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<div class="chapter">
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<div><h2 class="title">
<a name="optional"></a>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; 2014-2018 Andrzej Krzemie&#324;ski</p></div>
<div><div class="legalnotice">
<a name="optional.legal"></a><p>
        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>)
      </p>
</div></div>
</div></div>
<div class="toc">
<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/quick_start.html">Quick Start</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional
      return values</a></span></dt>
<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional
      automatic variables</a></span></dt>
<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional
      data members</a></span></dt>
<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing
      unnecessary default construction</a></span></dt>
<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage
      in containers</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to
      use Optional</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational
      operators</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional
      references</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place
      Factories</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception
      Safety Guarantees</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance
      considerations</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header
      &lt;boost/none.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
      &lt;boost/optional/bad_optional_access.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header &lt;boost/optional/optional_io.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
      &lt;boost/optional/optional_fwd.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
      &lt;boost/optional/optional.hpp&gt;</a></span></dt>
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
      &lt;boost/optional.hpp&gt;</a></span></dt>
</dl></dd>
<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.dependencies">Dependencies</a></span></dt>
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
      operations in older compilers</a></span></dt>
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional
      Reference Binding</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt>
<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</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>
<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>
<h4>
<a name="optional.introduction.h0"></a>
      <span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="index.html#optional.introduction.problem">Problem</a>
    </h4>
<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 differently. 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 just 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>
<h4>
<a name="optional.introduction.h1"></a>
      <span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="index.html#optional.introduction.solution">Solution</a>
    </h4>
<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: December 10, 2019 at 00:22:19 GMT</small></p></td>
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