ceph/src/boost/libs/optional/doc/html/boost_optional/tutorial/gotchas/moved_from__optional_.html

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  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">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;&gt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="number">1</span><span class="special">)};</span>
  37 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  46           to be trivially copyable when <code class="computeroutput"><span class="identifier">T</span></code>
  47           is trivial.
  48         </p>
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  52 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<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>)
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