# std::multiset

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< cpp‎ | container

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.

### 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`

`Allocator::reference` | (until C++11)
`value_type&` | (since C++11)

`const_reference`

`Allocator::const_reference` | (until C++11)
`const value_type&` | (since C++11)

`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
`const_iterator` | Constant LegacyBidirectionalIterator
`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 | returns an iterator to the beginning (public member function)
end cend | returns an iterator to the end (public member function)
rbegin crbegin | returns a reverse iterator to the beginning (public member function)
rend crend | 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)
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>= | lexicographically compares the values in the multiset (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. Since `iterator` is convertible to `const_iterator`, `const_iterator` should be used in function parameter lists to avoid violations of the One Definition Rule.

### 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

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