boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Quick Start</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="up" href="../index.html" title="Boost.Optional"> <link rel="prev" href="../index.html" title="Boost.Optional"> <link rel="next" href="quick_start/optional_automatic_variables.html" title="Optional automatic variables"> </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="p" href="../index.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="quick_start/optional_automatic_variables.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a> </h2></div></div></div> <div class="toc"><dl class="toc"> <dt><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional return values</a></dt> <dt><a href="quick_start/optional_automatic_variables.html">Optional automatic variables</a></dt> <dt><a href="quick_start/optional_data_members.html">Optional data members</a></dt> <dt><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing unnecessary default construction</a></dt> <dt><a href="quick_start/storage_in_containers.html">Storage in containers</a></dt> </dl></div> <div class="section"> <div class="titlepage"><div><div><h3 class="title"> <a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values" title="Optional return values">Optional return values</a> </h3></div></div></div> Let's write and use a converter function that converts a <code class="computeroutput">std::string</code> to an <code class="computeroutput">int</code>. It is possible that for a given string (e.g. <code class="computeroutput">"cat"</code>) there exists no value of type <code class="computeroutput">int</code> capable of representing the conversion result. We do not consider such situation an error. We expect that the converter can be used only to check if the conversion is possible. A natural signature for this function can be: <pre class="programlisting">#include <boost/optional.hpp> boost::optional<int> convert(const std::string& text); </pre> All necessary functionality can be included with one header <code class="computeroutput"><boost/optional.hpp></code>. The above function signature means that the function can either return a value of type <code class="computeroutput">int</code> or a flag indicating that no value of <code class="computeroutput">int</code> is available. This does not indicate an error. It is like one additional value of <code class="computeroutput">int</code>. This is how we can use our function: <pre class="programlisting">const std::string& text = /*... */; boost::optional<int> oi = convert(text); // move-construct if (oi) // contextual conversion to bool int i = *oi; // operator* </pre> In order to test if <code class="computeroutput">optional</code> contains a value, we use the contextual conversion to type <code class="computeroutput">bool</code>. Because of this we can combine the initialization of the optional object and the test into one instruction: <pre class="programlisting">if (boost::optional<int> oi = convert(text)) int i = *oi; </pre> We extract the contained value with <code class="computeroutput">operator*</code> (and with <code class="computeroutput">operator-></code> where it makes sense). An attempt to extract the contained value of an uninitialized optional object is an undefined behaviour (UB). This implementation guards the call with <code class="computeroutput">BOOST_ASSERT</code>. Therefore you should be sure that the contained value is there before extracting. For instance, the following code is reasonably UB-safe: <pre class="programlisting">int i = *convert("100"); </pre> This is because we know that string value <code class="computeroutput">"100"</code> converts to a valid value of <code class="computeroutput">int</code>. If you do not like this potential UB, you can use an alternative way of extracting the contained value: <pre class="programlisting">try { int j = convert(text).value(); } catch (const boost::bad_optional_access&) { // deal with it } </pre> This version throws an exception upon an attempt to access a non-existent contained value. If your way of dealing with the missing value is to use some default, like <code class="computeroutput">0</code>, there exists a yet another alternative: <pre class="programlisting">int k = convert(text).value_or(0); </pre> This uses the <code class="computeroutput">atoi</code>-like approach to conversions: if <code class="computeroutput">text</code> does not represent an integral number just return <code class="computeroutput">0</code>. Finally, you can provide a callback to be called when trying to access the contained value fails: <pre class="programlisting">int fallback_to_default() { cerr << "could not convert; using -1 instead" << endl; return -1; } int l = convert(text).value_or_eval(fallback_to_default); </pre> This will call the provided callback and return whatever the callback returns. The callback can have side effects: they will only be observed when the optional object does not contain a value. Now, let's consider how function <code class="computeroutput">convert</code> can be implemented. <pre class="programlisting">boost::optional<int> convert(const std::string& text) { std::stringstream s(text); int i; if ((s >> i) && s.get() == std::char_traits<char>::eof()) return i; else return boost::none; } </pre> Observe the two return statements. <code class="computeroutput">return i</code> uses the converting constructor that can create <code class="computeroutput">optional<T></code> from <code class="computeroutput">T</code>. Thus constructed optional object is initialized and its value is a copy of <code class="computeroutput">i</code>. The other return statement uses another converting constructor from a special tag <code class="computeroutput">boost::none</code>. It is used to indicate that we want to create an uninitialized optional object. </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal Copyright © 2014 Andrzej Krzemieński 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>) </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="../index.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="quick_start/optional_automatic_variables.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>