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  27 <a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional
  28       automatic variables</a>
  29 </h3></div></div></div>
  30 <p>
  31         We could write function <code class="computeroutput"><span class="identifier">convert</span></code>
  32         in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement:
  33       </p>
  34 <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&amp;</span> <span class="identifier">text</span><span class="special">)</span>
  35 <span class="special">{</span>
  36   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">ans</span><span class="special">;</span>
  37   <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>
  38   <span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
  39   <span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">&gt;&gt;</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&amp;&amp;</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">&lt;</span><span class="keyword">char</span><span class="special">&gt;::</span><span class="identifier">eof</span><span class="special">())</span>
  40     <span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
  41
  42   <span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span>
  43 <span class="special">}</span>
  44 </pre>
  45 <p>
  46         The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code>
  47         creates an unitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s
  48         you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
  49         in an indeterminate state. Its state is always well defined. Instruction
  50         <code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span>
  51         <span class="identifier">i</span></code> initializes the optional object.
  52         It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>.
  53         In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>,
  54         when an assignment from <code class="computeroutput"><span class="identifier">T</span></code>
  55         is invoked, it can do two things. If the optional object is not initialized
  56         (our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s
  57         copy constructor. If the optional object is already initialized, it assigns
  58         the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s
  59         copy assignment.
  60       </p>
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  63 <td align="left"></td>
  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>)
  67       </p>
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