std:: multiset

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

std::multiset is an associative container that contains a sorted set of objects of type Key. Unlike set, multiple keys with equivalent values are allowed. Sorting is done using the key comparison function Compare. Search, insertion, and removal operations have logarithmic complexity.

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 ) .

The order of the elements that compare equivalent is the order of insertion and does not change.

(since C++11)

std::multiset 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
value_type	Key
size_type	Unsigned integer type (usually std::size_t )
difference_type	Signed integer type (usually std::ptrdiff_t )
key_compare	Compare
value_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	Constant 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 functions

(constructor)	constructs the multiset (public member function)
(destructor)	destructs the 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 (C++11)	returns an iterator to the beginning (public member function)
end cend (C++11)	returns an iterator to the end (public member function)
rbegin crbegin (C++11)	returns a reverse iterator to the beginning (public member function)
rend crend (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 multiset s (function template)
std::swap (std::multiset)	specializes the std::swap algorithm (function template)
erase_if (std::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 103	C++98	iterator allows modification of keys	iterator made constant
LWG 230	C++98	Key was not required to be CopyConstructible (a key of type Key might not be able to be constructed)	Key is also required to be CopyConstructible

See also

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