NAME¶

std::multiset - std::multiset

Synopsis¶

Defined in header <set>
template<

class Key,
class Compare = std::less<Key>, (1)
class Allocator = std::allocator<Key>

> class multiset;
namespace pmr {

template <class Key, class Compare = std::less<Key>>
using multiset = std::multiset<Key, Compare, (2) (since C++17)
std::pmr::polymorphic_allocator<Key>>;

}

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.

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)
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)
Allocator::const_pointer (until
const_pointer 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 returns an iterator to the beginning
cbegin (public member function)
(C++11)
end returns an iterator to the end
cend (public member function)
(C++11)
rbegin returns a reverse iterator to the beginning
crbegin (public member function)
(C++11)
rend returns a reverse iterator to the end
crend (public member function)
(C++11)

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)
inserts elements
insert or nodes
(since C++17)
(public member function)
emplace constructs element in-place
(C++11) (public member function)
emplace_hint constructs elements in-place using a hint
(C++11) (public member function)
erase erases elements
(public member function)
swap swaps the contents
(public member function)
extract extracts nodes from the container
(C++17) (public member function)
merge splices nodes from another container
(C++17) (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 checks if the container contains element with specific key
(C++20) (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>= lexicographically compares the values in the multiset
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
std::swap(std::multiset) specializes the std::swap algorithm
(function template)
erase_if(std::multiset) Erases all elements satisfying specific criteria
(C++20) (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.

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