1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Boost.Optional</title> 5<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="index.html" title="Boost.Optional"> 8<link rel="next" href="boost_optional/quick_start.html" title="Quick Start"> 9</head> 10<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 11<table cellpadding="2" width="100%"><tr> 12<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 13<td align="center"><a href="../../../../index.html">Home</a></td> 14<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 15<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 16<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 17<td align="center"><a href="../../../../more/index.htm">More</a></td> 18</tr></table> 19<hr> 20<div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> 21<div class="chapter"> 22<div class="titlepage"><div> 23<div><h2 class="title"> 24<a name="optional"></a>Boost.Optional</h2></div> 25<div><div class="author"><h3 class="author"> 26<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span> 27</h3></div></div> 28<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div> 29<div><p class="copyright">Copyright © 2014-2018 Andrzej Krzemieński</p></div> 30<div><div class="legalnotice"> 31<a name="optional.legal"></a><p> 32 Distributed under the Boost Software License, Version 1.0. (See accompanying 33 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>) 34 </p> 35</div></div> 36</div></div> 37<div class="toc"> 38<p><b>Table of Contents</b></p> 39<dl class="toc"> 40<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt> 41<dt><span class="section"><a href="boost_optional/quick_start.html">Quick Start</a></span></dt> 42<dd><dl> 43<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional 44 return values</a></span></dt> 45<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional 46 automatic variables</a></span></dt> 47<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional 48 data members</a></span></dt> 49<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing 50 unnecessary default construction</a></span></dt> 51<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage 52 in containers</a></span></dt> 53</dl></dd> 54<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt> 55<dd><dl> 56<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt> 57<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt> 58<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to 59 use Optional</a></span></dt> 60<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational 61 operators</a></span></dt> 62<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt> 63<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional 64 references</a></span></dt> 65<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place 66 Factories</a></span></dt> 67<dt><span class="section"><a href="boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt> 68<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception 69 Safety Guarantees</a></span></dt> 70<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt> 71<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance 72 considerations</a></span></dt> 73</dl></dd> 74<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt> 75<dd><dl> 76<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header 77 <boost/none.hpp></a></span></dt> 78<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header 79 <boost/optional/bad_optional_access.hpp></a></span></dt> 80<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt> 81<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header 82 <boost/optional/optional_fwd.hpp></a></span></dt> 83<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header 84 <boost/optional/optional.hpp></a></span></dt> 85<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header 86 <boost/optional.hpp></a></span></dt> 87</dl></dd> 88<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies 89 and Portability</a></span></dt> 90<dd><dl> 91<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt> 92<dt><span class="section"><a href="boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace 93 operations in older compilers</a></span></dt> 94<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional 95 Reference Binding</a></span></dt> 96</dl></dd> 97<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt> 98<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</a></span></dt> 99</dl> 100</div> 101<div class="section"> 102<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 103<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a> 104</h2></div></div></div> 105<p> 106 Class template <code class="computeroutput"><span class="identifier">optional</span></code> is 107 a wrapper for representing 'optional' (or 'nullable') objects who may not (yet) 108 contain a valid value. Optional objects offer full value semantics; they are 109 good for passing by value and usage inside STL containers. This is a header-only 110 library. 111 </p> 112<h4> 113<a name="optional.introduction.h0"></a> 114 <span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="index.html#optional.introduction.problem">Problem</a> 115 </h4> 116<p> 117 Suppose we want to read a parameter form a config file which represents some 118 integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>. 119 It is possible that this parameter is not specified; such situation is no error. 120 It is valid to not specify the parameter and in that case the program is supposed 121 to behave slightly differently. Also, suppose that any possible value of type 122 <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 123 <code class="computeroutput"><span class="special">-</span><span class="number">1</span></code> 124 to represent the absence of the parameter in the config file. 125 </p> 126<h4> 127<a name="optional.introduction.h1"></a> 128 <span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="index.html#optional.introduction.solution">Solution</a> 129 </h4> 130<p> 131 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>: 132 </p> 133<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> 134 135<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> 136 137<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> 138<span class="special">{</span> 139 <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> 140 <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> 141 <span class="keyword">else</span> 142 <span class="identifier">runWithNoMax</span><span class="special">();</span> 143<span class="special">}</span> 144</pre> 145</div> 146</div> 147<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 148<td align="left"><p><small>Last revised: August 11, 2020 at 14:59:48 GMT</small></p></td> 149<td align="right"><div class="copyright-footer"></div></td> 150</tr></table> 151<hr> 152<div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> 153</body> 154</html> 155
