| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Optional automatic variables</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="../quick_start.html" title="Quick Start"> |
| <link rel="prev" href="../quick_start.html" title="Quick Start"> |
| <link rel="next" href="optional_data_members.html" title="Optional data members"> |
| </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="../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> |
| </div> |
| <div class="section"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional |
| automatic variables</a> |
| </h3></div></div></div> |
| <p> |
| We could write function <code class="computeroutput"><span class="identifier">convert</span></code> |
| in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement: |
| </p> |
| <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> |
| <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">ans</span><span class="special">;</span> |
| <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> |
| <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> |
| <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> |
| <span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> |
| |
| <span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code> |
| creates an unitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s |
| 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> |
| in an indeterminate state. Its state is always well defined. Instruction |
| <code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span> |
| <span class="identifier">i</span></code> initializes the optional object. |
| It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>. |
| 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>, |
| when an assignment from <code class="computeroutput"><span class="identifier">T</span></code> |
| is invoked, it can do two things. If the optional object is not initialized |
| (our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s |
| copy constructor. If the optional object is already initialized, it assigns |
| the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s |
| copy assignment. |
| </p> |
| </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<br>Copyright © 2014, 2015 Andrzej Krzemieński<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></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <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> |
| </div> |
| </body> |
| </html> |
