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- <div class="titlepage"><div><div><h3 class="title">
- <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
- unnecessary default construction</a>
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- <p>
- Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
- which does not have a default constructor: there is no good candidate for
- 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>:
- </p>
- <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">></span> <span class="identifier">getPeriod</span><span class="special">();</span>
- </pre>
- <p>
- In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
- but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
- and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
- something like 'multiple return values':
- </p>
- <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>
- <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>
- </pre>
- <p>
- The second line works already, this is the capability of <a href="../../../../../tuple/index.html" target="_top">Boost.Tuple</a>
- library, but the first line won't work. We could set some invented initial
- dates, but it is confusing and may be an unacceptable cost, given that these
- values will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
- </p>
- <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Date</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="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>
- </pre>
- <p>
- It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
- move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
- invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
- which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
- </p>
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- <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 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>)
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