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trivially-copyable optional<T> for trivial T

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Andrzej Krzemienski
2017-10-28 16:51:13 +02:00
parent e95f9fc254
commit 2d2c3c3f6f
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29
doc/1A_on_performance.qbk
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@ -1,7 +1,7 @@

[section Performance considerations]
Technical details aside, the memory layout of `optional<T>` is more-less this:
Technical details aside, the memory layout of `optional<T>` for a generic `T` is more-less this:
template <typename T>
class optional
@ -9,8 +9,33 @@ Technical details aside, the memory layout of `optional<T>` is more-less this:
bool _initialized;
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`:
But for the purpose of this analysis, considering memory layouts, we can think of it as:
template <typename T>
class optional
{
bool _initialized;
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`:
struct X // not trivial
{
X() {}
};
namespace boost { namespace optional_config {
template <> struct is_type_trivial<X> : boost::true_type {};
}}
[heading Controlling the size]
For the purpose of the followin analysis, considering memory layouts, we can think of it as:
template <typename T>
class optional

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@ -11,6 +11,11 @@
[section:relnotes Release Notes]
[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.
[heading Boost Release 1.63]
* Added two new in-place constructors. They work similarly to `emplace()` functions: they initialize the contained value by perfect-forwarding the obtained arguments. One constructor always initializes the contained value, the other based on a boolean condition.
* Syntax `o = {}` now correctly un-initializes optional, just like in `std::optional`.

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@ -28,6 +28,19 @@
</h2></div></div></div>
<h4>
<a name="boost_optional.relnotes.h0"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_66"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_66">Boost
Release 1.66</a>
</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.
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.
</li></ul></div>
<h4>
<a name="boost_optional.relnotes.h1"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_63"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_63">Boost
Release 1.63</a>
</h4>
@ -51,7 +64,7 @@
</li>
</ul></div>
<h4>
<a name="boost_optional.relnotes.h1"></a>
<a name="boost_optional.relnotes.h2"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_62"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_62">Boost
Release 1.62</a>
</h4>
@ -59,7 +72,7 @@
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12179" target="_top">Trac #12179</a>.
</li></ul></div>
<h4>
<a name="boost_optional.relnotes.h2"></a>
<a name="boost_optional.relnotes.h3"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_61"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_61">Boost
Release 1.61</a>
</h4>
@ -102,7 +115,7 @@
</li>
</ul></div>
<h4>
<a name="boost_optional.relnotes.h3"></a>
<a name="boost_optional.relnotes.h4"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_60">Boost
Release 1.60</a>
</h4>
@ -113,7 +126,7 @@
#11203</a>.
</li></ul></div>
<h4>
<a name="boost_optional.relnotes.h4"></a>
<a name="boost_optional.relnotes.h5"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_59">Boost
Release 1.59</a>
</h4>
@ -127,7 +140,7 @@
</li>
</ul></div>
<h4>
<a name="boost_optional.relnotes.h5"></a>
<a name="boost_optional.relnotes.h6"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_58">Boost
Release 1.58</a>
</h4>
@ -163,7 +176,7 @@
</li>
</ul></div>
<h4>
<a name="boost_optional.relnotes.h6"></a>
<a name="boost_optional.relnotes.h7"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_57">Boost
Release 1.57</a>
</h4>
@ -173,7 +186,7 @@
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
</li></ul></div>
<h4>
<a name="boost_optional.relnotes.h7"></a>
<a name="boost_optional.relnotes.h8"></a>
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_56">Boost
Release 1.56</a>
</h4>

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@ -29,7 +29,8 @@
</h3></div></div></div>
<p>
Technical details aside, the memory layout of <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 more-less this:
for a generic <code class="computeroutput"><span class="identifier">T</span></code> is more-less
this:
</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>
<span class="keyword">class</span> <span class="identifier">optional</span>
@ -39,8 +40,50 @@
<span class="special">};</span>
</pre>
<p>
But for the purpose of this analysis, considering memory layouts, we can
think of it as:
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
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>
<span class="keyword">class</span> <span class="identifier">optional</span>
<span class="special">{</span>
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
<span class="special">};</span>
</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>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="comment">// not trivial</span>
<span class="special">{</span>
<span class="identifier">X</span><span class="special">()</span> <span class="special">{}</span>
<span class="special">};</span>
<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="special">}}</span>
</pre>
<h5>
<a name="boost_optional.tutorial.performance_considerations.h0"></a>
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.controlling_the_size"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.controlling_the_size">Controlling
the size</a>
</h5>
<p>
For the purpose of the followin analysis, considering memory layouts, we
can think of it as:
</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>
<span class="keyword">class</span> <span class="identifier">optional</span>
@ -143,7 +186,7 @@
<span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
</p>
<h5>
<a name="boost_optional.tutorial.performance_considerations.h0"></a>
<a name="boost_optional.tutorial.performance_considerations.h1"></a>
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.optional_function_parameters"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.optional_function_parameters">Optional
function parameters</a>
</h5>

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</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: May 17, 2017 at 23:01:39 GMT</small></p></td>
<td align="left"><p><small>Last revised: October 28, 2017 at 14:38:14 GMT</small></p></td>
<td align="right"><div class="copyright-footer"></div></td>
</tr></table>
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