Defined in header
template<classKey,classT,classHash=std::hash<Key>,classKeyEqual=std::equal_to<Key>,classAllocator=std::allocator<std::pair<constKey,T>>>classunordered_multimap; (1) (since C++11)namespacepmr{template<classKey,classT,classHash=std::hash<Key>,classPred=std::equal_to<Key>>usingunordered_multimap=std::unordered_multimap<Key,T,Hash,Pred,std::pmr::polymorphic_allocator<std::pair<constKey,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,
cannot be used to compare two std::unordered_multimaps), except that every group of elements whose keys compare equivalent (compare equal with
as the comparator) forms a contiguous subrange in the iteration order, also accessible with
.
std::unordered_multimap meets the requirements of
,
,
.
All member functions of std::unordered_multimap are constexpr: it is possible to create and use std::unordered_multimap objects in the evaluation of a constant expression.
However, defining a constexprstd::unordered_multimap variable is generally an error, because constant evaluation requires any dynamically allocated storage to be released in the same evaluation, which is usually not the case with std::unordered_multimap's initializer.
(since C++26)Template parameters
Member types
Type Definition key_typeKey
mapped_typeT
value_typestd::pair<constKey,T>
size_typeunsigned integer type (usually
)
difference_typesigned integer type (usually
)
hasherHash
key_equalKeyEqual
allocator_typeAllocator
referencevalue_type&
const_referenceconstvalue_type&
pointerstd::allocator_traits<Allocator>::pointer
const_pointerstd::allocator_traits<Allocator>::const_pointer
iterator
and
(since C++26) to value_type
const_iterator
and
(since C++26) to constvalue_type
local_iteratoran 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_iteratoran 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
representing a container node
Member functions
constructs the unordered_multimap
(public member function)
destructs the unordered_multimap
(public member function)
assigns values to the container
(public member function)
returns the associated allocator
(public member function)
Iterators
returns an iterator to the beginning
(public member function)
returns an iterator to the end
(public member function)
Capacity
checks whether the container is empty
(public member function)
returns the number of elements
(public member function)
returns the maximum possible number of elements
(public member function)
Modifiers
clears the contents
(public member function)
inserts elements or nodes(since C++17)
(public member function)
(C++23)
inserts a range of elements
(public member function)
constructs element in-place
(public member function)
constructs elements in-place using a hint
(public member function)
erases elements
(public member function)
swaps the contents
(public member function)
(C++17)
extracts nodes from the container
(public member function)
(C++17)
splices nodes from another container
(public member function)
Lookup
returns the number of elements matching specific key
(public member function)
finds element with specific key
(public member function)
(C++20)
checks if the container contains element with specific key
(public member function)
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)
returns an iterator to the end of the specified bucket
(public member function)
returns the number of buckets
(public member function)
returns the maximum number of buckets
(public member function)
returns the number of elements in specific bucket
(public member function)
returns the bucket for specific key
(public member function)
Hash policy
returns average number of elements per bucket
(public member function)
manages maximum average number of elements per bucket
(public member function)
reserves at least the specified number of buckets and regenerates the hash table
(public member function)
reserves space for at least the specified number of elements and regenerates the hash table
(public member function)
Observers
returns function used to hash the keys
(public member function)
returns the function used to compare keys for equality
(public member function)
Non-member functions
Notes
macro ValueStdFeature
(C++23)Ranges construction and insertion for containers
__cpp_lib_constexpr_unordered_map
(C++26)Constexpr std::unordered_multimapExample
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++11 the definitions of reference, const_reference, pointer
and const_pointer were based on allocator_typebased on value_type and
See also
(C++11)
collection of key-value pairs, hashed by keys, keys are unique
(class template)
collection of key-value pairs, sorted by keys
(class template)
(C++23)
adapts two containers to provide a collection of key-value pairs, sorted by keys
(class template)