explicit multimap(const Compare& comp, const Allocator& = Allocator());
Effects: Constructs an empty multimap using the specified comparison object and allocator.
Complexity: Constant.
template <class InputIterator>
multimap(InputIterator first, InputIterator last,
const Compare& comp = Compare(),
const Allocator& = Allocator());
Effects: Constructs an empty multimap using the specified comparison object and allocator, and inserts elements from the range [first, last).
Complexity: Linear in N if the range [first, last) is already sorted using comp and otherwise N logN, where N is last - first.