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Tutorial updates. Added virtual functions with default implementations and reshufled the overloading topics to discuss manual overloading first.

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[SVN r17597]
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Joel de Guzman
2003-02-23 02:23:15 +00:00
parent acdad5caf3
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<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
<title>Deriving a Python Class</title>
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<table width="100%" height="48" border="0" cellspacing="2">
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<td><img src="theme/c%2B%2Bboost.gif">
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Deriving a Python Class</b></font>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="class_virtual_functions.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<p>
Continuing, now, at last, we can even derive from our base class Base in
Python. Before we can do that, we have to set up our <tt>class_</tt> wrapper as:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</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>&gt;(</span><span class=string>&quot;Base&quot;</span><span class=special>)
;
</span></pre></code>
<p>
Otherwise, we have to suppress the Base class' <tt>no_init</tt> by adding an
<tt>__init__()</tt> method to all our derived classes. <tt>no_init</tt> actually adds
an <tt>__init__</tt> method that raises a Python RuntimeError exception.</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=keyword>class </span><span class=identifier>Derived</span><span class=special>(</span><span class=identifier>Base</span><span class=special>):
... </span><span class=identifier>def </span><span class=identifier>f</span><span class=special>(</span><span class=identifier>self</span><span class=special>):
... </span><span class=keyword>return </span><span class=number>42
</span><span class=special>...
</span></pre></code>
<p>
Cool eh? A Python class deriving from a C++ class!</p>
<p>
Let's now make an instance of our Python class <tt>Derived</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>derived </span><span class=special>= </span><span class=identifier>Derived</span><span class=special>()
</span></pre></code>
<p>
Calling <tt>derived.f()</tt>:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>derived</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
</span><span class=number>42
</span></pre></code>
<p>
Will yield the expected result. Finally, calling calling the free function
<tt>call_f</tt> with <tt>derived</tt> as argument:</p>
<code><pre>
<span class=special>&gt;&gt;&gt; </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>derived</span><span class=special>)
</span><span class=number>42
</span></pre></code>
<p>
Will also yield the expected result.</p>
<p>
Here's what's happening:</p>
<ol><li><tt>call_f(derived)</tt> is called in Python</li><li>This corresponds to <tt>def(&quot;call_f&quot;, call_f);</tt>. Boost.Python dispatches this call.</li><li><tt>int call_f(Base&amp; 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&lt;int&gt;(self, &quot;f&quot;);</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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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
<td width="30"><a href="class_virtual_functions.html"><img src="theme/l_arr.gif" border="0"></a></td>
<td width="20"><a href="virtual_functions_with_default_implementations.html"><img src="theme/r_arr.gif" border="0"></a></td>
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<br>
<hr size="1"><p class="copyright">Copyright &copy; 2002-2003 David Abrahams<br>Copyright &copy; 2002-2003 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
is granted provided this copyright notice appears in all copies. This document
is provided &quot;as is&quot; without express or implied warranty, and with
no claim as to its suitability for any purpose. </font> </p>
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