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<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
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<title>Deriving a Python Class</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Deriving a Python Class</b></font>
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<p>
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Continuing, we can derive from our base class Base in Python and override
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the virtual function in Python. Before we can do that, we have to set up
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our <tt>class_</tt> wrapper as:</p>
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<code><pre>
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<span class=identifier>class_</span><span class=special><</span><span class=identifier>Base</span><span class=special>, </span><span class=identifier>BaseWrap</span><span class=special>, </span><span class=identifier>boost</span><span class=special>::</span><span class=identifier>noncopyable</span><span class=special>>(</span><span class=string>"Base"</span><span class=special>)
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;
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</span></pre></code>
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<p>
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Otherwise, we have to suppress the Base class' <tt>no_init</tt> by adding an
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<tt>__init__()</tt> method to all our derived classes. <tt>no_init</tt> actually adds
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an <tt>__init__</tt> method that raises a Python RuntimeError exception.</p>
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<code><pre>
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<span class=special>>>> </span><span class=keyword>class </span><span class=identifier>Derived</span><span class=special>(</span><span class=identifier>Base</span><span class=special>):
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... </span><span class=identifier>def </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
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... </span><span class=keyword>return </span><span class=number>42
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</span><span class=special>...
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</span></pre></code>
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<p>
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Cool eh? A Python class deriving from a C++ class!</p>
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<p>
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Let's now make an instance of our Python class <tt>Derived</tt>:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>derived </span><span class=special>= </span><span class=identifier>Derived</span><span class=special>()
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</span></pre></code>
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<p>
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Calling <tt>derived.f()</tt>:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>derived</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
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</span><span class=number>42
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</span></pre></code>
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<p>
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Will yield the expected result. Finally, calling calling the free function
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<tt>call_f</tt> with <tt>derived</tt> as argument:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>derived</span><span class=special>)
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</span><span class=number>42
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</span></pre></code>
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<p>
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Will also yield the expected result.</p>
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<p>
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Here's what's happening:</p>
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<ol><li><tt>call_f(derived)</tt> is called in Python</li><li>This corresponds to <tt>def("call_f", call_f);</tt>. Boost.Python dispatches this call.</li><li><tt>int call_f(Base& b) { return b.f(); }</tt> accepts the call.</li><li>The overridden virtual function <tt>f</tt> of <tt>BaseWrap</tt> is called.</li><li><tt>call_method<int>(self, "f");</tt> dispatches the call back to Python.</li><li><tt>def f(self): return 42</tt> is finally called.</li></ol><table border="0">
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<hr size="1"><p class="copyright">Copyright © 2002-2003 David Abrahams<br>Copyright © 2002-2003 Joel de Guzman<br><br>
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<font size="2">Permission to copy, use, modify, sell and distribute this document
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is granted provided this copyright notice appears in all copies. This document
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is provided "as is" without express or implied warranty, and with
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