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167 lines
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<link rel="stylesheet" type="text/css" href="../boost.css">
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<title>Boost.Python - <boost/python/from_python.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="../../../../c++boost.gif" 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/from_python.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="#classes">Classes</a>
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<dd>
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<dl class="page-index">
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<dt><a href="#from_python-spec">Class
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Template<code>from_python</code></a>
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<dd>
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<dl class="page-index">
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<dt><a href="#from_python-spec-synopsis">Class Template
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<code>from_python</code> synopsis</a>
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<dt><a href="#from_python-spec-ctors">Class Template
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<code>from_python</code> constructor</a>
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<dt><a href="#from_python-spec-observers">Class Template
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<code>from_python</code> observer functions</a>
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</dl>
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</dl>
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<dt><a href="#examples">Example</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/from_python.hpp></code> introduces a class
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template <code>from_python<T></code> for extracting a C++ object of
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type <code>T</code> from a Python object.
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="from_python-spec"></a>Class Template
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<code>from_python<class T></code></h3>
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<p><code>from_python<T></code> is the type used internally by
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Boost.Python to extract C++ function arguments from a Python argument tuple
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when calling a wrapped function. It can also be used directly to make
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similar conversions in other contexts.
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<h4><a name="from_python-spec-synopsis"></a>Class Template
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<code>from_python</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>
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struct from_python : private <a href=
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"../../../utility/utility.htm#Class noncopyable">boost::noncopyable</a> // Exposition only.
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// from_python<T> meets the <a href=
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"NonCopyable.html">NonCopyable</a> requirements
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{
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from_python(PyObject*);
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bool convertible() const;
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<i>convertible-to-T</i> operator()(PyObject*) const;
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};
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}
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</pre>
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<h4><a name="from_python-spec-ctors"></a>Class Template
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<code>from_python</code> constructor</h4>
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<pre>
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from_python(PyObject* p);
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> <code>p != 0</code>
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<dt><b>Effects:</b> Constructs a <code>from_python</code> object suitable
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for extracting a C++ object of type <code>T</code> from <code>p</code>.
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</dl>
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<h4><a name="from_python-spec-observers"></a>Class Template
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<code>from_python</code> observer functions</h4>
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<pre>
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bool convertible() const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Returns:</b> <code>false</code> if the conversion cannot succeed.
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This indicates that either:
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<dd>
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<ol>
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<li>No <code>from_python_converter</code> was registered for
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<code>T</code>, or
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<li>any such converter rejected the constructor argument
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<code>p</code> by returning <code>0</code> from its
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<code>convertible()</code> function
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</ol>
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Note that conversion may still fail in <code>operator()</code> due to
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an exception.
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<dt><b>Throws:</b> nothing
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<dt><b>Rationale:</b> Because <code>from_python<></code> is used in
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overload resolution, and throwing an exception can be slow, it is useful
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to be able to rule out a broad class of unsuccessful conversions without
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throwing an exception.
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</dl>
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<pre>
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<i>convertible-to-T</i> operator()(PyObject* p) const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> <code>*p</code> refers to the same object which was
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passed to the constructor, and <code>convertible()</code> returns
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<code>true</code>.
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<dt><b>Effects:</b> performs the conversion
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<dt><b>Returns:</b> an object convertible to <code>T</code>.
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</dl>
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<h2><a name="examples"></a>Example</h2>
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<pre>
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#include <string>
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#include <boost/python/from_python.hpp>
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// If a std::string can be extracted from p, return its
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// length. Otherwise, return 0.
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std::size_t length_if_string(PyObject* p)
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{
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from_python<std::string> converter(p);
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if (!converter.convertible())
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return 0;
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else
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return converter(p).size();
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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. All Rights Reserved.</i>
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