *x outlives the argument to f() is ***</span>
<span class="comment">// *** up to the user! Failure to do so could result in a crash! ***</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">call</span><span class="special"><</span><span class="keyword">void</span><span class="special">>(</span><span class="identifier">f</span><span class="special">,</span> <span class="identifier">ptr</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
<span class="special">}</span>
<span class="special">...</span>
</pre>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
</p>
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