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155 lines
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<title>Boost.Optional</title>
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td>
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<td align="center"><a href="../../../../index.html">Home</a></td>
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<div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div>
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<div class="chapter">
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<div class="titlepage"><div>
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<div><h2 class="title">
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<a name="optional"></a>Boost.Optional</h2></div>
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<div><div class="author"><h3 class="author">
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<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
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</h3></div></div>
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<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div>
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<div><p class="copyright">Copyright © 2014-2018 Andrzej Krzemieński</p></div>
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<div><div class="legalnotice">
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<a name="optional.legal"></a><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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</p>
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</div></div>
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</div></div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl class="toc">
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<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
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<dt><span class="section"><a href="boost_optional/quick_start.html">Quick Start</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional
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return values</a></span></dt>
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<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional
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automatic variables</a></span></dt>
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<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional
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data members</a></span></dt>
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<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing
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unnecessary default construction</a></span></dt>
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<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage
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in containers</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to
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use Optional</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational
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operators</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional
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references</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place
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Factories</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception
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Safety Guarantees</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
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<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance
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considerations</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header
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<boost/none.hpp></a></span></dt>
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<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
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<boost/optional/bad_optional_access.hpp></a></span></dt>
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<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
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<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
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<boost/optional/optional_fwd.hpp></a></span></dt>
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<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
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<boost/optional/optional.hpp></a></span></dt>
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<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
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<boost/optional.hpp></a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
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and Portability</a></span></dt>
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<dd><dl>
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<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
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<dt><span class="section"><a href="boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
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operations in older compilers</a></span></dt>
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<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional
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Reference Binding</a></span></dt>
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</dl></dd>
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<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt>
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<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</a></span></dt>
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</dl>
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</div>
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<div class="section">
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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="index.html#optional.introduction" 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="index.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 differently. 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 just use
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<code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
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to represent 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="index.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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</div>
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</div>
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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"><p><small>Last revised: November 08, 2018 at 17:44:53 GMT</small></p></td>
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