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<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="boost_optional/motivation.html">Motivation</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="index.html#optional.introduction.problem">Problem</a></span></dt>
<dt><span class="section"><a href="index.html#optional.introduction.solution">Solution</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial.html#boost_optional.tutorial.bypassing_unnecessary_default_construction">Bypassing
unnecessary default construction</a></span></dt>
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<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_models">The models</a></span></dt>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="index.html#optional.introduction.problem">Problem</a></span></dt>
<dt><span class="section"><a href="index.html#optional.introduction.solution">Solution</a></span></dt>
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<p>
This library can be used to represent 'optional' (or 'nullable') objects that
can be safely passed by value:
Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
contain a valid value. Optional objects offer full value semantics; they are
good for passing by value and usage inside STL containers. This is a header-only
library.
</p>
<pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">readInt</span><span class="special">();</span> <span class="comment">// this function may return either an int or a not-an-int</span>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="optional.introduction.problem"></a><a class="link" href="index.html#optional.introduction.problem" title="Problem">Problem</a>
</h3></div></div></div>
<p>
Suppose we want to read a parameter form a config file which represents some
integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
It is possible that this parameter is not specified; such situation is no
error. It is valid to not specify the parameter and in that case the program
is supposed to behave slightly different. Also suppose that any possible
value of type <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 the absence of the parameter in the config file.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="optional.introduction.solution"></a><a class="link" href="index.html#optional.introduction.solution" title="Solution">Solution</a>
</h3></div></div></div>
<p>
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>:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
<span class="keyword">if</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">oi</span> <span class="special">=</span> <span class="identifier">readInt</span><span class="special">())</span> <span class="comment">// did I get a real int</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"my int is: "</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// use my int</span>
<span class="keyword">else</span>
<span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"I have no int"</span><span class="special">;</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="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>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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>
<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>
<span class="keyword">else</span>
<span class="identifier">runWithNoMax</span><span class="special">();</span>
<span class="special">}</span>
</pre>
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<td align="left"><p><small>Last revised: May 23, 2014 at 14:35:08 GMT</small></p></td>
<td align="left"><p><small>Last revised: June 03, 2014 at 14:35:30 GMT</small></p></td>
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