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Boost.Container flat_[multi]map/set containers are ordered, vector-like container based, associative containers following Austern's and Alexandrescu's guidelines. These ordered vector containers have also benefited with the addition of move semantics to C++11, speeding up insertion and erasure times considerably. Flat associative containers have the following attributes:
Faster lookup than standard associative containers
Much faster iteration than standard associative containers. Random-access iterators instead of bidirectional iterators.
Less memory consumption for small objects (and for big objects if shrink_to_fit is used)
Improved cache performance (data is stored in contiguous memory)
Non-stable iterators (iterators are invalidated when inserting and erasing elements)
Non-copyable and non-movable values types can't be stored
Weaker exception safety than standard associative containers (copy/move constructors can throw when shifting values in erasures and insertions)
Slower insertion and erasure than standard associative containers (specially for non-movable types)