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<title>Class Properties</title>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="tutorial.class_properties"></a>Class Properties</h3></div></div></div>
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<p>
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In C++, classes with public data members are usually frowned upon. Well designed
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classes that take advantage of encapsulation hide the class' data members.
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The only way to access the class' data is through access (getter/setter)
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functions. Access functions expose class properties. Here's an example:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Num</span>
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<span class="special">{</span>
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<span class="identifier">Num</span><span class="special">();</span>
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<span class="keyword">float</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span><span class="special">;</span>
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<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">);</span>
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<span class="special">...</span>
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<span class="special">};</span>
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</pre>
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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 attributes
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can just be a different syntax for a method call. Wrapping our <code class="literal">Num</code>
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class using Boost.Python:
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</p>
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<pre class="programlisting"><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">)</span>
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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="special">&</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">get</span><span class="special">)</span>
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<span class="special">.</span><span class="identifier">add_property</span><span class="special">(</span><span class="string">"value"</span><span class="special">,</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="special">&</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">set</span><span class="special">);</span>
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</pre>
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<p>
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And at last, in Python:
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</p>
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<pre class="programlisting"><span class="special">>>></span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">Num</span><span class="special">()</span>
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<span class="special">>>></span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">value</span> <span class="special">=</span> <span class="number">3.14</span>
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<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</span>
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<span class="special">(</span><span class="number">3.14</span><span class="special">,</span> <span class="number">3.14</span><span class="special">)</span>
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<span class="special">>>></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="comment"># error!</span>
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</pre>
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<p>
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Take note that the class property <code class="literal">rovalue</code> is exposed as
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<span class="bold"><strong>read-only</strong></span> since the <code class="literal">rovalue</code>
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setter member function is not passed in:
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</p>
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<pre class="programlisting"><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="special">&</span><span class="identifier">Num</span><span class="special">::</span><span class="identifier">get</span><span class="special">)</span>
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</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2002-2005 Joel
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de Guzman, David Abrahams<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at [@http://www.boost.org/LICENSE_1_0.txt]
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