boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Boost.Optional</title> <link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="index.html" title="Boost.Optional"> <link rel="next" href="boost_optional/quick_start.html" title="Quick Start"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> <td align="center"><a href="../../../../index.html">Home</a></td> <td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> <div class="chapter"> <div class="titlepage"><div> <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 Andrzej Krzemieński</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/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> </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.synopsis">Synopsis</a></span></dt> <dt><span class="section"><a href="boost_optional/reference/detailed_semantics.html">Detailed Semantics</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/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 jut 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: September 12, 2014 at 09:54:26 GMT</small></p></td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"><a accesskey="n" href="boost_optional/quick_start.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a></div> </body> </html>