std::unordered_multiset (3) - Linux Manuals

std::unordered_multiset: std::unordered_multiset


std::unordered_multiset - std::unordered_multiset


Defined in header <unordered_set>
class Key,
class Hash = std::hash<Key>, (1) (since C++11)
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>> (2) (since C++17)
using unordered_multiset = std::unordered_multiset<Key, Hash, Pred,

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.

Member types

Member 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 (until C++20)
                       Hash::transparent_key_equal if defined and names a type, otherwise KeyEqual (since C++20)

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
const_iterator Constant LegacyForwardIterator
                       An iterator type whose category, value, difference, pointer and
local_iterator reference types are the same as iterator. This iterator
                       can be used to iterate through a single bucket but not across buckets
                       An iterator type whose category, value, difference, pointer and
const_local_iterator 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

                  constructs the unordered_multiset
constructor (public member function)
                  destructs the unordered_multiset
destructor (public member function)
                  assigns values to the container
operator= (public member function)
                  returns the associated allocator
get_allocator (public member function)


begin returns an iterator to the beginning
cbegin (public member function)

end_ returns an iterator to the end
cend (public member function)


                  checks whether the container is empty
empty (public member function)
                  returns the number of elements
size (public member function)
                  returns the maximum possible number of elements
max_size (public member function)


                  clears the contents
clear (public member function)
                  inserts elements
                  or nodes
insert (since C++17)
                  (public member function)
                  constructs element in-place
emplace (public member function)
                  constructs elements in-place using a hint
emplace_hint (public member function)
                  erases elements
erase (public member function)
                  swaps the contents
swap (public member function)

extract extracts nodes from the container
                  (public member function)

merge splices nodes from another container
                  (public member function)


                  returns the number of elements matching specific key
count (public member function)
                  finds element with specific key
find (public member function)

contains checks if the container contains element with specific key
                  (public member function)
                  returns range of elements matching a specific key
equal_range (public member function)

Bucket interface

                  returns an iterator to the beginning of the specified bucket
begin(size_type) (public member function)
                  returns an iterator to the end of the specified bucket
end(size_type) (public member function)
                  returns the number of buckets
bucket_count (public member function)
                  returns the maximum number of buckets
max_bucket_count (public member function)
                  returns the number of elements in specific bucket
bucket_size (public member function)
                  returns the bucket for specific key
bucket (public member function)

Hash policy

                  returns average number of elements per bucket
load_factor (public member function)
                  manages maximum average number of elements per bucket
max_load_factor (public member function)
                  reserves at least the specified number of buckets.
rehash This regenerates the hash table.
                  (public member function)
                  reserves space for at least the specified number of elements.
reserve This regenerates the hash table.
                  (public member function)


                  returns function used to hash the keys
hash_function (public member function)
                  returns the function used to compare keys for equality
key_eq (public member function)

Non-member functions

                                   compares the values in the unordered_multiset
operator== (function template)

std::swap(std::unordered_multiset) specializes the std::swap algorithm
                                   (function template)

erase_if(std::unordered_multiset) Erases all elements satisfying specific criteria
                                   (function template)

Deduction_guides(since C++17)


The member types iterator and const_iterator may be aliases to the same type. Since iterator is convertible to const_iterator, const_iterator should be used in function parameter lists to avoid violations of the One Definition Rule.