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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage |
| in containers</a> |
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| <p> |
| Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>). |
| One of the valid responses is to choose nothing, which is represented by |
| an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>. |
| You want to make a histogram showing how many times each choice was made. |
| You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>: |
| </p> |
| <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">choices</span><span class="special">;</span> |
| |
| <span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span> |
| <span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> |
| is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. In this case |
| the state of being uninitialized is treated as a yet another value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared less than any value |
| of <code class="computeroutput"><span class="identifier">T</span></code>. |
| </p> |
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| Distributed under the Boost Software License, Version 1.0. (See accompanying |
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