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<title>Bypassing unnecessary default construction</title>
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<title>Optional automatic variables</title>
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<title>Optional data members</title>
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<title>Storage in containers</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="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
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in containers</a>
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</h3></div></div></div>
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<p>
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Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
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One of the valid responses is to choose nothing, which is represented by
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an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
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You want to make a histogram showing how many times each choice was made.
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You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
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</p>
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<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>
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<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>
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<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>
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<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><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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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>
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is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code> whenever
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<code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
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In this case the state of being uninitialized is treated as a yet another
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value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared
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less than any value of <code class="computeroutput"><span class="identifier">T</span></code>.
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So the set of values that type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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can assume is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, -2147483648, -2147483647, ..., -1,
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0, 1, ..., 2147483647} (assuming a 32-bit <code class="computeroutput"><span class="keyword">int</span></code>).
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</p>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<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 <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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