ceph/src/boost/libs/optional/doc/html/boost_optional/quick_start/bypassing_unnecessary_default_construction.html

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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_optional.quick_start.bypassing_unnecessary_default_construction"></a><a class="link" href="bypassing_unnecessary_default_construction.html" title="Bypassing unnecessary default construction">Bypassing
  28       unnecessary default construction</a>
  29 </h3></div></div></div>
  30 <p>
  31         Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
  32         which does not have a default constructor: there is no good candidate for
  33         a default date. We have a function that returns two dates in form of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code>:
  34       </p>
  35 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">&gt;</span> <span class="identifier">getPeriod</span><span class="special">();</span>
  36 </pre>
  37 <p>
  38         In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
  39         but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
  40         and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
  41         something like 'multiple return values':
  42       </p>
  43 <pre class="programlisting"><span class="identifier">Date</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span> <span class="comment">// Error: no default ctor!</span>
  44 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
  45 </pre>
  46 <p>
  47         The second line works already, this is the capability of <a href="../../../../../tuple/index.html" target="_top">Boost.Tuple</a>
  48         library, but the first line won't work. We could set some invented initial
  49         dates, but it is confusing and may be an unacceptable cost, given that these
  50         values will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
  51       </p>
  52 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">Date</span><span class="special">&gt;</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span>
  53 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
  54 </pre>
  55 <p>
  56         It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
  57         move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
  58         invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>,
  59         which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
  60       </p>
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  64 <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>
  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>)
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