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<title>Optional data members</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.quick_start.optional_data_members"></a><a class="link" href="optional_data_members.html" title="Optional data members">Optional
      data members</a>
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<p>
        Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization.
        This is because we do not want to perform an expensive initialization of
        our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at
        all) it is really used. We can do it this way:
      </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</span>
<span class="special">{</span>
  <span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&gt;</span> <span class="identifier">resource_</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
  <span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span>

  <span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&amp;</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">// not thread-safe</span>
  <span class="special">{</span>
    <span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span>
        <span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span>

    <span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span>
  <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
        <code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor
        creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s
        default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code>
        doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>. In function
        <code class="computeroutput"><span class="identifier">getResource</span></code> we first check
        if <code class="computeroutput"><span class="identifier">resource_</span></code> is initialized.
        This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>,
        but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>.
        These two ways are equivalent. Function <code class="computeroutput"><span class="identifier">emplace</span></code>
        initializes the optional in-place by perfect-forwarding the arguments to
        the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>.
        No copy- or move-construction is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code>
        doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
      </p>
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<th align="left">Note</th>
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<tr><td align="left" valign="top"><p>
          Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only
          available on compilers that support rvalue references and variadic templates.
          If your compiler does not support these features and you still need to
          avoid any move-constructions, use <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
          Factories</a>.
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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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