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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 © 2003-2007 Fernando Luis Cacciola Carballal</p></div> |
| <div><p class="copyright">Copyright © 2014, 2015 Andrzej Krzemieński</p></div> |
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| <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/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/tutorial/in_place_factories.html">In-Place |
| Factories</a></span></dt> |
| <dt><span class="section"><a href="boost_optional/tutorial/a_note_about_optional_bool_.html">A |
| note about optional<bool></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 |
| <boost/none.hpp></a></span></dt> |
| <dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header |
| <boost/optional/bad_optional_access.hpp></a></span></dt> |
| <dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt> |
| <dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header |
| <boost/optional/optional_fwd.hpp></a></span></dt> |
| <dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header |
| <boost/optional/optional.hpp></a></span></dt> |
| <dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header |
| <boost/optional.hpp></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/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"><</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> |
| |
| <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> |
| |
| <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"><</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> |
| <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> |
| </div> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"><p><small>Last revised: March 13, 2015 at 21:52:15 GMT</small></p></td> |
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