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<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
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<title>Virtual Functions with Default Implementations</title>
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Virtual Functions with Default Implementations</b></font>
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<p>
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Recall that in the <a href="class_virtual_functions.html">
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previous section</a>, we
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wrapped a class with a pure virtual function that we then implemented in
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C++ or Python classes derived from it. Our base class:</p>
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<code><pre>
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<span class=keyword>struct </span><span class=identifier>Base
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</span><span class=special>{
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</span><span class=keyword>virtual </span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() = </span><span class=number>0</span><span class=special>;
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};
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</span></pre></code>
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<p>
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had a pure virtual function <tt>f</tt>. If, however, its member function <tt>f</tt> was
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not declared as pure virtual:</p>
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<code><pre>
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<span class=keyword>struct </span><span class=identifier>Base
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</span><span class=special>{
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</span><span class=keyword>virtual </span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() { </span><span class=keyword>return </span><span class=number>0</span><span class=special>; }
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};
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</span></pre></code>
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<p>
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and instead had a default implementation that returns <tt>0</tt>, as shown above,
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we need to add a forwarding function that calls the <tt>Base</tt> default virtual
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function <tt>f</tt> implementation:</p>
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<code><pre>
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<span class=keyword>struct </span><span class=identifier>BaseWrap </span><span class=special>: </span><span class=identifier>Base
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</span><span class=special>{
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</span><span class=identifier>BaseWrap</span><span class=special>(</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self_</span><span class=special>)
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: </span><span class=identifier>self</span><span class=special>(</span><span class=identifier>self_</span><span class=special>) {}
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</span><span class=keyword>int </span><span class=identifier>f</span><span class=special>() { </span><span class=keyword>return </span><span class=identifier>call_method</span><span class=special><</span><span class=keyword>int</span><span class=special>>(</span><span class=identifier>self</span><span class=special>, </span><span class=string>"f"</span><span class=special>); }
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</span><span class=keyword>int </span><span class=identifier>default_f</span><span class=special>() { </span><span class=keyword>return </span><span class=identifier>Base</span><span class=special>::</span><span class=identifier>f</span><span class=special>(); } // <<=== ***</span><span class=identifier>ADDED</span><span class=special>***
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</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self</span><span class=special>;
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};
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</span></pre></code>
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<p>
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Then, Boost.Python needs to keep track of 1) the dispatch function <tt>f</tt> and
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2) the forwarding function to its default implementation <tt>default_f</tt>.
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There's a special <tt>def</tt> function for this purpose. Here's how it is
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applied to our example above:</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=string>"Base"</span><span class=special>)
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.</span><span class=identifier>def</span><span class=special>(</span><span class=string>"f"</span><span class=special>, &</span><span class=identifier>Base</span><span class=special>::</span><span class=identifier>f</span><span class=special>, &</span><span class=identifier>BaseWrap</span><span class=special>::</span><span class=identifier>default_f</span><span class=special>)
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</span></pre></code>
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<p>
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Note that we are allowing <tt>Base</tt> objects to be instantiated this time,
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unlike before where we specifically defined the <tt>class_<Base></tt> with
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<tt>no_init</tt>.</p>
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<p>
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In Python, the results would be as expected:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>base </span><span class=special>= </span><span class=identifier>Base</span><span class=special>()
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>>> </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><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>base.f()</tt>:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>base</span><span class=special>.</span><span class=identifier>f</span><span class=special>()
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</span><span class=number>0
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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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Calling <tt>call_f</tt>, passing in a <tt>base</tt> object:</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>base</span><span class=special>)
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</span><span class=number>0
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</span></pre></code>
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<p>
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Calling <tt>call_f</tt>, passing in a <tt>derived</tt> object:</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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<hr size="1"><p class="copyright">Copyright © 2002-2003 David Abrahams<br>Copyright © 2002-2003 Joel de Guzman<br><br>
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