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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Class Operators/Special Functions</b></font>
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<a name="python_operators"></a><h2>Python Operators</h2><p>
C is well known for the abundance of operators. C++ extends this to the
extremes by allowing operator overloading. Boost.Python takes advantage of
this and makes it easy to wrap C++ operator-powered classes.</p>
<p>
Consider a file position class <tt>FilePos</tt> and a set of operators that take
on FilePos instances:</p>
<code><pre>
<span class=keyword>class </span><span class=identifier>FilePos </span><span class=special>{ /*...*/ };
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>+(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>+(</span><span class=keyword>int</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span><span class=keyword>int </span><span class=keyword>operator</span><span class=special>-(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span><span class=identifier>FilePos </span><span class=keyword>operator</span><span class=special>-(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>+=(</span><span class=identifier>FilePos</span><span class=special>&amp;, </span><span class=keyword>int</span><span class=special>);
</span><span class=identifier>FilePos</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>-=(</span><span class=identifier>FilePos</span><span class=special>&amp;, </span><span class=keyword>int</span><span class=special>);
</span><span class=keyword>bool </span><span class=keyword>operator</span><span class=special>&lt;(</span><span class=identifier>FilePos</span><span class=special>, </span><span class=identifier>FilePos</span><span class=special>);
</span></pre></code>
<p>
The class and the various operators can be mapped to Python rather easily
and intuitively:</p>
<code><pre>
<span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>FilePos</span><span class=special>&gt;(</span><span class=string>&quot;FilePos&quot;</span><span class=special>)
.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>+ </span><span class=keyword>int</span><span class=special>()) // </span><span class=identifier>__add__
</span><span class=special>.</span><span class=identifier>def</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=identifier>__radd__
</span><span class=special>.</span><span class=identifier>def</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>__sub__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>- </span><span class=keyword>int</span><span class=special>()) // </span><span class=identifier>__sub__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>+= </span><span class=keyword>int</span><span class=special>()) // </span><span class=identifier>__iadd__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>-= </span><span class=identifier>other</span><span class=special>&lt;</span><span class=keyword>int</span><span class=special>&gt;())
.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>self </span><span class=special>&lt; </span><span class=identifier>self</span><span class=special>); // </span><span class=identifier>__lt__
</span></pre></code>
<p>
The code snippet above is very clear and needs almost no explanation at
all. It is virtually the same as the operators' signatures. Just take
note that <tt>self</tt> refers to FilePos object. Also, not every class <tt>T</tt> that
you might need to interact with in an operator expression is (cheaply)
default-constructible. You can use <tt>other&lt;T&gt;()</tt> in place of an actual
<tt>T</tt> instance when writing &quot;self expressions&quot;.</p>
<a name="special_methods"></a><h2>Special Methods</h2><p>
Python has a few more <i>Special Methods</i>. Boost.Python supports all of the
standard special method names supported by real Python class instances. A
similar set of intuitive interfaces can also be used to wrap C++ functions
that correspond to these Python <i>special functions</i>. Example:</p>
<code><pre>
<span class=keyword>class </span><span class=identifier>Rational
</span><span class=special>{ </span><span class=keyword>operator </span><span class=keyword>double</span><span class=special>() </span><span class=keyword>const</span><span class=special>; };
</span><span class=identifier>Rational </span><span class=identifier>pow</span><span class=special>(</span><span class=identifier>Rational</span><span class=special>, </span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>Rational </span><span class=identifier>abs</span><span class=special>(</span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>ostream</span><span class=special>&amp; </span><span class=keyword>operator</span><span class=special>&lt;&lt;(</span><span class=identifier>ostream</span><span class=special>&amp;,</span><span class=identifier>Rational</span><span class=special>);
</span><span class=identifier>class_</span><span class=special>&lt;</span><span class=identifier>Rational</span><span class=special>&gt;()
.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>float_</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) // </span><span class=identifier>__float__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>pow</span><span class=special>(</span><span class=identifier>self</span><span class=special>, </span><span class=identifier>other</span><span class=special>&lt;</span><span class=identifier>Rational</span><span class=special>&gt;)) // </span><span class=identifier>__pow__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>abs</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) // </span><span class=identifier>__abs__
</span><span class=special>.</span><span class=identifier>def</span><span class=special>(</span><span class=identifier>str</span><span class=special>(</span><span class=identifier>self</span><span class=special>)) // </span><span class=identifier>__str__
</span><span class=special>;
</span></pre></code>
<p>
Need we say more?</p>
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<td class="note_box">
<img src="theme/lens.gif"></img> What is the business of <tt>operator&lt;&lt;</tt> <tt>.def(str(self))</tt>?
Well, the method <tt>str</tt> requires the <tt>operator&lt;&lt;</tt> to do its work (i.e.
<tt>operator&lt;&lt;</tt> is used by the method defined by def(str(self)). </td>
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<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
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is provided &quot;as is&quot; without express or implied warranty, and with
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