[unord.multimap]

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

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

### 23.5.4 Class template unordered_multimap [unord.multimap]

#### [23.5.4.1](#overview) Overview [[unord.multimap.overview]](unord.multimap.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14202)

An unordered_multimap is an unordered associative container
that supports equivalent keys (an instance of unordered_multimap may contain
multiple copies of each key value) and that associates values of
another type mapped_type with the keys[.](#overview-1.sentence-1)

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

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14212)

An unordered_multimap 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_multimap supports the a_eq operations in that table, not the a_uniq operations[.](#overview-2.sentence-2)

For an unordered_multimap<Key, T> the key_type is Key,
the mapped_type is T,
and the value_type is pair<const Key, T>[.](#overview-2.sentence-3)

[3](#overview-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14224)

Subclause [unord.multimap] only describes operations on unordered_multimap 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#L14229)

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_multimap_)

namespace std {template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>>class unordered_multimap {public:// typesusing key_type = Key; using mapped_type = T; using value_type = pair<const Key, T>; 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.multimap.cnstr]](#cnstr "23.5.4.2 Constructors"), construct/copy/destroyconstexpr unordered_multimap(); constexpr explicit unordered_multimap(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator>constexpr unordered_multimap(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_multimap(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_multimap(const unordered_multimap&); constexpr unordered_multimap(unordered_multimap&&); constexpr explicit unordered_multimap(const Allocator&); constexpr unordered_multimap(const unordered_multimap&, const type_identity_t<Allocator>&); constexpr unordered_multimap(unordered_multimap&&, const type_identity_t<Allocator>&); constexpr unordered_multimap(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_multimap(size_type n, const allocator_type& a): unordered_multimap(n, hasher(), key_equal(), a) { }constexpr unordered_multimap(size_type n, const hasher& hf, const allocator_type& a): unordered_multimap(n, hf, key_equal(), a) { }template<class InputIterator>constexpr unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a): unordered_multimap(f, l, n, hasher(), key_equal(), a) { }template<class InputIterator>constexpr unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a): unordered_multimap(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_multimap(from_range_t, R&& rg, size_type n, const allocator_type& a): unordered_multimap(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_multimap(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a): unordered_multimap(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { }constexpr unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a): unordered_multimap(il, n, hasher(), key_equal(), a) { }constexpr unordered_multimap(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a): unordered_multimap(il, n, hf, key_equal(), a) { }constexpr ~unordered_multimap(); constexpr unordered_multimap& operator=(const unordered_multimap&); constexpr unordered_multimap& operator=(unordered_multimap&&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); constexpr unordered_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 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; // [[unord.multimap.modifiers]](#modifiers "23.5.4.3 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& obj); constexpr iterator insert(value_type&& obj); template<class P> constexpr iterator insert(P&& obj); constexpr iterator insert(const_iterator hint, const value_type& obj); constexpr iterator insert(const_iterator hint, value_type&& obj); template<class P> constexpr iterator insert(const_iterator hint, P&& 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); 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_multimap&)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_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source); template<class H2, class P2>constexpr void merge(unordered_map<Key, T, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_map<Key, T, H2, P2, Allocator>&& source); // observersconstexpr hasher hash_function() const; constexpr key_equal key_eq() const; // map 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-key-type*<InputIterator>>, class Pred = equal_to<*iter-key-type*<InputIterator>>, class Allocator = allocator<*iter-to-alloc-type*<InputIterator>>> unordered_multimap(InputIterator, InputIterator, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multimap<*iter-key-type*<InputIterator>, *iter-mapped-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<*range-key-type*<R>>, class Pred = equal_to<*range-key-type*<R>>, class Allocator = allocator<*range-to-alloc-type*<R>>> unordered_multimap(from_range_t, R&&, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multimap<*range-key-type*<R>, *range-mapped-type*<R>, Hash, Pred, Allocator>; template<class Key, class T, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<pair<const Key, T>>> unordered_multimap(initializer_list<pair<Key, T>>, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_multimap<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, typename *see below*::size_type, Allocator)-> unordered_multimap<*iter-key-type*<InputIterator>, *iter-mapped-type*<InputIterator>,
 hash<*iter-key-type*<InputIterator>>,
 equal_to<*iter-key-type*<InputIterator>>, Allocator>; template<class InputIterator, class Allocator> unordered_multimap(InputIterator, InputIterator, Allocator)-> unordered_multimap<*iter-key-type*<InputIterator>, *iter-mapped-type*<InputIterator>,
 hash<*iter-key-type*<InputIterator>>,
 equal_to<*iter-key-type*<InputIterator>>, Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_multimap(InputIterator, InputIterator, typename *see below*::size_type, Hash,
 Allocator)-> unordered_multimap<*iter-key-type*<InputIterator>, *iter-mapped-type*<InputIterator>, Hash,
 equal_to<*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> unordered_multimap(from_range_t, R&&, typename *see below*::size_type, Allocator)-> unordered_multimap<*range-key-type*<R>, *range-mapped-type*<R>, hash<*range-key-type*<R>>,
 equal_to<*range-key-type*<R>>, Allocator>; template<ranges::[input_range](range.refinements#concept:input_range "25.4.6 Other range refinements [range.refinements]") R, class Allocator> unordered_multimap(from_range_t, R&&, Allocator)-> unordered_multimap<*range-key-type*<R>, *range-mapped-type*<R>, hash<*range-key-type*<R>>,
 equal_to<*range-key-type*<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_multimap(from_range_t, R&&, typename *see below*::size_type, Hash, Allocator)-> unordered_multimap<*range-key-type*<R>, *range-mapped-type*<R>, Hash,
 equal_to<*range-key-type*<R>>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, typename *see below*::size_type,
 Allocator)-> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, Allocator)-> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_multimap(initializer_list<pair<Key, T>>, typename *see below*::size_type,
 Hash, Allocator)-> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>;}

[5](#overview-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14493)

A size_type parameter type in an unordered_multimap deduction guide
refers to the size_type member type of the type deduced by the deduction guide[.](#overview-5.sentence-1)

#### [23.5.4.2](#cnstr) Constructors [[unord.multimap.cnstr]](unord.multimap.cnstr)

[🔗](#lib:unordered_multimap,constructor)

`constexpr unordered_multimap() : unordered_multimap(size_type(see below)) { }
constexpr explicit unordered_multimap(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#L14508)

*Effects*: Constructs an empty unordered_multimap 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#L14517)

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

[🔗](#lib:unordered_multimap,constructor_)

`template<class InputIterator>
  constexpr unordered_multimap(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_multimap(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_multimap(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#L14541)

*Effects*: Constructs an empty unordered_multimap 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#L14552)

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

#### [23.5.4.3](#modifiers) Modifiers [[unord.multimap.modifiers]](unord.multimap.modifiers)

[🔗](#lib:unordered_multimap,insert)

`template<class P>
  constexpr iterator insert(P&& obj);
`

[1](#modifiers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14566)

*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#L14570)

*Effects*: Equivalent to: return emplace(std​::​forward<P>(obj));

[🔗](#lib:unordered_multimap,insert_)

`template<class P>
  constexpr iterator insert(const_iterator hint, P&& obj);
`

[3](#modifiers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14582)

*Constraints*: is_constructible_v<value_type, P&&> is true[.](#modifiers-3.sentence-1)

[4](#modifiers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14586)

*Effects*: Equivalent to:return emplace_hint(hint, std​::​forward<P>(obj));

#### [23.5.4.4](#erasure) Erasure [[unord.multimap.erasure]](unord.multimap.erasure)

[🔗](#lib:erase_if,unordered_multimap)

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

[1](#erasure-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14602)

*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();