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e70bbe4791
Also port some "glaringly obvious" bugfixes from HEAD. Hope it doesn't cause problems. [SVN r35237]
266 lines
8.6 KiB
HTML
266 lines
8.6 KiB
HTML
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - <boost/python/ptr.hpp></title>
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<table border="0" cellpadding="7" cellspacing="0" width="100%" summary=
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"header">
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<tr>
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<td valign="top" width="300">
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<h3><a href="../../../../index.htm"><img height="86" width="277" alt=
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"C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">Header <boost/python/ptr.hpp></h2>
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</table>
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<hr>
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<h2>Contents</h2>
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<dl class="page-index">
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<dt><a href="#introduction">Introduction</a>
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<dt><a href="#functions">Functions</a>
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<dd>
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<dl class="page-index">
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<dt><a href="#ptr-spec">ptr</a>
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</dl>
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<dt><a href="#classes">Classes</a>
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<dd>
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<dl class="page-index">
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<dt><a href="#pointer_wrapper-spec">Class template <code>pointer_wrapper</code></a>
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<dd>
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<dl class="page-index">
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<dt><a href="#pointer_wrapper-spec-synopsis">Class template <code>pointer_wrapper</code> synopsis</a>
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<dt><a href="#pointer_wrapper-spec-types">Class
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<code>pointer_wrapper</code> types</a>
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<dt><a href="#pointer_wrapper-spec-ctors">Class
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<code>pointer_wrapper</code> constructors and destructor</a>
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<dt><a href="#pointer_wrapper-spec-observers">Class
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<code>pointer_wrapper</code> observer functions</a>
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</dl>
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</dl>
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<dt><a href="#metafunctions">Metafunctions</a>
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<dd>
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<dl class="page-index">
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<dt><a href="#is_pointer_wrapper-spec">Class template <code>is_pointer_wrapper</code></a>
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<dd>
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<dl class="page-index">
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<dt><a href="#is_pointer_wrapper-spec-synopsis">Class template <code>is_pointer_wrapper</code> synopsis</a>
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</dl>
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<dt><a href="#unwrap_pointer-spec">Class template <code>unwrap_pointer</code></a>
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<dd>
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<dl class="page-index">
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<dt><a href="#unwrap_pointer-spec-synopsis">Class template <code>unwrap_pointer</code> synopsis</a>
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</dl>
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</dl>
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<dt><a href="#examples">Example(s)</a>
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</dl>
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<hr>
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<h2><a name="introduction"></a>Introduction</h2>
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<p><code><boost/python/ptr.hpp></code> defines the
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<code>ptr()</code> function template, which allows users to
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specify how to convert C++ pointer values to python in the context
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of implementing overridable virtual functions, invoking Python
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callable objects, or explicitly converting C++ objects to
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Python. Normally, when passing pointers to Python callbacks, the
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pointee is copied to ensure that the Python object
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never holds a dangling reference. To specify that the new Python
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object should merely contain a copy of a pointer <code>p</code>,
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the user can pass <code><a href="#ptr-spec">ptr</a>(p)</code> instead of passing
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<code>p</code> directly. This interface is meant to mirror the use
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of <a href="../../../bind/ref.html"><code>boost::ref()</code></a>,
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which can be similarly used to prevent copying of referents.
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<p><code>ptr(p)</code> returns an instance of <code><a
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href="#pointer_wrapper-spec">pointer_wrapper<></a></code>, which
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can be detected using the <code><a
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href="#is_pointer_wrapper-spec">is_pointer_wrapper<></a></code>
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metafunction; <code><a
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href="#unwrap_pointer-spec">unwrap_pointer<></a></code> is a
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metafunction which extracts the original pointer type from a
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<code>pointer_wrapper<></code>. These classes can be thought
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of as implementation details.
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<h2><a name="functions"></a>Functions</h2>
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<pre>
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<a name="ptr-spec">template <class T></a>
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pointer_wrapper<T> ptr(T x);
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</pre>
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<dl class="ptr-semantics">
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<dt><b>Requires:</b> <code>T</code> is a pointer type.
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<dt><b>Returns:</b> <code><a href="#pointer_wrapper-spec">pointer_wrapper</a><T>(x)</code>
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<dt><b>Throws:</b> nothing.
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</dl>
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="pointer_wrapper-spec"></a>Class template <code>pointer_wrapper</code></h3>
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<p>A "type envelope" which is returned by <a
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href="#ptr-spec">ptr()</a>, used to indicate reference semantics
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for pointers passed to Python callbacks.
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<h4><a name="pointer_wrapper-spec-synopsis"></a>Class
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<code>pointer_wrapper</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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template<class Ptr> class pointer_wrapper
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{
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public:
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typedef Ptr type;
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explicit pointer_wrapper(Ptr x);
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operator Ptr() const;
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Ptr get() const;
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};
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}}
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</pre>
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<h4><a name="pointer_wrapper-spec-types"></a>Class template <code>pointer_wrapper</code> types</h4>
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<pre>
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typedef Ptr type;
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</pre>
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The type of the pointer being wrapped.
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<h4><a name="pointer_wrapper-spec-ctors"></a>Class template <code>pointer_wrapper</code> constructors and
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destructor</h4>
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<pre>
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explicit pointer_wrapper(Ptr x);
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> <code>Ptr</code> is a pointer type.
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<dt><b>Effects:</b> Stores <code>x</code> in a the <code>pointer_wrapper<></code>.
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<dt><b>Throws:</b> nothing.
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</dl>
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<h4><a name="pointer_wrapper-spec-observers"></a>Class template <code>pointer_wrapper</code> observer
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functions</h4>
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<pre>
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operator Ptr() const;
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Ptr get() const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Returns:</b> a copy of the stored pointer.
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<dt><b>Rationale:</b> <code>pointer_wrapper</code> is intended
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to be a stand-in for the actual pointer type, but sometimes it's
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better to have an explicit way to retrieve the pointer.
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</dl>
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<h2><a name="metafunctions"></a>Metafunctions</h2>
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<h3><a name="is_pointer_wrapper-spec"></a>Class template <code>is_pointer_wrapper</code></h3>
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<p>A unary metafunction whose <code>value</code> is true iff its
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argument is a <code>pointer_wrapper<></code>.
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<h4><a name="is_pointer_wrapper-spec-synopsis"></a>Class template <code>is_pointer_wrapper</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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template<class T> class is_pointer_wrapper
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{
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static <i>unspecified</i> value = ...;
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};
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}}
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</pre>
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<dl class="metafunction-semantics">
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<dt><b>Returns:</b> <code>true</code> iff <code>T</code> is a
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specialization of
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<code>pointer_wrapper<></code>.
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<dt><code>value</code> is an integral constant convertible to bool of
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unspecified type
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</dl>
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<h3><a name="unwrap_pointer-spec"></a>Class template <code>unwrap_pointer</code></h3>
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A unary metafunction which extracts the wrapped pointer type from a
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specialization of <code>pointer_wrapper<></code>.
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<h4><a name="unwrap_pointer-spec-synopsis"></a>Class template <code>unwrap_pointer</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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template<class T> class unwrap_pointer
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{
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typedef <i>unspecified</i> type;
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};
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}}
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</pre>
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<dl class="metafunction-semantics">
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<dt><b>Returns:</b> <code>T::type</code> if <code>T</code> is a
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specialization of
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<code>pointer_wrapper<></code>, <code>T</code> otherwise
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</dl>
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<h2><a name="examples"></a>Example(s)</h2>
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This example illustrates the use of <code>ptr()</code> to prevent an
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object from being copied:
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<pre>
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#include <boost/python/call.hpp>
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#include <boost/python/ptr.hpp>
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class expensive_to_copy
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{
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...
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};
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void pass_as_arg(expensive_to_copy* x, PyObject* f)
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{
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// call the Python function f, passing a Python object built around
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// which refers to *x by-pointer.
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//
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// *** Note: ensuring that *x outlives the argument to f() is ***
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// *** up to the user! Failure to do so could result in a crash! ***
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boost::python::call<void>(f, ptr(x));
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}
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...
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</pre>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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13 November, 2002
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<!--webbot bot="Timestamp" endspan i-checksum="39359" -->
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<p><i>© Copyright <a href="../../../../people/dave_abrahams.htm">Dave
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Abrahams</a> 2002. </i> Distributed
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under the Boost Software License, Version 1.0. (See accompanying file
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LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)</p>
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