Added has_value()

This commit is contained in:
Andrzej Krzemienski
2018-03-24 00:04:15 +01:00
parent 33c7a6aa2b
commit 5182f7f30f
49 changed files with 119 additions and 59 deletions

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

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