forked from boostorg/unordered
96 lines
3.6 KiB
Plaintext
96 lines
3.6 KiB
Plaintext
[/ Copyright 2011 Daniel James.
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]
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[section:compliance C++11 Compliance]
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[section:allocator_compliance Use of allocators]
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C++11 introduced a new, mostly backwards compatible, allocator system.
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This uses a traits class, `allocator_traits` to handle the allocator
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adding extra functionality, and making some methods and types optional.
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At the time of writing there isn't a stable release of a standard library
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with `allocator_traits` (libc++ has `allocator_traits` but it hasn't been
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released yet) so a partial implementation is always used.
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A full implementation of `allocator_traits` requires sophisticated
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member function detection which requires support for SFINAE expressions,
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or something close. This is available on GCC from version 4.4, Clang and
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Visual C++ 2008 (with a little hacking) or later.
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On these compilers, the `construct`, `destroy` and `max_size` member functions
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are optional, as per C++11. On other compilers they are still required.
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`propagate_on_container_copy_assignment`,
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`propagate_on_container_move_assignment` and
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`propagate_on_container_swap` are supported on most compilers
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(/TODO/: which ones don't support them?).
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`select_on_container_copy_construction` is also supported, but on
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compilers without full member function detection it must have exactly
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the right function signature, and can't be declared in a base class
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in order for it to be detected.
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The use of the allocator's construct and destruct methods might be a bit
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surprising.
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Nodes are constructed and destructed using the allocator, but the objects
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contained in the node are stored in aligned space within the node
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and constructed and destructed by calling the constructor and destructor
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directly. So `construct` and `destroy` are called for the node, but not for
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the object.
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`pointer_traits` aren't used. Instead, pointer types are obtained from
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rebound allocators.
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[endsect]
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[section:move Move emulation]
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Move emulation is implemented using Boost.Move. If rvalue references are
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available it will use them, but if not it uses a close, but imperfect emulation
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and to get the advantage of using movable container elements, you'll need to
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use Boost.Move.
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* Non-copyable objects can be stored in the containers, but without support
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for rvalue references the container will not be movable.
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* The number of arguments used in `emplace` is limited to /TODO/.
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* Argument forwarding is not perfect.
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[endsect]
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[section:pairs Pairs]
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Since the containers use `std::pair` they're limited to the version
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from the current standard library. But since C++11 `std::pair`'s
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`piecewise_construct` based constructor is very useful, `emplace`
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emulates it with a `piecewise_construct` in the `boost::unordered`
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namespace. So for example, the following will work:
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boost::unordered_multimap<std::string, std::complex> x;
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x.emplace(
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boost::unordered::piecewise_construct,
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boost::make_tuple("key"), boost::make_tuple(1, 2));
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Older drafts of the standard also supported variadic constructors
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for `std::pair`, where the first argument would be used for the
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first part of the pair, and the remaining for the second part.
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For the same example:
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x.emplace("key", 1, 2);
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This is emulated in Boost.Unordered, but will be deprecated soon.
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While it is a lot more compact, it lead to ambiguities so it was
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removed.
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[endsect]
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[section:swap Swapping]
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When swapping, `Pred` and `Hash` are not currently swapped by calling
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`swap`, their copy constructors are used. As a consequence when swapping
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an exception may be throw from their copy constructor.
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[endsect]
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[endsect]
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