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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">&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>
<span class="special">{</span>
  <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>
  <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">&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>
    <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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<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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