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<title>Chapter&#160;1.&#160;Boost.Optional</title>
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<div><h2 class="title">
<a name="optional"></a>Chapter&#160;1.&#160;Boost.Optional</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
</h3></div></div>
<div><p class="copyright">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal</p></div>
<div><p class="copyright">Copyright &#169; 2014 Andrzej Krzemie&#324;ski</p></div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</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>
</dl></dd>
<dt><span class="section"><a href="boost_optional/tutorial.html">Tutorial</a></span></dt>
<dd><dl>
<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">Development</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>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
<dt><span class="section"><a href="boost_optional/development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/synopsis.html">Synopsis</a></span></dt>
<dt><span class="section"><a href="boost_optional/detailed_semantics.html">Detailed Semantics</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html">Examples</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_return_values">Optional
return values</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_local_variables">Optional
local variables</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.optional_data_members">Optional
data members</a></span></dt>
<dt><span class="section"><a href="boost_optional/examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
expensive unnecessary default construction</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/optional_references.html">Optional references</a></span></dt>
<dt><span class="section"><a href="boost_optional/rebinding_semantics_for_assignment_of_optional_references.html">Rebinding
semantics for assignment of optional references</a></span></dt>
<dt><span class="section"><a href="boost_optional/in_place_factories.html">In-Place Factories</a></span></dt>
<dt><span class="section"><a href="boost_optional/a_note_about_optional_bool_.html">A note about
optional&lt;bool&gt;</a></span></dt>
<dt><span class="section"><a href="boost_optional/exception_safety_guarantees.html">Exception Safety
Guarantees</a></span></dt>
<dt><span class="section"><a href="boost_optional/type_requirements.html">Type requirements</a></span></dt>
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
and Portability</a></span></dt>
<dd><dl><dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.optional_reference_binding">Optional
Reference Binding</a></span></dt></dl></dd>
<dt><span class="section"><a href="boost_optional/acknowledgments.html">Acknowledgments</a></span></dt>
</dl>
</div>
<div class="section">
<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>
</dl></div>
<p>
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>
<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="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: June 03, 2014 at 14:35:30 GMT</small></p></td>
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