[multimap]

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

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

### 23.4.4 Class template multimap [multimap]

#### [23.4.4.1](#overview) Overview [[multimap.overview]](multimap.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12203)

Amultimap is an associative container that supports equivalent keys (i.e., possibly containing multiple copies of
the same key value) and provides for fast retrieval of values of another typeT based on the keys[.](#overview-1.sentence-1)

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

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12216)

A multimap 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)

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

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

For amultimap<Key,T> thekey_type isKey and thevalue_type ispair<const Key,T>[.](#overview-2.sentence-4)

Descriptions are provided here only for operations onmultimap that are not described in one of those tables
or 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#L12249)

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)

[🔗](#lib:comp,multimap::value_compare)

namespace std {template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>>class multimap {public:// typesusing key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; using key_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*; class value_compare {protected: Compare comp; constexpr value_compare(Compare c) : comp(c) { }public:constexpr bool operator()(const value_type& x, const value_type& y) const {return comp(x.first, y.first); }}; // [[multimap.cons]](#cons "23.4.4.2 Constructors"), construct/copy/destroyconstexpr multimap() : multimap(Compare()) { }constexpr explicit multimap(const Compare& comp, const Allocator& = Allocator()); template<class InputIterator>constexpr multimap(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 multimap(from_range_t, R&& rg, const Compare& comp = Compare(), const Allocator& = Allocator()); constexpr multimap(const multimap& x); constexpr multimap(multimap&& x); constexpr explicit multimap(const Allocator&); constexpr multimap(const multimap&, const type_identity_t<Allocator>&); constexpr multimap(multimap&&, const type_identity_t<Allocator>&); constexpr multimap(initializer_list<value_type>, const Compare& = Compare(), const Allocator& = Allocator()); template<class InputIterator>constexpr multimap(InputIterator first, InputIterator last, const Allocator& a): multimap(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 multimap(from_range_t, R&& rg, const Allocator& a)): multimap(from_range, std::forward<R>(rg), Compare(), a) { }constexpr multimap(initializer_list<value_type> il, const Allocator& a): multimap(il, Compare(), a) { }constexpr ~multimap(); constexpr multimap& operator=(const multimap& x); constexpr multimap& operator=(multimap&& x)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Compare>); constexpr multimap& 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; // [[multimap.modifiers]](#modifiers "23.4.4.3 Modifiers"), 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); template<class P> constexpr iterator insert(P&& x); constexpr iterator insert(const_iterator position, const value_type& x); constexpr iterator insert(const_iterator position, value_type&& x); template<class P> constexpr iterator insert(const_iterator position, P&& 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> 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); 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(multimap&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Compare>); constexpr void clear() noexcept; template<class C2>constexpr void merge(multimap<Key, T, C2, Allocator>& source); template<class C2>constexpr void merge(multimap<Key, T, C2, Allocator>&& source); template<class C2>constexpr void merge(map<Key, T, C2, Allocator>& source); template<class C2>constexpr void merge(map<Key, T, C2, Allocator>&& source); // observersconstexpr key_compare key_comp() const; constexpr value_compare value_comp() const; // map 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-key-type*<InputIterator>>, class Allocator = allocator<*iter-to-alloc-type*<InputIterator>>> multimap(InputIterator, InputIterator, Compare = Compare(), Allocator = Allocator())-> multimap<*iter-key-type*<InputIterator>, *iter-mapped-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<*range-key-type*<R>>, class Allocator = allocator<*range-to-alloc-type*<R>>> multimap(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())-> multimap<*range-key-type*<R>, *range-mapped-type*<R>, Compare, Allocator>; template<class Key, class T, class Compare = less<Key>, class Allocator = allocator<pair<const Key, T>>> multimap(initializer_list<pair<Key, T>>, Compare = Compare(), Allocator = Allocator())-> multimap<Key, T, Compare, Allocator>; template<class InputIterator, class Allocator> multimap(InputIterator, InputIterator, Allocator)-> multimap<*iter-key-type*<InputIterator>, *iter-mapped-type*<InputIterator>,
 less<*iter-key-type*<InputIterator>>, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") R, class Allocator> multimap(from_range_t, R&&, Allocator)-> multimap<*range-key-type*<R>, *range-mapped-type*<R>, less<*range-key-type*<R>>, Allocator>; template<class Key, class T, class Allocator> multimap(initializer_list<pair<Key, T>>, Allocator)-> multimap<Key, T, less<Key>, Allocator>;}

#### [23.4.4.2](#cons) Constructors [[multimap.cons]](multimap.cons)

[🔗](#lib:multimap,constructor)

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

[1](#cons-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12459)

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

[2](#cons-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12465)

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

[🔗](#lib:multimap,constructor_)

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

[3](#cons-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12478)

*Effects*: Constructs an emptymultimap 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#L12486)

*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:multimap,constructor__)

`template<[container-compatible-range](container.intro.reqmts#concept:container-compatible-range "23.2.2.1 Introduction [container.intro.reqmts]")<value_type> R>
  constexpr multimap(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#L12504)

*Effects*: Constructs an empty multimap 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#L12510)

*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)

#### [23.4.4.3](#modifiers) Modifiers [[multimap.modifiers]](multimap.modifiers)

[🔗](#lib:insert,multimap)

`template<class P> constexpr iterator insert(P&& x);
template<class P> constexpr iterator insert(const_iterator position, P&& x);
`

[1](#modifiers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12525)

*Constraints*: is_constructible_v<value_type, P&&> is true[.](#modifiers-1.sentence-1)

[2](#modifiers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12529)

*Effects*: The first form is equivalent toreturn emplace(std​::​forward<P>(x))[.](#modifiers-2.sentence-1)

The second form is
equivalent to return emplace_hint(position, std​::​forward<P>(x))[.](#modifiers-2.sentence-2)

#### [23.4.4.4](#erasure) Erasure [[multimap.erasure]](multimap.erasure)

[🔗](#lib:erase_if,multimap)

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

[1](#erasure-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L12546)

*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();