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<p>
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We call it a <span class="emphasis"><em>direct</em></span> storage. This makes <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
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trivially-copyable type for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
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This only works for compilers that support defaulted functions. On compilers
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without defaulted functions we still use the direct storage, but <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
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no longer recognized as trivially-copyable. Apart from scalar types, we leave
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the programmer a way of customizing her type, so that it is reconized by
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<code class="computeroutput"><span class="identifier">optional</span></code> as candidate for
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optimized storage, by specializing type trait <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">opitonal_config</span><span class="special">::</span><span class="identifier">optional_uses_direct_storage_for</span></code>:
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This only works for compilers that support defaulted functions (including
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defaulted move assignment and constructor). On compilers without defaulted
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functions we still use the direct storage, but <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
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is no longer recognized as trivially-copyable. Apart from scalar types, we
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leave the programmer a way of customizing her type, so that it is reconized
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by <code class="computeroutput"><span class="identifier">optional</span></code> as candidate
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for optimized storage, by specializing type trait <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">opitonal_config</span><span class="special">::</span><span class="identifier">optional_uses_direct_storage_for</span></code>:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="comment">// not trivial</span>
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<span class="special">{</span>
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