Subject: [Boost-commit] svn:boost r56561 - trunk/libs/unordered/doc
Date: 2009-10-03 12:42:01
Date: 2009-10-03 12:42:00 EDT (Sat, 03 Oct 2009)
New Revision: 56561
Update the unordered rationale.
Text files modified:
trunk/libs/unordered/doc/intro.qbk | 2
trunk/libs/unordered/doc/rationale.qbk | 122 +++++++++------------------------------
2 files changed, 31 insertions(+), 93 deletions(-)
--- trunk/libs/unordered/doc/intro.qbk (original)
+++ trunk/libs/unordered/doc/intro.qbk 2009-10-03 12:42:00 EDT (Sat, 03 Oct 2009)
@@ -9,7 +9,7 @@
Working Draft of the C++ Standard]]
[def __hash-table__ [@http://en.wikipedia.org/wiki/Hash_table
--- trunk/libs/unordered/doc/rationale.qbk (original)
+++ trunk/libs/unordered/doc/rationale.qbk 2009-10-03 12:42:00 EDT (Sat, 03 Oct 2009)
@@ -5,12 +5,6 @@
Thomas Wang's article on integer hash functions]]
- N2345, 'Placement Insert for Containers']]
- the August 2007 version of the working draft standard]]
[section:rationale Implementation Rationale]
@@ -99,105 +93,49 @@
[h2 Equality operators]
`operator==` and `operator!=` are not included in the standard, but I've
-added them as I think they could be useful and can be efficiently
-implemented. They are specified
-differently to the standard associative containers, comparing keys
-using the equality predicate rather than `operator==`. This is inconsistent
-with the other containers but it is probably closer to user's expectations.
+added them as I think they could be useful and can be implemented
+fairly efficiently. They are specified differently to the other standard
+containers, comparing keys using the equality predicate rather than
+It's also different to the proposal
+which uses the equality operators for the whole of `value_type`. This
+implementation just uses the key equality function for the key,
+and `mapped_type`'s equality operator in `unordered_map` and
+`unordered_multimap` for the mapped part of the element.
+Also, in `unordered_multimap`, the mapped values for a group of elements with
+equivalent keys are only considered equal if they are in the same order,
+in n2944 they just need to be a permutation of each other. Since the
+order of elements with equal keys is now defined to be stable, it seems to me
+that their order can be considered part of the container's value.
[h2 Active Issues and Proposals]
-[h3 Removing unused allocator functions]
+[h3 C++0x allocators]
-N2257, removing unused allocator functions],
-Matt Austern suggests removing the `construct`, `destroy` and `address` member
-functions - all of which Boost.Unordered calls. Changing this will simplify the
-implementation, as well as make supporting `emplace` easier, but means that the
-containers won't support allocators which require these methods to be called.
-Detlef Vollmann opposed this change in
+Recent drafts have included an overhaul of the allocators, but this was
+dependent on concepts which are no longer in the standard.
+attempts to respecify them without concepts. I'll try to implement this (or
+an appropriate later version) in a future version of boost, possibly changed
+a little to accomodate non-C++0x compilers.
[h3 Swapping containers with unequal allocators]
It isn't clear how to swap containers when their allocators aren't equal.
-Issue 431: Swapping containers with unequal allocators].
-Howard Hinnant wrote about this in
-and suggested swapping both the allocators and the containers' contents.
-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.
-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. Weâve identified two such
-models: the âMoves with Valueâ allocator model and the âScopedâ allocator model.
-In the âMoves with Valueâ 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 itemâs type.
-In the âScopedâ 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 âMoves with Valueâ allocator model and the
-âScopedâ 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 âMoves with Valueâ allocator model and
-absolutely the wrong thing to do if the containers conform to the âScopedâ 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
-Since there is currently neither have a swappable trait or concept for
-allocators this implementation always performs a slow swap.
+Issue 431: Swapping containers with unequal allocators]. This has been resolved
+with the new allocator specification, so this should be fixed when
+support is added.
[h3 Are insert and erase stable for unordered_multiset and unordered_multimap?]
-It is not specified if `unordered_multiset` and `unordered_multimap` preserve the order
+It wan't specified if `unordered_multiset` and `unordered_multimap` preserve the order
of elements with equivalent keys (i.e. if they're stable under `insert` and `erase`).
-This is [@http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#518 issue 581].
-The current proposal is that insert, erase and rehash are stable - so they are here.
-(Update: during the release of this version, this requirement was added to
-the lastest working draft]).
-[h3 const_local_iterator cbegin, cend missing from TR1]
-Issue 691] is that `cbegin` and `cend` are missing for local iterators.
-The current resolution is that they'll be added, so I've added them.
+n2691] it's been specified that they do and this implementation follows that.
Boost-Commit list run by bdawes at acm.org, david.abrahams at rcn.com, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk