1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Optional automatic variables</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="up" href="../quick_start.html" title="Quick Start"> 9<link rel="prev" href="../quick_start.html" title="Quick Start"> 10<link rel="next" href="optional_data_members.html" title="Optional data members"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="optional_data_members.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional 28 automatic variables</a> 29</h3></div></div></div> 30<p> 31 We could write function <code class="computeroutput"><span class="identifier">convert</span></code> 32 in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement: 33 </p> 34<pre class="programlisting"><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">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span> 35<span class="special">{</span> 36 <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">ans</span><span class="special">;</span> 37 <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> 38 <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 39 <span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span> 40 <span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> 41 42 <span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span> 43<span class="special">}</span> 44</pre> 45<p> 46 The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code> 47 creates an unitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s 48 you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> 49 in an indeterminate state. Its state is always well defined. Instruction 50 <code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span> 51 <span class="identifier">i</span></code> initializes the optional object. 52 It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>. 53 In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>, 54 when an assignment from <code class="computeroutput"><span class="identifier">T</span></code> 55 is invoked, it can do two things. If the optional object is not initialized 56 (our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s 57 copy constructor. If the optional object is already initialized, it assigns 58 the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s 59 copy assignment. 60 </p> 61</div> 62<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 63<td align="left"></td> 64<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p> 65 Distributed under the Boost Software License, Version 1.0. (See accompanying 66 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>) 67 </p> 68</div></td> 69</tr></table> 70<hr> 71<div class="spirit-nav"> 72<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="optional_data_members.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 73</div> 74</body> 75</html> 76
