[unord.set]

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

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

### 23.5.6 Class template unordered_set [unord.set]

#### [23.5.6.1](#overview) Overview [[unord.set.overview]](unord.set.overview)

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14691)

An unordered_set is an unordered associative container that
supports unique keys (an unordered_set contains at most one of each
key value) and in which the elements' keys are the elements
themselves[.](#overview-1.sentence-1)

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

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14701)

An unordered_set 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 unique keys;
that is, an unordered_set supports the a_uniq operations in that table,
not the a_eq operations[.](#overview-2.sentence-2)

For an unordered_set<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#L14714)

Subclause [unord.set] only describes operations on unordered_set 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#L14719)

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_set_)

namespace std {template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>>class unordered_set {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*; using insert_return_type = *insert-return-type*<iterator, node_type>; // [[unord.set.cnstr]](#cnstr "23.5.6.2 Constructors"), construct/copy/destroyconstexpr unordered_set(); constexpr explicit unordered_set(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); template<class InputIterator>constexpr unordered_set(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_set(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_set(const unordered_set&); constexpr unordered_set(unordered_set&&); constexpr explicit unordered_set(const Allocator&); constexpr unordered_set(const unordered_set&, const type_identity_t<Allocator>&); constexpr unordered_set(unordered_set&&, const type_identity_t<Allocator>&); constexpr unordered_set(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_set(size_type n, const allocator_type& a): unordered_set(n, hasher(), key_equal(), a) { }constexpr unordered_set(size_type n, const hasher& hf, const allocator_type& a): unordered_set(n, hf, key_equal(), a) { }template<class InputIterator>constexpr unordered_set(InputIterator f, InputIterator l, size_type n, const allocator_type& a): unordered_set(f, l, n, hasher(), key_equal(), a) { }template<class InputIterator>constexpr unordered_set(InputIterator f, InputIterator l, size_type n, const hasher& hf, const allocator_type& a): unordered_set(f, l, n, hf, key_equal(), a) { }constexpr unordered_set(initializer_list<value_type> il, size_type n, const allocator_type& a): unordered_set(il, 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_set(from_range_t, R&& rg, size_type n, const allocator_type& a): unordered_set(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_set(from_range_t, R&& rg, size_type n, const hasher& hf, const allocator_type& a): unordered_set(from_range, std::forward<R>(rg), n, hf, key_equal(), a) { }constexpr unordered_set(initializer_list<value_type> il, size_type n, const hasher& hf, const allocator_type& a): unordered_set(il, n, hf, key_equal(), a) { }constexpr ~unordered_set(); constexpr unordered_set& operator=(const unordered_set&); constexpr unordered_set& operator=(unordered_set&&)noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_move_assignable_v<Hash> && is_nothrow_move_assignable_v<Pred>); constexpr unordered_set& 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.set.modifiers]](#modifiers "23.5.6.4 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 K> constexpr pair<iterator, bool> insert(K&& obj); constexpr iterator insert(const_iterator hint, const value_type& obj); constexpr iterator insert(const_iterator hint, value_type&& obj); template<class K> constexpr iterator insert(const_iterator hint, K&& 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); 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_set&)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_set<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_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2>constexpr void merge(unordered_multiset<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_set(InputIterator, InputIterator, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_set<*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_set(from_range_t, R&&, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_set<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_set(initializer_list<T>, typename *see below*::size_type = *see below*,
 Hash = Hash(), Pred = Pred(), Allocator = Allocator())-> unordered_set<T, Hash, Pred, Allocator>; template<class InputIterator, class Allocator> unordered_set(InputIterator, InputIterator, typename *see below*::size_type, Allocator)-> unordered_set<*iter-value-type*<InputIterator>,
 hash<*iter-value-type*<InputIterator>>,
 equal_to<*iter-value-type*<InputIterator>>,
 Allocator>; template<class InputIterator, class Hash, class Allocator> unordered_set(InputIterator, InputIterator, typename *see below*::size_type,
 Hash, Allocator)-> unordered_set<*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_set(from_range_t, R&&, typename *see below*::size_type, Allocator)-> unordered_set<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_set(from_range_t, R&&, Allocator)-> unordered_set<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_set(from_range_t, R&&, typename *see below*::size_type, Hash, Allocator)-> unordered_set<ranges::range_value_t<R>, Hash,
 equal_to<ranges::range_value_t<R>>, Allocator>; template<class T, class Allocator> unordered_set(initializer_list<T>, typename *see below*::size_type, Allocator)-> unordered_set<T, hash<T>, equal_to<T>, Allocator>; template<class T, class Hash, class Allocator> unordered_set(initializer_list<T>, typename *see below*::size_type, Hash, Allocator)-> unordered_set<T, Hash, equal_to<T>, Allocator>;}

[5](#overview-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L14970)

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

#### [23.5.6.2](#cnstr) Constructors [[unord.set.cnstr]](unord.set.cnstr)

[🔗](#lib:unordered_set,constructor)

`constexpr unordered_set() : unordered_set(size_type(see below)) { }
constexpr explicit unordered_set(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#L14986)

*Effects*: Constructs an empty unordered_set 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#L14995)

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

[🔗](#lib:unordered_set,constructor_)

`template<class InputIterator>
  constexpr unordered_set(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_set(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#L15019)

*Effects*: Constructs an empty unordered_set 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#L15030)

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

#### [23.5.6.3](#erasure) Erasure [[unord.set.erasure]](unord.set.erasure)

[🔗](#lib:erase_if,unordered_set)

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

[1](#erasure-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15045)

*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();

#### [23.5.6.4](#modifiers) Modifiers [[unord.set.modifiers]](unord.set.modifiers)

[🔗](#lib:insert,unordered_set)

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

[1](#modifiers-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15070)

*Constraints*: The [*qualified-id*](expr.prim.id.qual#nt:qualified-id "7.5.5.3 Qualified names [expr.prim.id.qual]")*s* Hash​::​is_transparent andPred​::​is_transparent are valid and denote types[.](#modifiers-1.sentence-1)

For the second overload,is_convertible_v<K&&, const_iterator> andis_convertible_v<K&&, iterator> are both false[.](#modifiers-1.sentence-2)

[2](#modifiers-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15078)

*Preconditions*: value_type is *Cpp17EmplaceConstructible* into unordered_set from std​::​forward<K>  
(obj)[.](#modifiers-2.sentence-1)

[3](#modifiers-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15083)

*Effects*: If the set already contains an element that is equivalent to obj,
there is no effect[.](#modifiers-3.sentence-1)

Otherwise, let h be hash_function()(obj)[.](#modifiers-3.sentence-2)

Constructs an object u of type value_type with std​::​forward<K>(obj)[.](#modifiers-3.sentence-3)

If hash_function()(u) != h || contains(u) is true,
the behavior is undefined[.](#modifiers-3.sentence-4)

Inserts u into *this[.](#modifiers-3.sentence-5)

[4](#modifiers-4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15094)

*Returns*: For the first overload,
the bool component of the returned pair is true if and only if the insertion took place[.](#modifiers-4.sentence-1)

The returned iterator points to the set element
that is equivalent to obj[.](#modifiers-4.sentence-2)

[5](#modifiers-5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L15102)

*Complexity*: Average case constant, worst case linear[.](#modifiers-5.sentence-1)