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  27 <a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
  28       in containers</a>
  29 </h3></div></div></div>
  30 <p>
  31         Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
  32         One of the valid responses is to choose nothing, which is represented by
  33         an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>.
  34         You want to make a histogram showing how many times each choice was made.
  35         You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
  36       </p>
  37 <pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;,</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choices</span><span class="special">;</span>
  38
  39 <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">&lt;</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>
  40   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
  41   <span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
  42 <span class="special">}</span>
  43 </pre>
  44 <p>
  45         This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  46         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
  47         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
  48         of <code class="computeroutput"><span class="identifier">T</span></code>.
  49       </p>
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  53 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
  54         Distributed under the Boost Software License, Version 1.0. (See accompanying
  55         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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