Boost C++ Libraries

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C++11 Compliance

Move emulation
Use of allocators
Pairs
Miscellaneous

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 you'll need to use Boost.Move to take advantage of using movable container elements, also note that:

  • Non-copyable objects can be stored in the containers, but without support for rvalue references the container will not be movable.
  • Argument forwarding is not perfect.

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.

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.

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.


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