std::multimap

Defined in header <map>
template < class Key, class T, class Compare = std::less <Key>, class Allocator = std::allocator < std::pair < const Key, T>> > class multimap;	(1)
namespace pmr { template < class Key, class T, class Compare = std::less <Key> > using multimap = std:: multimap <Key, T, Compare, std::pmr::polymorphic_allocator < std::pair < const Key, T>>> ; }	(2)	(since C++17)

std::multimap is an associative container that contains a sorted list of key-value pairs, while permitting multiple entries with the same key. Sorting is done according to the comparison function Compare

Iterators of std::multimap iterate in non-descending order of keys, where non-descending is defined by the comparison that was used for construction. That is, given

• m, a std::multimap
• it_l and it_r, dereferenceable iterators to m, with it_l < it_r

m.value_comp ( ) ( *it_r, *it_l) == false

The order of the key-value pairs whose keys compare equivalent is the order of insertion and does not change.

(since C++11)

Everywhere the standard library uses the Compare requirements, equivalence is determined by using the equivalence relation as described on Compare. In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a)

std::multimap meets the requirements of Container, AllocatorAwareContainer, AssociativeContainer and ReversibleContainer

Template parameters

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

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)
key_compare	Compare
allocator_type	Allocator
reference	value_type&
const_reference	const value_type&
pointer	Allocator::pointer (until C++11) std::allocator_traits <Allocator> :: pointer (since C++11)
const_pointer	Allocator::const_pointer (until C++11) std::allocator_traits <Allocator> :: const_pointer (since C++11)
iterator	LegacyBidirectionalIterator to value_type
const_iterator	LegacyBidirectionalIterator to const value_type
reverse_iterator	std::reverse_iterator<iterator>
const_reverse_iterator	std::reverse_iterator<const_iterator>
node_type (since C++17)	a specialization of node handle representing a container node

Member classes

value_compare

compares objects of type value_type (class)

Member functions

(constructor)	constructs the multimap (public member function)
(destructor)	destructs the multimap (public member function)
operator=	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Iterators
begincbegin (C++11)	returns an iterator to the beginning (public member function)
endcend (C++11)	returns an iterator to the end (public member function)
rbegincrbegin (C++11)	returns a reverse iterator to the beginning (public member function)
rendcrend (C++11)	returns a reverse 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 (C++11)	constructs element in-place (public member function)
emplace_hint (C++11)	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)
lower_bound	returns an iterator to the first element not less than the given key (public member function)
upper_bound	returns an iterator to the first element greater than the given key (public member function)
Observers
key_comp	returns the function that compares keys (public member function)
value_comp	returns the function that compares keys in objects of type value_type (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (removed in C++20) (C++20)	lexicographically compares the values of two multimaps (function template)
std::swap(std::multimap)	specializes the std::swap algorithm (function template)
erase_if(std::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

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 230	C++98	Key was not required to be CopyConstructible (a key of type Key	Key is also required to be CopyConstructible

See also

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