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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"><</span><span class=
"keyword">int
</span><span class=
"special">></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>:
37 <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>
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"><</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"><</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>
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>
45 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>
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>.
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53 <td align=
"right"><div class=
"copyright-footer">Copyright
© 2003-
2007 Fernando Luis Cacciola Carballal
<br>Copyright
© 2014-
2016 Andrzej Krzemie
ński
<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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