[unord.multiset]

# 23 Containers library [[containers]](./#containers)

## 23.5 Unordered associative containers [[unord]](unord#multiset)

### 23.5.7 Class template unordered_multiset [unord.multiset]

#### [23.5.7.1](#overview) Overview [[unord.multiset.overview]](unord.multiset.overview)

[1](#overview-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[.](#overview-1.sentence-1)

The unordered_multiset class
supports forward iterators[.](#overview-1.sentence-2)

[2](#overview-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 "23.2.2.2 Container requirements")),
of an allocator-aware container ([[container.alloc.reqmts]](container.alloc.reqmts "23.2.2.5 Allocator-aware containers")), and
of an unordered associative container ([[unord.req]](unord.req "23.2.8 Unordered associative containers"))[.](#overview-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[.](#overview-2.sentence-2)

For an unordered_multiset<Key> the key_type and the value_type are
both Key[.](#overview-2.sentence-3)

The iterator and const_iterator types are both
constant iterator types[.](#overview-2.sentence-4)

It is unspecified whether they are the same type[.](#overview-2.sentence-5)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15134)

Subclause [unord.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[.](#overview-3.sentence-1)

[4](#overview-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 "24.3.1 General"))[.](#overview-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 "23.2 Requirements")using difference_type = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using const_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using local_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using const_local_iterator = *implementation-defined*; // see [[container.requirements]](container.requirements "23.2 Requirements")using node_type = *unspecified*; // [[unord.multiset.cnstr]](#cnstr "23.5.7.2 Constructors"), 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 "23.2.2.1 Introduction [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 "23.2.2.1 Introduction [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 "23.2.2.1 Introduction [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 "23.2.2.1 Introduction [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 "18.4.2 Concept 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 "25.4.6 Other 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 "25.4.6 Other 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 "25.4.6 Other 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 "25.4.6 Other 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](#overview-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[.](#overview-5.sentence-1)

#### [23.5.7.2](#cnstr) Constructors [[unord.multiset.cnstr]](unord.multiset.cnstr)

[🔗](#lib:unordered_multiset,constructor)

`constexpr unordered_multiset() : unordered_multiset(size_type(see below)) { }
constexpr explicit unordered_multiset(size_type n, const hasher& hf = hasher(),
                                      const key_equal& eql = key_equal(),
                                      const allocator_type& a = allocator_type());
`

[1](#cnstr-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15404)

*Effects*: Constructs an empty unordered_multiset using the
specified hash function, key equality predicate, and allocator, and
using at least n buckets[.](#cnstr-1.sentence-1)

For the default constructor,
the number of buckets is implementation-defined[.](#cnstr-1.sentence-2)

max_load_factor() returns 1.0[.](#cnstr-1.sentence-3)

[2](#cnstr-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15413)

*Complexity*: Constant[.](#cnstr-2.sentence-1)

[🔗](#lib:unordered_multiset,constructor_)

`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 "23.2.2.1 Introduction [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(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());
`

[3](#cnstr-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15437)

*Effects*: Constructs an empty unordered_multiset using the
specified hash function, key equality predicate, and allocator, and
using at least n buckets[.](#cnstr-3.sentence-1)

If n is not
provided, the number of buckets is implementation-defined[.](#cnstr-3.sentence-2)

Then
inserts elements from the range [f, l), rg, or il,
respectively[.](#cnstr-3.sentence-3)

max_load_factor() returns 1.0[.](#cnstr-3.sentence-4)

[4](#cnstr-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15448)

*Complexity*: Average case linear, worst case quadratic[.](#cnstr-4.sentence-1)

#### [23.5.7.3](#erasure) Erasure [[unord.multiset.erasure]](unord.multiset.erasure)

[🔗](#lib:erase_if,unordered_multiset)

`template<class K, class H, class P, class A, class Predicate>
  constexpr typename unordered_multiset<K, H, P, A>::size_type
    erase_if(unordered_multiset<K, H, P, A>& c, Predicate pred);
`

[1](#erasure-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15463)

*Effects*: Equivalent to:auto original_size = c.size();for (auto i = c.begin(), last = c.end(); i != last; ) {if (pred(*i)) { i = c.erase(i); } else {++i; }}return original_size - c.size();