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Reverted specialization for trivial types

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It caused too many problems. I left only specialiation for scalar types.
I will need to devise clever type traits for reconizing trivial types with working constructor.
This commit is contained in:
Andrzej Krzemienski
2017-11-04 17:42:22 +01:00
parent cbf3cd05af
commit 06dea2cb9b
11 changed files with 293 additions and 123 deletions
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20
doc/html/boost_optional/tutorial/performance_considerations.html
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@ -43,7 +43,7 @@
Lifetime of the <code class="computeroutput"><span class="identifier">T</span></code> inside
<code class="computeroutput"><span class="identifier">_storage</span></code> is manually controlled
with placement-<code class="computeroutput"><span class="keyword">new</span></code>s and pseudo-destructor
calls. However, for trivial <code class="computeroutput"><span class="identifier">T</span></code>s
calls. However, for scalar <code class="computeroutput"><span class="identifier">T</span></code>s
we use a different way of storage, by simply holding a <code class="computeroutput"><span class="identifier">T</span></code>:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
@ -55,15 +55,13 @@
</pre>
<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 trivial <code class="computeroutput"><span class="identifier">T</span></code>s.
This only works for compilers that support defaulted functions and type traits.
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. On compilers that do not fully
support type traits, we still use the direct storage for scalar types, but
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 trivial, 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">is_type_trivial</span></code>:
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>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="comment">// not trivial</span>
<span class="special">{</span>
@ -72,7 +70,7 @@
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">optional_config</span> <span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;&gt;</span> <span class="keyword">struct</span> <span class="identifier">is_type_trivial</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">true_type</span> <span class="special">{};</span>
<span class="keyword">template</span> <span class="special">&lt;&gt;</span> <span class="keyword">struct</span> <span class="identifier">optional_uses_direct_storage_for</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">true_type</span> <span class="special">{};</span>
<span class="special">}}</span>
</pre>