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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 &#169; 2003-2007 Fernando Luis Cacciola Carballal</p></div>
29<div><p class="copyright">Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski</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 &lt;boost/none.hpp&gt;</a></span></dt>
78<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
79 &lt;boost/optional/bad_optional_access.hpp&gt;</a></span></dt>
80<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header &lt;boost/optional/optional_io.hpp&gt;</a></span></dt>
81<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
82 &lt;boost/optional/optional_fwd.hpp&gt;</a></span></dt>
83<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
84 &lt;boost/optional/optional.hpp&gt;</a></span></dt>
85<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
86 &lt;boost/optional.hpp&gt;</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">&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>
134
135<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>
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">&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>
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>
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148<td align="left"><p><small>Last revised: July 02, 2018 at 21:10:01 GMT</small></p></td>
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