std:: unordered_multiset

Defined in header <unordered_set>
template < class Key, class Hash = std:: hash < Key > , class KeyEqual = std:: equal_to < Key > , class Allocator = std:: allocator < Key > > class unordered_multiset ;	(1)	(since C++11)
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 >> ; }	(2)	(since C++17)

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_multiset s), 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

This section is incomplete Reason: Add descriptions of the template parameters.

Member types

Type	Definition
key_type	Key
value_type	Key
size_type	Unsigned integer type (usually std::size_t )
difference_type	Signed integer type (usually std::ptrdiff_t )
hasher	Hash
key_equal	KeyEqual
allocator_type	Allocator
reference	value_type &
const_reference	const value_type &
pointer	std:: allocator_traits < Allocator > :: pointer
const_pointer	std:: allocator_traits < Allocator > :: const_pointer
iterator	Constant LegacyForwardIterator to value_type
const_iterator	LegacyForwardIterator to const value_type
local_iterator	An iterator type whose category, value, difference, pointer and reference types are the same as iterator . This iterator can be used to iterate through a single bucket but not across buckets
const_local_iterator	An iterator type whose category, value, difference, pointer and reference types are the same as const_iterator . This iterator can be used to iterate through a single bucket but not across buckets
node_type (since C++17)	a specialization of node handle representing a container node

Member functions

(constructor)	constructs the unordered_multiset (public member function)
(destructor)	destructs the unordered_multiset (public member function)
operator=	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Iterators
begin cbegin	returns an iterator to the beginning (public member function)
end cend	returns an iterator to the end (public member function)
Capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size	returns the maximum possible number of elements (public member function)
Modifiers
clear	clears the contents (public member function)
insert	inserts elements or nodes (since C++17) (public member function)
insert_range (C++23)	inserts a range of elements (public member function)
emplace	constructs element in-place (public member function)
emplace_hint	constructs elements in-place using a hint (public member function)
erase	erases elements (public member function)
swap	swaps the contents (public member function)
extract (C++17)	extracts nodes from the container (public member function)
merge (C++17)	splices nodes from another container (public member function)
Lookup
count	returns the number of elements matching specific key (public member function)
find	finds element with specific key (public member function)
contains (C++20)	checks if the container contains element with specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)
Bucket interface
begin (size_type) cbegin (size_type)	returns an iterator to the beginning of the specified bucket (public member function)
end (size_type) cend (size_type)	returns an iterator to the end of the specified bucket (public member function)
bucket_count	returns the number of buckets (public member function)
max_bucket_count	returns the maximum number of buckets (public member function)
bucket_size	returns the number of elements in specific bucket (public member function)
bucket	returns the bucket for specific key (public member function)
Hash policy
load_factor	returns average number of elements per bucket (public member function)
max_load_factor	manages maximum average number of elements per bucket (public member function)
rehash	reserves at least the specified number of buckets and regenerates the hash table (public member function)
reserve	reserves space for at least the specified number of elements and regenerates the hash table (public member function)
Observers
hash_function	returns function used to hash the keys (public member function)
key_eq	returns the function used to compare keys for equality (public member function)

Non-member functions

operator== operator!= (C++11) (C++11) (removed in C++20)	compares the values in the unordered_multiset (function template)
std::swap (std::unordered_multiset) (C++11)	specializes the std::swap algorithm (function template)
erase_if (std::unordered_multiset) (C++20)	erases all elements satisfying specific criteria (function template)

Deduction guides

(since C++17)

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.

Feature-test macro	Value	Std	Feature
__cpp_lib_containers_ranges	202202L	(C++23)	Ranges construction and insertion for containers

Example

This section is incomplete Reason: no example

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 2050	C++11	the definitions of reference , const_reference , pointer and const_pointer were based on allocator_type	based on value_type and std::allocator_traits

See also

unordered_set (C++11)	collection of unique keys, hashed by keys (class template)
multiset	collection of keys, sorted by keys (class template)
flat_multiset (C++23)	adapts a container to provide a collection of keys, sorted by keys (class template)