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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="optional.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
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</h2></div></div></div>
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<p>
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Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
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a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
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contain a valid value. Optional objects offer full value semantics; they are
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good for passing by value and usage inside STL containers. This is a header-only
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library.
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</p>
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<h4>
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<a name="optional.introduction.h0"></a>
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<span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="introduction.html#optional.introduction.problem">Problem</a>
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</h4>
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<p>
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Suppose we want to read a parameter form a config file which represents some
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integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
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It is possible that this parameter is not specified; such situation is no error.
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It is valid to not specify the parameter and in that case the program is supposed
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to behave slightly different. Also suppose that any possible value of type
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<code class="computeroutput"><span class="keyword">int</span></code> is a valid value for <code class="computeroutput"><span class="string">"MaxValue"</span></code>, so we cannot jut use <code class="computeroutput"><span class="special">-</span><span class="number">1</span></code> to represent
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the absence of the parameter in the config file.
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</p>
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<h4>
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<a name="optional.introduction.h1"></a>
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<span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="introduction.html#optional.introduction.solution">Solution</a>
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</h4>
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<p>
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This is how you solve it with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>:
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</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="identifier">hpp</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">getConfigParam</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">);</span> <span class="comment">// return either an int or a `not-an-int`</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="keyword">if</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">oi</span> <span class="special">=</span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="string">"MaxValue"</span><span class="special">))</span> <span class="comment">// did I get a real int?</span>
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<span class="identifier">runWithMax</span><span class="special">(*</span><span class="identifier">oi</span><span class="special">);</span> <span class="comment">// use my int</span>
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<span class="keyword">else</span>
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<span class="identifier">runWithNoMax</span><span class="special">();</span>
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<span class="special">}</span>
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</pre>
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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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