std::unordered_multiset

template<
    class Key,
    class Hash = std::hash<Key>,
    class KeyEqual = std::equal_to<Key>,
    class Allocator = std::allocator<Key>
> class unordered_multiset;

namespace pmr {
    template<
        class Key,
        class Hash = std::hash<Key>,
        class Pred = std::equal_to<Key>
    > using unordered_multiset = std::unordered_multiset<Key, Hash, Pred,
                                     std::pmr::polymorphic_allocator<Key>>;
}

std::unordered_multiset is an associative container that contains set of possibly non-unique objects of type Key. Search, insertion, and removal have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its value. This allows fast access to individual elements, since once hash is computed, it refers to the exact bucket the element is placed into.

The iteration order of this container is not required to be stable (so, for example, std::equal cannot be used to compare two std::unordered_multisets), except that every group of elements whose keys compare equivalent (compare equal with key_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible with equal_range().

std::unordered_multiset meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Template parameters

Member types

Member functions

Non-member functions

Notes

The member types iterator and const_iterator may be aliases to the same type. This means defining a pair of function overloads using the two types as parameter types may violate the One Definition Rule. Since iterator is convertible to const_iterator, a single function with a const_iterator as parameter type will work instead.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also