[unord.map.overview]

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

## 23.5 Unordered associative containers [[unord]](unord#map.overview)

### 23.5.3 Class template unordered_map [[unord.map]](unord.map#overview)

#### 23.5.3.1 Overview [unord.map.overview]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13407)

An unordered_map is an unordered associative container that
supports unique keys (an unordered_map contains at most one of each
key value) and that associates values of another typemapped_type with the keys[.](#1.sentence-1)

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

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13417)

An unordered_map 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"))[.](#2.sentence-1)

It provides the operations described in the preceding requirements table for unique keys;
that is, an unordered_map supports the a_uniq operations in that table,
not the a_eq operations[.](#2.sentence-2)

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

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13429)

Subclause [[unord.map]](unord.map "23.5.3 Class template unordered_­map") only describes operations on unordered_map 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#L13434)

The types iterator and const_iterator meet
the constexpr iterator requirements ([[iterator.requirements.general]](iterator.requirements.general "24.3.1 General"))[.](#4.sentence-1)

[ð](#lib:unordered_map)

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_map {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*; using insert_return_type = *insert-return-type*<iterator, node_type>; // [[unord.map.cnstr]](unord.map.cnstr "23.5.3.2 Constructors"), construct/copy/destroyconstexpr unordered_map(); constexpr explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator>constexpr unordered_map(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_map(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_map(const unordered_map&); constexpr unordered_map(unordered_map&&); constexpr explicit unordered_map(const Allocator&); constexpr unordered_map(const unordered_map&, const type_identity_t<Allocator>&); constexpr unordered_map(unordered_map&&, const type_identity_t<Allocator>&); constexpr unordered_map(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_map(size_type n, const allocator_type& a): unordered_map(n, hasher(), key_equal(), a) { }constexpr unordered_map(size_type n, const hasher& hf, const allocator_type& a): unordered_map(n, hf, key_equal(), a) { }template<class InputIterator>constexpr unordered_map(InputIterator f, InputIterator l, size_type n, const allocator_type& a): unordered_map(f, l, n, hasher(), key_equal(), a) { }template<class InputIterator>constexpr unordered_map(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a): unordered_map(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_map(from_range_t, R&& rg, size_type n, const allocator_type& a): unordered_map(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_map(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a): unordered_map(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { }constexpr unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a): unordered_map(il, n, hasher(), key_equal(), a) { }constexpr unordered_map(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a): unordered_map(il, n, hf, key_equal(), a) { }constexpr ~unordered_map(); constexpr unordered_map& operator=(const unordered_map&); constexpr unordered_map& operator=(unordered_map&&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); constexpr unordered_map& 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.map.modifiers]](unord.map.modifiers "23.5.3.4 Modifiers"), modifierstemplate<class... Args> constexpr pair<iterator, bool> emplace(Args&&... args); template<class... Args>constexpr iterator emplace_hint(const_iterator position, Args&&... args); constexpr pair<iterator, bool> insert(const value_type& obj); constexpr pair<iterator, bool> insert(value_type&& obj); template<class P> constexpr pair<iterator, bool> 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 insert_return_type insert(node_type&& nh); constexpr iterator insert(const_iterator hint, node_type&& nh); template<class... Args>constexpr pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template<class... Args>constexpr pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template<class K, class... Args>constexpr pair<iterator, bool> try_emplace(K&& k, Args&&... args); template<class... Args>constexpr iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args); template<class... Args>constexpr iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args); template<class K, class... Args>constexpr iterator try_emplace(const_iterator hint, K&& k, Args&&... args); template<class M>constexpr pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template<class M>constexpr pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template<class K, class M>constexpr pair<iterator, bool> insert_or_assign(K&& k, M&& obj); template<class M>constexpr iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj); template<class M>constexpr iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj); template<class K, class M>constexpr iterator insert_or_assign(const_iterator hint, K&& k, M&& obj); 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_map&)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_map<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_multimap<Key, T, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_multimap<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; // [[unord.map.elem]](unord.map.elem "23.5.3.3 Element access"), element accessconstexpr mapped_type& operator[](const key_type& k); constexpr mapped_type& operator[](key_type&& k); template<class K> constexpr mapped_type& operator[](K&& k); constexpr mapped_type& at(const key_type& k); constexpr const mapped_type& at(const key_type& k) const; template<class K> constexpr mapped_type& at(const K& k); template<class K> constexpr const mapped_type& at(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_map(InputIterator, InputIterator, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_map<*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_map(from_range_t, R&&, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_map<*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_map(initializer_list<pair<Key, T>>, typename *see below*::size_type = *see below*, Hash = Hash(),
 Pred = Pred(), Allocator = Allocator())-> unordered_map<Key, T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_map(InputIterator, InputIterator, typename *see below*::size_type, Allocator)-> unordered_map<*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_map(InputIterator, InputIterator, Allocator)-> unordered_map<*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_map(InputIterator, InputIterator, typename *see below*::size_type, Hash, Allocator)-> unordered_map<*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_map(from_range_t, R&&, typename *see below*::size_type, Allocator)-> unordered_map<*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_map(from_range_t, R&&, Allocator)-> unordered_map<*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_map(from_range_t, R&&, typename *see below*::size_type, Hash, Allocator)-> unordered_map<*range-key-type*<R>, *range-mapped-type*<R>, Hash,
 equal_to<*range-key-type*<R>>, Allocator>; template<class Key, class T, class Allocator> unordered_map(initializer_list<pair<Key, T>>, typename *see below*::size_type,
 Allocator)-> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Allocator> unordered_map(initializer_list<pair<Key, T>>, Allocator)-> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>; template<class Key, class T, class Hash, class Allocator> unordered_map(initializer_list<pair<Key, T>>, typename *see below*::size_type, Hash,
 Allocator)-> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;}

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L13730)

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