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233 lines
7.2 KiB
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<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<html>
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<head>
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<meta name="generator" content=
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"HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - <boost/python/extract.hpp></title>
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</head>
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<body>
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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"
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alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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</td>
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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/extract.hpp></h2>
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</td>
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</tr>
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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></dt>
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<dt><a href="#classes">Classes</a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#extract-spec">Class <code>extract</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#extract-spec-synopsis">Class <code>extract</code>
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synopsis</a></dt>
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<dt><a href="#extract-spec-ctors">Class <code>extract</code>
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constructors and destructor</a></dt>
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<dt><a href="#extract-spec-observers">Class
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<code>extract</code> observer functions</a></dt>
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</dl>
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</dd>
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</dl>
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</dd>
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<dt><a href="#examples">Example</a></dt>
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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>Exposes a mechanism for extracting C++ object values from
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generalized Python objects. Note that
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<code>extract<</code>...<code>></code> can also be used to
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"downcast" an <a
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href="object.html#object-spec">object</a> to some specific <a
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href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>. Because
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invoking a mutable python type with an argument of the same type
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(e.g. <code>list([1,2])</code> typically makes a <em>copy</em> of
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the argument object, this may be the only way to access the <a
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href="ObjectWrapper.html#ObjectWrapper-concept">ObjectWrapper</a>'s
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interface on the original object.
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="extract-spec"></a>Class template <code>extract</code></h3>
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<p><code>extract<T></code> can be used to extract a value of
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an arbitrary C++ type from an instance of <code><a
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href="object.html#object-spec">object</a></code>. Two usages are supported:
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<ol>
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<li><b><code>extract<T>(o)</code></b> is a temporary object
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which is implicitly convertible to <code>T</code> (explicit conversion
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is also available through the object's function-call
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operator). However, if no conversion is available which can convert
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<code>o</code> to an object of type <code>T</code>, a Python
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<code>TypeError</code> exception will be <a
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href="definitions.html#raise">raised</a>.
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<li><b><code>extract<T> x(o);</code></b> constructs an extractor
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whose <code>check()</code> member function can be used to ask whether
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a conversion is available without causing an exception to be thrown.
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</ol>
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<h4><a name="extract-spec-synopsis"></a>Class template <code>extract</code>
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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>
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struct extract
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{
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typedef <i>unspecified</i> result_type;
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extract(PyObject*);
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extract(object const&);
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result_type operator()() const;
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operator result_type() const;
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bool check() const;
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};
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}}
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</pre>
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<h4><a name="extract-spec-ctors"></a>Class <code>extract</code>
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constructors and destructor</h4>
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<pre>
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extract(PyObject* p);
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extract(object const&);
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> The first form requires that <code>p</code> is non-null.</dt>
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<dt><b>Effects:</b>Stores a pointer to the Python object managed
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by its constructor argument. In particular, the reference
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count of the object is not incremented. The onus is on the user
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to be sure it is not destroyed before the extractor's conversion
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function is called.</dt>
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</dl>
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<h4><a name="extract-spec-observers"></a>Class <code>extract</code>
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observer functions</h4>
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<pre>
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result_type operator()() const;
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operator result_type() const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Effects:</b> Converts the stored pointer to
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<code>result_type</code>, which is either <code>T</code> or
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<code>T const&</code>.
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</dt>
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<dt><b>Returns:</b> An object of <code>result_type</code>
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corresponding to the one referenced by the stored pointer.</dt>
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<dt><b>Throws:</b> <code><a
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href="errors.html#error_already_set-spec">error_already_set</a></code>
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and sets a <code>TypeError</code> if no such conversion is
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available. May also emit other unspecified exceptions thrown by
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the converter which is actually used.</dt>
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</dl>
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<pre>
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bool check() const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Postconditions:</b> None. In particular, note that a
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return value of <code>true</code> does not preclude an exception
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being thrown from <code>operator result_type()</code> or
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<code>operator()()</code>.</dt>
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<dt><b>Returns:</b> <code>false</code> <i>only</i> if no conversion from the
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stored pointer to <code>T</code> is available.</dt>
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</dl>
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<h2><a name="examples"></a>Examples</h2>
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<pre>
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#include <cstdio>
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using namespace boost::python;
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int Print(str s)
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{
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// extract a C string from the Python string object
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char const* c_str = extract<char const*>(s);
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// Print it using printf
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std::printf("%s\n", c_str);
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// Get the Python string's length and convert it to an int
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return extract<int>(s.attr("__len__")())
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}
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</pre>
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The following example shows how extract can be used along with
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<code><a
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href="class.html#class_-spec">class_</a><</code>...<code>></code>
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to create and access an instance of a wrapped C++ class.
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<pre>
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struct X
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{
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X(int x) : v(x) {}
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int value() { return v; }
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private:
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int v;
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};
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BOOST_PYTHON_MODULE(extract_ext)
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{
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object x_class(
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class_<X>("X", init<int>())
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.def("value", &X::value))
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;
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// Instantiate an X object through the Python interface.
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// Its lifetime is now managed by x_obj.
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object x_obj = x_class(3);
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// Get a reference to the C++ object out of the Python object
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X& x = extract<X&>(x_obj);
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assert(x.value() == 3);
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}
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</pre>
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<p>Revised 15 November, 2002</p>
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<p><i>© Copyright <a href=
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"http://www.boost.org/people/dave_abrahams.htm">Dave Abrahams</a> 2002.</i></p>
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</body>
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</html>
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