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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="caveat.html" 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/class_list.html" 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/class_list.html" 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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<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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