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- <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>
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- <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>
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- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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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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