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<title>ObjectWrapper</title>
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<link rel="home" href="../index.html" title="Boost.Python Reference Manual">
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<link rel="up" href="../concepts.html" title="Chapter 1. Concepts">
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<link rel="next" href="../high_level_components.html" title="Chapter 2. High Level Components">
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<table cellpadding="2" width="100%"><tr><td valign="top"><img alt="" width="" height="" src="../../images/boost.png"></td></tr></table>
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<a accesskey="p" href="resultconverter.html"><img src="../../images/prev.png" alt="Prev"></a><a accesskey="u" href="../concepts.html"><img src="../../images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../images/home.png" alt="Home"></a><a accesskey="n" href="../high_level_components.html"><img src="../../images/next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="concepts.objectwrapper"></a><a class="link" href="objectwrapper.html" title="ObjectWrapper">ObjectWrapper</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen">ObjectWrapper
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Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements">TypeWrapper
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Concept Requirements</a></span></dt>
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<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.caveat">Caveat</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.objectwrapper.introduction"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.introduction" title="Introduction">Introduction</a>
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</h3></div></div></div>
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<p>
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This page defines two concepts used to describe classes which manage a
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Python objects, and which are intended to support usage with a Python-like
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syntax.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.objectwrapper.objectwrapper_concept_requiremen"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper
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Concept Requirements</a>
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</h3></div></div></div>
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<p>
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Models of the ObjectWrapper concept have <a class="link" href="../object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object">object</a>
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as a publicly-accessible base class, and are used to supply special construction
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behavior and/or additional convenient functionality through (often templated)
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member functions. Except when the return type R is itself an <a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper</a>,
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a member function invocation of the form
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</p>
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<pre class="programlisting"><span class="identifier">x</span><span class="special">.</span><span class="identifier">some_function</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre>
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<p>
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always has semantics equivalent to:
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</p>
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<pre class="programlisting"><span class="identifier">extract</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"some_function"</span><span class="special">)(</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)))()</span></pre>
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<p>
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(see <a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">caveat</a> below).
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.objectwrapper.typewrapper_concept_requirements"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper
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Concept Requirements</a>
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</h3></div></div></div>
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<p>
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TypeWrapper is a refinement of <a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper</a>
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which is associated with a particular Python type <code class="computeroutput"><span class="identifier">X</span></code>.
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For a given TypeWrapper <code class="computeroutput"><span class="identifier">T</span></code>,
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a valid constructor expression
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</p>
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<pre class="programlisting"><span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre>
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<p>
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builds a new T object managing the result of invoking X with arguments
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corresponding to
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</p>
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<pre class="programlisting"><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)</span></pre>
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<p>
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. When used as arguments to wrapped C++ functions, or as the template parameter
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to <a class="link" href="../to_from_python_type_conversion.html#to_from_python_type_conversion.boost_python_extract_hpp.class_template_extract" title="Class template extract">extract<></a>,
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only instances of the associated Python type will be considered a match.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="concepts.objectwrapper.caveat"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">Caveat</a>
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</h3></div></div></div>
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<p>
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The upshot of the special member function invocation rules when the return
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type is a TypeWrapper is that it is possible for the returned object to
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manage a Python object of an inappropriate type. This is not usually a
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serious problem; the worst-case result is that errors will be detected
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at runtime a little later than they might otherwise be. For an example
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of how this can occur, note that the <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">dict</a>
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member function <code class="computeroutput"><span class="identifier">items</span></code> returns
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an object of type <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">list</a>.
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Now suppose the user defines this <code class="computeroutput"><span class="identifier">dict</span></code>
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subclass in Python:
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</p>
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<pre class="programlisting"><span class="special">>>></span> <span class="keyword">class</span> <span class="identifier">mydict</span><span class="special">(</span><span class="identifier">dict</span><span class="special">):</span>
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<span class="special">...</span> <span class="identifier">def</span> <span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
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<span class="special">...</span> <span class="keyword">return</span> <span class="identifier">tuple</span><span class="special">(</span><span class="identifier">dict</span><span class="special">.</span><span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="special">#</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">tuple</span>
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</pre>
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<p>
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Since an instance of <code class="computeroutput"><span class="identifier">mydict</span></code>
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is also an instance of <code class="computeroutput"><span class="identifier">dict</span></code>,
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when used as an argument to a wrapped C++ function, <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">boost::python::dict</a>
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can accept objects of Python type <code class="computeroutput"><span class="identifier">mydict</span></code>.
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Invoking <code class="computeroutput"><span class="identifier">items</span><span class="special">()</span></code>
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on this object can result in an instance of <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">boost::python::list</a>
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which actually holds a Python <code class="computeroutput"><span class="identifier">tuple</span></code>.
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Subsequent attempts to use <code class="computeroutput"><span class="identifier">list</span></code>
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methods (e.g. <code class="computeroutput"><span class="identifier">append</span></code>, or
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any other mutating operation) on this object will raise the same exception
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that would occur if you tried to do it from Python.
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</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>
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</p>
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</div></td>
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</tr></table>
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<hr>
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