# std::unordered_multiset

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< cpp‎ | container

Defined in header `<unordered_set>` |  |
template< class Key, class Hash = std::hash <Key>, class KeyEqual = std::equal_to <Key>, class Allocator = std::allocator <Key> > class unordered_multiset; | (1) | (since C++11)
namespace pmr { template <class Key, class Hash = std::hash <Key>, class Pred = std::equal_to <Key>> using unordered_multiset = std::unordered_multiset<Key, Hash, Pred, std::pmr::polymorphic_allocator <Key>> } | (2) | (since C++17)

Unordered multiset is an associative container that contains set of possibly non-unique objects of type Key. 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 value. This allows fast access to individual elements, since once 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, std::equal cannot be used to compare two `std::unordered_multiset`s), except that every group of elements whose keys compare equivalent (compare equal with key_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible with equal_range().

`std::unordered_multiset` meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

### 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)
`hasher` | `Hash`

`key_equal`

`KeyEqual` | (until C++20)
`Hash::transparent_key_equal` if defined and names a type, otherwise `KeyEqual` | (since C++20)

`allocator_type` | `Allocator`
`reference` | `value_type&`
`const_reference` | `const value_type&`
`pointer` | std::allocator_traits <Allocator>::pointer
`const_pointer` | std::allocator_traits <Allocator>::const_pointer
`iterator` | Constant LegacyForwardIterator
`const_iterator` | Constant LegacyForwardIterator
`local_iterator` | An 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_iterator` | An 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 node handle representing a container node

### Member functions

(constructor) | constructs the `unordered_multiset` (public member function)
(destructor) | destructs the `unordered_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)

##### 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 | constructs element in-place (public member function)
emplace_hint | 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)

##### Bucket interface

begin(size_type) cbegin(size_type) | returns an iterator to the beginning of the specified bucket (public member function)
end(size_type) cend(size_type) | returns an iterator to the end of the specified bucket (public member function)
bucket_count | returns the number of buckets (public member function)
max_bucket_count | returns the maximum number of buckets (public member function)
bucket_size | returns the number of elements in specific bucket (public member function)
bucket | returns the bucket for specific key (public member function)

##### Hash policy

load_factor | returns average number of elements per bucket (public member function)
max_load_factor | manages maximum average number of elements per bucket (public member function)
rehash | reserves at least the specified number of buckets. This regenerates the hash table. (public member function)
reserve | reserves space for at least the specified number of elements. This regenerates the hash table. (public member function)

##### Observers

hash_function | returns function used to hash the keys (public member function)
key_eq | returns the function used to compare keys for equality (public member function)

### Non-member functions

operator==operator!= | compares the values in the unordered_multiset (function template)
std::swap(std::unordered_multiset) (C++11) | specializes the std::swap algorithm (function template)
erase_if(std::unordered_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.

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