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When allocators aren't equal, use a slow swap.

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[SVN r42102]
This commit is contained in:
Daniel James
2007-12-16 13:17:44 +00:00
parent b542050fa0
commit 0ce212b5aa
5 changed files with 114 additions and 44 deletions
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63
doc/rationale.qbk
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@@ -116,19 +116,64 @@ It isn't clear how to swap containers when their allocators aren't equal.
This is
[@http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#431
Issue 431: Swapping containers with unequal allocators].
Howard Hinnant wrote about this in
[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1599.html N1599]
and suggested swapping both the allocators and the containers' contents.
There is currently a further issue - if the allocator's swap does throw there's
no guarantee what state the allocators will be in. The only solution seems to
be to double buffer the allocators. But I'm assuming that it won't throw for
now.
Update: The committee have now decided that `swap` should do a fast swap if the
But the committee have now decided that `swap` should do a fast swap if the
allocator is Swappable and a slow swap using copy construction otherwise. To
make this distinction requires concepts. For now I'm sticking with the current
implementation.
make this distinction requires concepts.
In
[@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2387.pdf
N2387, Omnibus Allocator Fix-up Proposals],
Pablo Halpern suggests that there are actually two distinct allocator models,
"Moves with Value" and "Scoped" which behave differently:
[:
When allocators are allowed to have state, it is necessary to have a model for
determining from where an object obtains its allocator. We’ve identified two such
models: the “Moves with Value” allocator model and the “Scoped” allocator model.
In the “Moves with Value” allocator model, the copy constructor of an allocator-aware
class will copy both the value and the allocator from its argument. This is the model
specified in the C++03 standard. With this model, inserting an object into a container
usually causes the new container item to copy the allocator from the object that was
inserted. This model can be useful in special circumstances, e.g., if the items within a
container use an allocator that is specially tuned to the item’s type.
In the “Scoped” allocator model, the allocator used to construct an object is determined
by the context of that object, much like a storage class. With this model, inserting an
object into a container causes the new container item to use the same allocator as the
container. To avoid allocators being used in the wrong context, the allocator is never
copied during copy or move construction. Thus, it is possible using this model to use
allocators based on short-lived resources without fear that an object will transfer its
allocator to a copy that might outlive the (shared) allocator resource. This model is
reasonably safe and generally useful on a large scale. There was strong support in the
2005 Tremblant meeting for pursuing an allocator model that propagates allocators
from container to contained objects.
]
With these models the choice becomes clearer:
[:
I introduced the “Moves with Value” allocator model and the
“Scoped” allocator model. In the former case, the allocator is copied when the container
is copy-constructed. In the latter case it is not. Swapping the allocators is the right thing
to do if the containers conform to the “Moves with Value” allocator model and
absolutely the wrong thing to do if the containers conform to the “Scoped” allocator
model. With the two allocator models well-defined, the desired behavior becomes clear.
]
The proposal is that allocators are swapped if the allocator follows the
"Moves with Value" model and the allocator is swappable. Otherwise a slow swap
is used. Since containers currently only support the "Moves with Value" model
this is consistent with the committee's current recommendation (although it
suggests using a trait to detect if the allocator is swappable rather than a
concept).
Since there is currently neither have a swappable trait or concept for
allocators this implementation always performs a slow swap.
[h3 Are insert and erase stable for unordered_multiset and unordered_multimap?]

48
doc/ref.xml
View File

@@ -385,8 +385,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</parameter>
<type>void</type>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</method>
</method-group>
<method-group name="observers">
@@ -586,8 +590,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
<para><code>x.swap(y)</code></para>
</effects>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</function>
</free-function-group>
</class>
@@ -964,8 +972,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</parameter>
<type>void</type>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</method>
</method-group>
<method-group name="observers">
@@ -1165,8 +1177,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
<para><code>x.swap(y)</code></para>
</effects>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</function>
</free-function-group>
</class>
@@ -1560,8 +1576,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</parameter>
<type>void</type>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</method>
</method-group>
<method-group name="observers">
@@ -1797,8 +1817,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
<para><code>x.swap(y)</code></para>
</effects>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</function>
</free-function-group>
</class>
@@ -2183,8 +2207,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</parameter>
<type>void</type>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>key_equal</code> or <code>hasher</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</method>
</method-group>
<method-group name="observers">
@@ -2386,8 +2414,12 @@ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
<para><code>x.swap(y)</code></para>
</effects>
<throws>
<para>Doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
<para>If the allocators are equal, doesn't throw an exception unless it is thrown by the copy constructor or copy assignment operator of <code>Hash</code> or <code>Pred</code>.</para>
</throws>
<notes>
<para>For a discussion of the behaviour when allocators aren't equal see
<link linkend="unordered.rationale.swapping_containers_with_unequal_allocators">the implementation details</link>.</para>
</notes>
</function>
</free-function-group>
</class>