std::unordered_multimap

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

Iterator invalidation

Member function table

Non-member function table

 

std::unordered_multimap

Member types
Member functions
unordered_multimap::unordered_multimap unordered_multimap::~unordered_multimap    unordered_multimap::operator= unordered_multimap::get_allocator Iterators unordered_multimap::beginunordered_multimap::cbegin unordered_multimap::endunordered_multimap::cend Capacity unordered_multimap::size unordered_multimap::max_size unordered_multimap::empty Modifiers unordered_multimap::clear unordered_multimap::insert unordered_multimap::insert_range (C++23) unordered_multimap::emplace unordered_multimap::emplace_hint unordered_multimap::erase unordered_multimap::swap unordered_multimap::extract (C++17) unordered_multimap::merge (C++17)	Lookup unordered_multimap::count unordered_multimap::find unordered_multimap::contains (C++20) unordered_multimap::equal_range Bucket interface unordered_multimap::begin(size_type)unordered_multimap::cbegin(size_type) unordered_multimap::end(size_type)unordered_multimap::cend(size_type) unordered_multimap::bucket_count unordered_multimap::max_bucket_count unordered_multimap::bucket_size unordered_multimap::bucket Hash policy unordered_multimap::load_factor unordered_multimap::max_load_factor unordered_multimap::rehash unordered_multimap::reserve Observers unordered_multimap::hash_function unordered_multimap::key_eq
Non-member functions
std::swap(std::unordered_multimap) erase_if(std::unordered_multimap) (C++20)	operator==operator!= (until C++20)
Deduction guides (C++17)

 

Defined in header <unordered_map>
template<     class Key,     class T,     class Hash = std::hash<Key>,     class KeyEqual = std::equal_to<Key>,     class Allocator = std::allocator<std::pair<const Key, T>> > class unordered_multimap;	(1)	(since C++11)
namespace pmr {     template<         class Key,         class T,         class Hash = std::hash<Key>,         class Pred = std::equal_to<Key>     > using unordered_multimap =           std::unordered_multimap<Key, T, Hash, Pred,               std::pmr::polymorphic_allocator<std::pair<const Key, T>>>; }	(2)	(since C++17)

std::unordered_multimap is an unordered associative container that supports equivalent keys (an unordered_multimap may contain multiple copies of each key value) and that associates values of another type with the keys. The unordered_multimap class supports forward iterators. 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 key. This allows fast access to individual elements, since once the 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_multimaps), 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_multimap meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Member types

Type	Definition
key_type	Key
mapped_type	T
value_type	std::pair<const Key, T>
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	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_multimap (public member function)
(destructor)	destructs the unordered_multimap (public member function)
operator=	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Iterators
begincbegin	returns an iterator to the beginning (public member function)
endcend	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_multimap (function template)
std::swap(std::unordered_multimap) (C++11)	specializes the std::swap algorithm (function template)
erase_if(std::unordered_multimap) (C++20)	erases all elements satisfying specific criteria (function template)

Deduction guides

(since C++17)

Notes

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

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_map (C++11)	collection of key-value pairs, hashed by keys, keys are unique (class template)
multimap	collection of key-value pairs, sorted by keys (class template)
flat_multimap (C++23)	adapts two containers to provide a collection of key-value pairs, sorted by keys (class template)