[multiset]

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

## 23.4 Associative containers [[associative]](associative#multiset)

### 23.4.7 Class template multiset [multiset]

#### [23.4.7.1](#overview) Overview [[multiset.overview]](multiset.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12994)

Amultiset is an associative container that supports equivalent keys (i.e., possibly contains multiple copies of
the same key value) and provides for fast retrieval of the keys themselves[.](#overview-1.sentence-1)

Themultiset class
supports bidirectional iterators[.](#overview-1.sentence-2)

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13004)

A multiset meets all of the requirements
of a container ([[container.reqmts]](container.reqmts "23.2.2.2 Container requirements")),
of a reversible container ([[container.rev.reqmts]](container.rev.reqmts "23.2.2.3 Reversible container requirements")),
of an allocator-aware container ([[container.alloc.reqmts]](container.alloc.reqmts "23.2.2.5 Allocator-aware containers")), and
of an associative container ([[associative.reqmts]](associative.reqmts "23.2.7 Associative containers"))[.](#overview-2.sentence-1)

multiset also provides most operations described in [[associative.reqmts]](associative.reqmts "23.2.7 Associative containers") for duplicate keys[.](#overview-2.sentence-2)

This means that amultiset supports thea_eq operations in [[associative.reqmts]](associative.reqmts "23.2.7 Associative containers") but not thea_uniq operations[.](#overview-2.sentence-3)

For amultiset<Key> both thekey_type andvalue_type areKey[.](#overview-2.sentence-4)

Descriptions are provided here only for operations onmultiset that are not described in one of these tables
and for operations where there is additional semantic information[.](#overview-2.sentence-5)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13034)

The types iterator and const_iterator meet
the constexpr iterator requirements ([[iterator.requirements.general]](iterator.requirements.general "24.3.1 General"))[.](#overview-3.sentence-1)

namespace std {template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>>class multiset {public:// typesusing key_type = Key; using key_compare = Compare; using value_type = Key; using value_compare = Compare; 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 reverse_iterator = std::reverse_iterator<iterator>; using const_reverse_iterator = std::reverse_iterator<const_iterator>; using node_type = *unspecified*; // [[multiset.cons]](#cons "23.4.7.2 Constructors"), construct/copy/destroyconstexpr multiset() : multiset(Compare()) { }constexpr explicit multiset(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator>constexpr multiset(InputIterator first, InputIterator last, const Compare& comp = Compare(), const Allocator& = Allocator()); template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<value_type> R>constexpr multiset(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); constexpr multiset(const multiset& x); constexpr multiset(multiset&& x); constexpr explicit multiset(const Allocator&); constexpr multiset(const multiset&, const type_identity_t<Allocator>&); constexpr multiset(multiset&&, const type_identity_t<Allocator>&); constexpr multiset(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator>constexpr multiset(InputIterator first, InputIterator last, const Allocator& a): multiset(first, last, Compare(), a) { }template<[*container-compatible-range*](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<value_type> R>constexpr multiset(from_range_t, R&& rg, const Allocator& a)): multiset(from_range, std::forward<R>(rg), Compare(), a) { }constexpr multiset(initializer_list<value_type> il, const Allocator& a): multiset(il, Compare(), a) { }constexpr ~multiset(); constexpr multiset& operator=(const multiset& x); constexpr multiset& operator=(multiset&& x)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); constexpr 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 reverse_iterator rbegin() noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr reverse_iterator rend() noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() 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& x); constexpr iterator insert(value_type&& x); constexpr iterator insert(const_iterator position, const value_type& x); constexpr iterator insert(const_iterator position, value_type&& x); 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& x); template<class K> constexpr size_type erase(K&& x); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(multiset&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); constexpr void clear() noexcept; template<class C2>constexpr void merge(multiset<Key, C2, Allocator>& source); template<class C2>constexpr void merge(multiset<Key, C2, Allocator>&& source); template<class C2>constexpr void merge(set<Key, C2, Allocator>& source); template<class C2>constexpr void merge(set<Key, C2, Allocator>&& source); // observersconstexpr key_compare key_comp() const; constexpr value_compare value_comp() const; // set operationsconstexpr iterator find(const key_type& x); constexpr const_iterator find(const key_type& x) const; template<class K> constexpr iterator find(const K& x); template<class K> constexpr const_iterator find(const K& x) const; constexpr size_type count(const key_type& x) const; template<class K> constexpr size_type count(const K& x) const; constexpr bool contains(const key_type& x) const; template<class K> constexpr bool contains(const K& x) const; constexpr iterator lower_bound(const key_type& x); constexpr const_iterator lower_bound(const key_type& x) const; template<class K> constexpr iterator lower_bound(const K& x); template<class K> constexpr const_iterator lower_bound(const K& x) const; constexpr iterator upper_bound(const key_type& x); constexpr const_iterator upper_bound(const key_type& x) const; template<class K> constexpr iterator upper_bound(const K& x); template<class K> constexpr const_iterator upper_bound(const K& x) const; constexpr pair<iterator, iterator> equal_range(const key_type& x); constexpr pair<const_iterator, const_iterator> equal_range(const key_type& x) const; template<class K>constexpr pair<iterator, iterator> equal_range(const K& x); template<class K>constexpr pair<const_iterator, const_iterator> equal_range(const K& x) const; }; template<class InputIterator, class Compare = less<*iter-value-type*<InputIterator>>, class Allocator = allocator<*iter-value-type*<InputIterator>>> multiset(InputIterator, InputIterator,
 Compare = Compare(), Allocator = Allocator())-> multiset<*iter-value-type*<InputIterator>, Compare, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") R, class Compare = less<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())-> multiset<ranges::range_value_t<R>, Compare, Allocator>; template<class Key, class Compare = less<Key>, class Allocator = allocator<Key>> multiset(initializer_list<Key>, Compare = Compare(), Allocator = Allocator())-> multiset<Key, Compare, Allocator>; template<class InputIterator, class Allocator> multiset(InputIterator, InputIterator, Allocator)-> multiset<*iter-value-type*<InputIterator>,
 less<*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> multiset(from_range_t, R&&, Allocator)-> multiset<ranges::range_value_t<R>, less<ranges::range_value_t<R>>, Allocator>; template<class Key, class Allocator> multiset(initializer_list<Key>, Allocator) -> multiset<Key, less<Key>, Allocator>;}

#### [23.4.7.2](#cons) Constructors [[multiset.cons]](multiset.cons)

[🔗](#lib:multiset,constructor)

`constexpr explicit multiset(const Compare& comp, const Allocator& = Allocator());
`

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13229)

*Effects*: Constructs an empty multiset using the specified comparison object and allocator[.](#cons-1.sentence-1)

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13233)

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

[🔗](#lib:multiset,constructor_)

`template<class InputIterator>
  constexpr multiset(InputIterator first, InputIterator last,
                     const Compare& comp = Compare(), const Allocator& = Allocator());
`

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13246)

*Effects*: Constructs an emptymultiset using the specified comparison object and allocator,
and inserts elements from the range
[first, last)[.](#cons-3.sentence-1)

[4](#cons-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13254)

*Complexity*: Linear in N if the range
[first, last)
is already sorted with respect to comp and otherwise NlogN,
where N islast - first[.](#cons-4.sentence-1)

[🔗](#lib:multiset,constructor__)

`template<[container-compatible-range](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<value_type> R>
  constexpr multiset(from_range_t, R&& rg, const Compare& comp = Compare(),
                     const Allocator& = Allocator());
`

[5](#cons-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13272)

*Effects*: Constructs an empty multiset using the specified comparison object and allocator, and
inserts elements from the range rg[.](#cons-5.sentence-1)

[6](#cons-6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13278)

*Complexity*: Linear in N if rg is already sorted with respect to comp and
otherwise NlogN, where N is ranges​::​distance(rg)[.](#cons-6.sentence-1)

#### [23.4.7.3](#erasure) Erasure [[multiset.erasure]](multiset.erasure)

[🔗](#lib:erase_if,multiset)

`template<class Key, class Compare, class Allocator, class Predicate>
  constexpr typename multiset<Key, Compare, Allocator>::size_type
    erase_if(multiset<Key, Compare, Allocator>& c, Predicate pred);
`

[1](#erasure-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13294)

*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();