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<!-- Generated by the Spirit (http://spirit.sf.net) QuickDoc -->
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<title>Class Properties</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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<td width="85%">
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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Class Properties</b></font>
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<p>
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In C++, classes with public data members are usually frowned
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upon. Well designed classes that take advantage of encapsulation hide
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the class' data members. The only way to access the class' data is
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through access (getter/setter) functions. Access functions expose class
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properties. Here's an example:</p>
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<code><pre>
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<span class=keyword>struct </span><span class=identifier>Num
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</span><span class=special>{
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</span><span class=identifier>Num</span><span class=special>();
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</span><span class=keyword>float </span><span class=identifier>get</span><span class=special>() </span><span class=keyword>const</span><span class=special>;
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</span><span class=keyword>void </span><span class=identifier>set</span><span class=special>(</span><span class=keyword>float </span><span class=identifier>value</span><span class=special>);
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...
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};
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</span></pre></code>
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<p>
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However, in Python attribute access is fine; it doesn't neccessarily break
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encapsulation to let users handle attributes directly, because the
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attributes can just be a different syntax for a method call. Wrapping our
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<tt>Num</tt> class using Boost.Python:</p>
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<code><pre>
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<span class=identifier>class_</span><span class=special><</span><span class=identifier>Num</span><span class=special>>(</span><span class=string>"Num"</span><span class=special>)
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.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>"rovalue"</span><span class=special>, &</span><span class=identifier>Num</span><span class=special>::</span><span class=identifier>get</span><span class=special>)
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.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>"value"</span><span class=special>, &</span><span class=identifier>Num</span><span class=special>::</span><span class=identifier>get</span><span class=special>, &</span><span class=identifier>Num</span><span class=special>::</span><span class=identifier>set</span><span class=special>);
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</span></pre></code>
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<p>
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And at last, in Python:</p>
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<code><pre>
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<span class=special>>>> </span><span class=identifier>x </span><span class=special>= </span><span class=identifier>Num</span><span class=special>()
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>>> </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value </span><span class=special>= </span><span class=number>3.14
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</span><span class=special>>>> </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>value</span><span class=special>, </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>rovalue
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</span><span class=special>(</span><span class=number>3.14</span><span class=special>, </span><span class=number>3.14</span><span class=special>)
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>>> </span><span class=identifier>x</span><span class=special>.</span><span class=identifier>rovalue </span><span class=special>= </span><span class=number>2.17 </span>##<span class=identifier>error</span><span class=special>!
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</span></pre></code>
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<p>
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Take note that the class property <tt>rovalue</tt> is exposed as <b>read-only</b>
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since the <tt>rovalue</tt> setter member function is not passed in:</p>
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<code><pre>
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<span class=special>.</span><span class=identifier>add_property</span><span class=special>(</span><span class=string>"rovalue"</span><span class=special>, &</span><span class=identifier>Num</span><span class=special>::</span><span class=identifier>get</span><span class=special>)
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</span></pre></code>
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<br>
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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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no claim as to its suitability for any purpose. </font> </p>
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