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[unord.multiset.overview]
# 23 Containers library [[containers]](./#containers)
## 23.5 Unordered associative containers [[unord]](unord#multiset.overview)
### 23.5.7 Class template unordered_multiset [[unord.multiset]](unord.multiset#overview)
#### 23.5.7.1 Overview [unord.multiset.overview]
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15112)
An unordered_multiset is an unordered associative container
that supports equivalent keys (an instance of unordered_multiset may contain
multiple copies of the same key value) and in which each element's key
is the element itself[.](#1.sentence-1)
The unordered_multiset class
supports forward iterators[.](#1.sentence-2)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15122)
An unordered_multiset meets all of the requirements
of a container ([[container.reqmts]](container.reqmts "23.2.2.2Container requirements")),
of an allocator-aware container ([[container.alloc.reqmts]](container.alloc.reqmts "23.2.2.5Allocator-aware containers")), and
of an unordered associative container ([[unord.req]](unord.req "23.2.8Unordered associative containers"))[.](#2.sentence-1)
It provides the operations described in the
preceding requirements table for equivalent keys; that is, an unordered_multiset supports the a_eq operations in that table, not the a_uniq operations[.](#2.sentence-2)
For an unordered_multiset<Key> the key_type and the value_type are
both Key[.](#2.sentence-3)
The iterator and const_iterator types are both
constant iterator types[.](#2.sentence-4)
It is unspecified whether they are the same type[.](#2.sentence-5)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15134)
Subclause [[unord.multiset]](unord.multiset "23.5.7Class template unordered_­multiset") only describes operations on unordered_multiset that
are not described in one of the requirement tables, or for which there
is additional semantic information[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15139)
The types iterator and const_iterator meet
the constexpr iterator requirements ([[iterator.requirements.general]](iterator.requirements.general "24.3.1General"))[.](#4.sentence-1)
[🔗](#lib:unordered_multiset)
namespace std {template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>>class unordered_multiset {public:// typesusing key_type = Key; using value_type = Key; using hasher = Hash; using key_equal = Pred; using allocator_type = Allocator; using pointer = typename allocator_traits<Allocator>::pointer; using const_pointer = typename allocator_traits<Allocator>::const_pointer; using reference = value_type&; using const_reference = const value_type&; using size_type = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using difference_type = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using const_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using local_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using const_local_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2Requirements")using node_type = *unspecified*; // [[unord.multiset.cnstr]](unord.multiset.cnstr "23.5.7.2Constructors"), construct/copy/destroyconstexpr unordered_multiset(); constexpr explicit unordered_multiset(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator>constexpr unordered_multiset(InputIterator f, InputIterator l,
size_type n = *see below*, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1Introduction[container.intro.reqmts]")<value_type> R>constexpr unordered_multiset(from_range_t, R&& rg,
size_type n = *see below*, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); constexpr unordered_multiset(const unordered_multiset&); constexpr unordered_multiset(unordered_multiset&&); constexpr explicit unordered_multiset(const Allocator&); constexpr unordered_multiset(const unordered_multiset&, const type_identity_t<Allocator>&); constexpr unordered_multiset(unordered_multiset&&, const type_identity_t<Allocator>&); constexpr unordered_multiset(initializer_list<value_type> il,
size_type n = *see below*, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); constexpr unordered_multiset(size_type n, const allocator_type& a): unordered_multiset(n, hasher(), key_equal(), a) { }constexpr unordered_multiset(size_type n, const hasher& hf, const allocator_type& a): unordered_multiset(n, hf, key_equal(), a) { }template<class InputIterator>constexpr unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a): unordered_multiset(f, l, n, hasher(), key_equal(), a) { }template<class InputIterator>constexpr unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a): unordered_multiset(f, l, n, hf, key_equal(), a) { }template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1Introduction[container.intro.reqmts]")<value_type> R>constexpr unordered_multiset(from_range_t, R&& rg, size_type n, const allocator_type& a): unordered_multiset(from_range, std::forward<R>(rg),
n, hasher(), key_equal(), a) { }template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1Introduction[container.intro.reqmts]")<value_type> R>constexpr unordered_multiset(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a): unordered_multiset(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { }constexpr unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a): unordered_multiset(il, n, hasher(), key_equal(), a) { }constexpr unordered_multiset(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a): unordered_multiset(il, n, hf, key_equal(), a) { }constexpr ~unordered_multiset(); constexpr unordered_multiset& operator=(const unordered_multiset&); constexpr unordered_multiset& operator=(unordered_multiset&&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); constexpr unordered_multiset& operator=(initializer_list<value_type>); constexpr allocator_type get_allocator() const noexcept; // iteratorsconstexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; // capacityconstexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; // modifierstemplate<class... Args> constexpr iterator emplace(Args&&... args); template<class... Args>constexpr iterator emplace_hint(const_iterator position, Args&&... args); constexpr iterator insert(const value_type& obj); constexpr iterator insert(value_type&& obj); constexpr iterator insert(const_iterator hint, const value_type& obj); constexpr iterator insert(const_iterator hint, value_type&& obj); template<class InputIterator> constexpr void insert(InputIterator first, InputIterator last); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1Introduction[container.intro.reqmts]")<value_type> R>constexpr void insert_range(R&& rg); constexpr void insert(initializer_list<value_type>); constexpr node_type extract(const_iterator position); constexpr node_type extract(const key_type& x); template<class K> constexpr node_type extract(K&& x); constexpr iterator insert(node_type&& nh); constexpr iterator insert(const_iterator hint, node_type&& nh); constexpr iterator erase(iterator position)requires (![same_as](concept.same#concept:same_as "18.4.2Concept same_­as[concept.same]")<iterator, const_iterator>); constexpr iterator erase(const_iterator position); constexpr size_type erase(const key_type& k); template<class K> constexpr size_type erase(K&& x); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(unordered_multiset&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); constexpr void clear() noexcept; template<class H2, class P2>constexpr void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); template<class H2, class P2>constexpr void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_set<Key, H2, P2, Allocator>&& source); // observersconstexpr hasher hash_function() const; constexpr key_equal key_eq() const; // set operationsconstexpr iterator find(const key_type& k); constexpr const_iterator find(const key_type& k) const; template<class K>constexpr iterator find(const K& k); template<class K>constexpr const_iterator find(const K& k) const; constexpr size_type count(const key_type& k) const; template<class K>constexpr size_type count(const K& k) const; constexpr bool contains(const key_type& k) const; template<class K>constexpr bool contains(const K& k) const; constexpr pair<iterator, iterator> equal_range(const key_type& k); constexpr pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K>constexpr pair<iterator, iterator> equal_range(const K& k); template<class K>constexpr pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interfaceconstexpr size_type bucket_count() const noexcept; constexpr size_type max_bucket_count() const noexcept; constexpr size_type bucket_size(size_type n) const; constexpr size_type bucket(const key_type& k) const; template<class K> constexpr size_type bucket(const K& k) const; constexpr local_iterator begin(size_type n); constexpr const_local_iterator begin(size_type n) const; constexpr local_iterator end(size_type n); constexpr const_local_iterator end(size_type n) const; constexpr const_local_iterator cbegin(size_type n) const; constexpr const_local_iterator cend(size_type n) const; // hash policyconstexpr float load_factor() const noexcept; constexpr float max_load_factor() const noexcept; constexpr void max_load_factor(float z); constexpr void rehash(size_type n); constexpr void reserve(size_type n); }; template<class InputIterator, class Hash = hash<*iter-value-type*<InputIterator>>, class Pred = equal_to<*iter-value-type*<InputIterator>>, class Allocator = allocator<*iter-value-type*<InputIterator>>> unordered_multiset(InputIterator, InputIterator, *see below*::size_type = *see below*,
Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multiset<*iter-value-type*<InputIterator>,
Hash, Pred, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6Other range refinements[range.refinements]") R, class Hash = hash<ranges::range_value_t<R>>, class Pred = equal_to<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> unordered_multiset(from_range_t, R&&, typename *see below*::size_type = *see below*,
Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multiset<ranges::range_value_t<R>, Hash, Pred, Allocator>; template<class T, class Hash = hash<T>, class Pred = equal_to<T>, class Allocator = allocator<T>> unordered_multiset(initializer_list<T>, typename *see below*::size_type = *see below*,
Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multiset<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multiset(InputIterator, InputIterator, typename *see below*::size_type, Allocator)-> unordered_multiset<*iter-value-type*<InputIterator>,
hash<*iter-value-type*<InputIterator>>,
equal_to<*iter-value-type*<InputIterator>>,
Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multiset(InputIterator, InputIterator, typename *see below*::size_type,
Hash, Allocator)-> unordered_multiset<*iter-value-type*<InputIterator>, Hash,
equal_to<*iter-value-type*<InputIterator>>,
Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6Other range refinements[range.refinements]") R, class Allocator> unordered_multiset(from_range_t, R&&, typename *see below*::size_type, Allocator)-> unordered_multiset<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>,
equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6Other range refinements[range.refinements]") R, class Allocator> unordered_multiset(from_range_t, R&&, Allocator)-> unordered_multiset<ranges::range_value_t<R>, hash<ranges::range_value_t<R>>,
equal_to<ranges::range_value_t<R>>, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6Other range refinements[range.refinements]") R, class Hash, class Allocator> unordered_multiset(from_range_t, R&&, typename *see below*::size_type, Hash, Allocator)-> unordered_multiset<ranges::range_value_t<R>, Hash, equal_to<ranges::range_value_t<R>>,
Allocator>; template<class T, class Allocator> unordered_multiset(initializer_list<T>, typename *see below*::size_type, Allocator)-> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_multiset(initializer_list<T>, typename *see below*::size_type, Hash, Allocator)-> unordered_multiset<T, Hash, equal_to<T>, Allocator>;}
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15388)
A size_type parameter type in an unordered_multiset deduction guide
refers to the size_type member type of
the type deduced by the deduction guide[.](#5.sentence-1)