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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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6
doc/1A_on_performance.qbk
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@ -10,7 +10,7 @@ Technical details aside, the memory layout of `optional<T>` for a generic `T` is
std::aligned_storage_t<sizeof(t), alignof(T)> _storage;
};
Lifetime of the `T` inside `_storage` is manually controlled with placement-`new`s and pseudo-destructor calls. However, for trivial `T`s we use a different way of storage, by simply holding a `T`:
Lifetime of the `T` inside `_storage` is manually controlled with placement-`new`s and pseudo-destructor calls. However, for scalar `T`s we use a different way of storage, by simply holding a `T`:
template <typename T>
class optional
@ -19,7 +19,7 @@ Lifetime of the `T` inside `_storage` is manually controlled with placement-`new
T _storage;
};
We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable type for trivial `T`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 `optional<T>` 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 `optional` as trivial, by specializing type trait `boost::opitonal_config::is_type_trivial`:
We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable type for scalar `T`s. This only works for compilers that support defaulted functions. On compilers without defaulted functions we still use the direct storage, but `optional<T>` 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 `optional` as candidate for optimized storage, by specializing type trait `boost::opitonal_config::optional_uses_direct_storage_for`:
struct X // not trivial
{
@ -28,7 +28,7 @@ We call it a ['direct] storage. This makes `optional<T>` a trivially-copyable ty
namespace boost { namespace optional_config {
template <> struct is_type_trivial<X> : boost::true_type {};
template <> struct optional_uses_direct_storage_for<X> : boost::true_type {};
}}

2
doc/91_relnotes.qbk
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@ -13,7 +13,7 @@
[heading Boost Release 1.66]
* On newer compilers `optional` is now trivially-copyable for trivial `T`s. This uses a different storage (just `T` rather than `aligned_storage`). We require the compiler to support defaulted functions and type traits. Otherwise, we still use the the plain storage for scalar types.
* On newer compilers `optional` is now trivially-copyable for scalar `T`s. This uses a different storage (just `T` rather than `aligned_storage`). We require the compiler to support defaulted functions.
[heading Boost Release 1.63]

5
doc/html/boost_optional/relnotes.html
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@ -33,11 +33,10 @@
</h4>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
On newer compilers <code class="computeroutput"><span class="identifier">optional</span></code>
is now trivially-copyable for trivial <code class="computeroutput"><span class="identifier">T</span></code>s.
is now trivially-copyable for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
This uses a different storage (just <code class="computeroutput"><span class="identifier">T</span></code>
rather than <code class="computeroutput"><span class="identifier">aligned_storage</span></code>).
We require the compiler to support defaulted functions and type traits.
Otherwise, we still use the the plain storage for scalar types.
We require the compiler to support defaulted functions.
</li></ul></div>
<h4>
<a name="boost_optional.relnotes.h1"></a>

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>

2
doc/html/index.html
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@ -146,7 +146,7 @@
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"><p><small>Last revised: October 30, 2017 at 22:34:46 GMT</small></p></td>
<td align="left"><p><small>Last revised: November 04, 2017 at 16:36:14 GMT</small></p></td>
<td align="right"><div class="copyright-footer"></div></td>
</tr></table>
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