void swap( multimap& other );
Exchanges the contents of the container with those of other. Does not invoke any move, copy, or swap operations on individual elements.
All iterators and references remain valid. The past-the-end iterator is invalidated.
The Pred objects must be Swappable, and they are exchanged using unqualified call to non-member swap.
If std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then the allocators are exchanged using a