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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Class Virtual Functions</b></font>
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<p>
In this section, we shall learn how to make functions behave
polymorphically through virtual functions. Continuing our example, let us
add a virtual function to our <tt>Base</tt> class:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>Base
</span><span class=special>{
</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>;
};
</span></pre></code>
<p>
Since <tt>f</tt> is a pure virtual function, <tt>Base</tt> is now an abstract
class. Given an instance of our class, the free function <tt>call_f</tt>
calls some implementation of this virtual function in a concrete
derived class:</p>
<code><pre>
<span class=keyword>int </span><span class=identifier>call_f</span><span class=special>(</span><span class=identifier>Base</span><span class=special>& </span><span class=identifier>b</span><span class=special>) { </span><span class=keyword>return </span><span class=identifier>b</span><span class=special>.</span><span class=identifier>f</span><span class=special>(); }
</span></pre></code>
<p>
To allow this function to be implemented in a Python derived class, we
need to create a class wrapper:</p>
<code><pre>
<span class=keyword>struct </span><span class=identifier>BaseWrap </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>PyObject</span><span class=special>* </span><span class=identifier>self_</span><span class=special>)
: </span><span class=identifier>self</span><span class=special>(</span><span class=identifier>self_</span><span class=special>) {}
</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>); }
</span><span class=identifier>PyObject</span><span class=special>* </span><span class=identifier>self</span><span class=special>;
};
</span></pre></code>
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<img src="theme/lens.gif"></img> <b>member function and methods</b><br><br> Python, like
many object oriented languages uses the term <b>methods</b>. Methods
correspond roughly to C++'s <b>member functions</b> </td>
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<p>
Our class wrapper <tt>BaseWrap</tt> is derived from <tt>Base</tt>. Its overridden
virtual member function <tt>f</tt> in effect calls the corresponding method
of the Python object <tt>self</tt>, which is a pointer back to the Python
<tt>Base</tt> object holding our <tt>BaseWrap</tt> instance.</p>
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<td class="note_box">
<img src="theme/note.gif"></img> <b>Why do we need BaseWrap?</b><br><br>
<i>You may ask</i>, "Why do we need the <tt>BaseWrap</tt> derived class? This could
have been designed so that everything gets done right inside of
Base."<br><br>
One of the goals of Boost.Python is to be minimally intrusive on an
existing C++ design. In principle, it should be possible to expose the
interface for a 3rd party library without changing it. To unintrusively
hook into the virtual functions so that a Python override may be called, we
must use a derived class.<br><br>
Note however that you don't need to do this to get methods overridden
in Python to behave virtually when called <i>from</i> <b>Python</b>. The only
time you need to do the <tt>BaseWrap</tt> dance is when you have a virtual
function that's going to be overridden in Python and called
polymorphically <i>from</i> <b>C++</b>. </td>
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<p>
Wrapping <tt>Base</tt> and the free function <tt>call_f</tt>:</p>
<code><pre>
<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>, </span><span class=identifier>no_init</span><span class=special>)
;
</span><span class=identifier>def</span><span class=special>(</span><span class=string>"call_f"</span><span class=special>, </span><span class=identifier>call_f</span><span class=special>);
</span></pre></code>
<p>
Notice that we parameterized the <tt>class_</tt> template with <tt>BaseWrap</tt> as the
second parameter. What is <tt>noncopyable</tt>? Without it, the library will try
to create code for converting Base return values of wrapped functions to
Python. To do that, it needs Base's copy constructor... which isn't
available, since Base is an abstract class.</p>
<p>
In Python, let us try to instantiate our <tt>Base</tt> class:</p>
<code><pre>
<span class=special>>>> </span><span class=identifier>base </span><span class=special>= </span><span class=identifier>Base</span><span class=special>()
</span><span class=identifier>RuntimeError</span><span class=special>: </span><span class=identifier>This </span><span class=keyword>class </span><span class=identifier>cannot </span><span class=identifier>be </span><span class=identifier>instantiated </span><span class=identifier>from </span><span class=identifier>Python
</span></pre></code>
<p>
Why is it an error? <tt>Base</tt> is an abstract class. As such it is advisable
to define the Python wrapper with <tt>no_init</tt> as we have done above. Doing
so will disallow abstract base classes such as <tt>Base</tt> to be instantiated.</p>
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<hr size="1"><p class="copyright">Copyright © 2002-2003 David Abrahams<br>Copyright © 2002-2003 Joel de Guzman<br><br>
<font size="2">Permission to copy, use, modify, sell and distribute this document
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