boost/libs/unordered/doc/compliance.qbk

[/ Copyright 2011 Daniel James.
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) ]

[section:compliance C++11 Compliance]

[section:move Move emulation]

Support for move semantics is implemented using Boost.Move. If rvalue
references are available it will use them, but if not it uses a close,
but imperfect emulation. On such compilers:

* Non-copyable objects can be stored in the containers.
  They can be constructed in place using `emplace`, or if they support
  Boost.Move, moved into place.
* The containers themselves are not movable.
* Argument forwarding is not perfect.

[endsect]

[section:allocator_compliance Use of allocators]

C++11 introduced a new allocator system. It's backwards compatible due to
the lax requirements for allocators in the old standard, but might need
some changes for allocators which worked with the old versions of the
unordered containers.
It uses a traits class, `allocator_traits` to handle the allocator
adding extra functionality, and making some methods and types optional.
During development a stable release of
`allocator_traits` wasn't available so an internal partial implementation
is always used in this version. Hopefully a future version will use the
standard implementation where available.

The member functions `construct`, `destroy` and `max_size` are now
optional, if they're not available a fallback is used.
A full implementation of `allocator_traits` requires sophisticated
member function detection so that the fallback is used whenever the
member function call is not well formed.
This requires support for SFINAE expressions, which are available on
GCC from version 4.4 and Clang.

On other compilers, there's just a test to see if the allocator has
a member, but no check that it can be called. So rather than using a
fallback there will just be a compile error.

`propagate_on_container_copy_assignment`,
`propagate_on_container_move_assignment`,
`propagate_on_container_swap` and
`select_on_container_copy_construction` are also supported.
Due to imperfect move emulation, some assignments might check
`propagate_on_container_copy_assignment` on some compilers and
`propagate_on_container_move_assignment` on others.

The use of the allocator's construct and destruct methods might be a bit
surprising.
Nodes are constructed and destructed using the allocator, but the elements
are stored in aligned space within the node and constructed and destructed
by calling the constructor and destructor directly.

In C++11 the allocator's construct function has the signature:

    template <class U, class... Args>
    void construct(U* p, Args&&... args);

which supports calling `construct` for the contained object, but
most existing allocators don't support this. If member function detection
was good enough then with old allocators it would fall back to calling
the element's constructor directly but in general, detection isn't good
enough to do this which is why Boost.Unordered just calls the constructor
directly every time. In most cases this will work okay.

`pointer_traits` aren't used. Instead, pointer types are obtained from
rebound allocators, this can cause problems if the allocator can't be
used with incomplete types. If `const_pointer` is not defined in the
allocator, `boost::pointer_to_other<pointer, const value_type>::type`
is used to obtain a const pointer.

[endsect]

[section:pairs Pairs]

Since the containers use `std::pair` they're limited to the version
from the current standard library. But since C++11 `std::pair`'s
`piecewise_construct` based constructor is very useful, `emplace`
emulates it with a `piecewise_construct` in the `boost::unordered`
namespace. So for example, the following will work:

    boost::unordered_multimap<std::string, std::complex> x;

    x.emplace(
        boost::unordered::piecewise_construct,
        boost::make_tuple("key"), boost::make_tuple(1, 2));

Older drafts of the standard also supported variadic constructors
for `std::pair`, where the first argument would be used for the
first part of the pair, and the remaining for the second part.

[endsect]

[section:misc Miscellaneous]

When swapping, `Pred` and `Hash` are not currently swapped by calling
`swap`, their copy constructors are used. As a consequence when swapping
an exception may be throw from their  copy constructor.

Variadic constructor arguments for `emplace` are only used when both
rvalue references and variadic template parameters are available.
Otherwise `emplace` can only take up to 10 constructors arguments.

[endsect]

[endsect]