1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Moved-from optional</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="../gotchas.html" title="Gotchas"> 9<link rel="prev" href="../gotchas.html" title="Gotchas"> 10<link rel="next" href="mixed_relational_comparisons.html" title="Mixed relational comparisons"> 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="../gotchas.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.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="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="boost_optional.tutorial.gotchas.moved_from__optional_"></a><a class="link" href="moved_from__optional_.html" title="Moved-from optional">Moved-from 28 <code class="computeroutput"><span class="identifier">optional</span></code></a> 29</h4></div></div></div> 30<p> 31 When an optional object that contains a value is moved from (is a source 32 of move constructor or assignment) it still contains a value and its contained 33 value is left in a moved-from state. This can be illustrated with the following 34 example. 35 </p> 36<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opi</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_unique</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="number">1</span><span class="special">)};</span> 37<span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opj</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">opi</span><span class="special">);</span> 38<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">opi</span><span class="special">);</span> 39<span class="identifier">assert</span> <span class="special">(*</span><span class="identifier">opi</span> <span class="special">==</span> <span class="keyword">nullptr</span><span class="special">);</span> 40</pre> 41<p> 42 Quite a lot of people expect that when an object that contains a value 43 is moved from, its contained value should be destroyed. This is not so, 44 for performance reasons. Current semantics allow the implementation of 45 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">opiotnal</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> 46 to be trivially copyable when <code class="computeroutput"><span class="identifier">T</span></code> 47 is trivial. 48 </p> 49</div> 50<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 51<td align="left"></td> 52<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p> 53 Distributed under the Boost Software License, Version 1.0. (See accompanying 54 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>) 55 </p> 56</div></td> 57</tr></table> 58<hr> 59<div class="spirit-nav"> 60<a accesskey="p" href="../gotchas.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.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="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 61</div> 62</body> 63</html> 64
