[unord.set]

24.5.6.1 Overview [unord.set.overview]

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.

The unordered_set class supports forward iterators.

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An unordered_set meets all of the requirements of a container ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), of an unordered associative container ([unord.req]).

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.

For an unordered_set<Key> the key_type and the value_type are both Key.

The iterator and const_iterator types are both constant iterator types.

It is unspecified whether they are the same type.

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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.

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namespace std { template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>, class Allocator = allocator<Key>> class unordered_set { public: // types using 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] using difference_type = implementation-defined; // see [container.requirements] using iterator = implementation-defined; // see [container.requirements] using const_iterator = implementation-defined; // see [container.requirements] using local_iterator = implementation-defined; // see [container.requirements] using const_local_iterator = implementation-defined; // see [container.requirements] using node_type = unspecified; using insert_return_type = insert-return-type<iterator, node_type>; // [unord.set.cnstr], construct/copy/destroy unordered_set(); 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> 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<value_type> R> 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()); unordered_set(const unordered_set&); unordered_set(unordered_set&&); explicit unordered_set(const Allocator&); unordered_set(const unordered_set&, const type_identity_t<Allocator>&); unordered_set(unordered_set&&, const type_identity_t<Allocator>&); 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()); unordered_set(size_type n, const allocator_type& a) : unordered_set(n, hasher(), key_equal(), a) { } unordered_set(size_type n, const hasher& hf, const allocator_type& a) : unordered_set(n, hf, key_equal(), a) { } template<class InputIterator> 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> 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) { } 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<value_type> R> 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<value_type> R> 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) { } 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) { } ~unordered_set(); unordered_set& operator=(const unordered_set&); 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>); unordered_set& operator=(initializer_list<value_type>); allocator_type get_allocator() const noexcept; // iterators iterator begin() noexcept; const_iterator begin() const noexcept; iterator end() noexcept; const_iterator end() const noexcept; const_iterator cbegin() const noexcept; const_iterator cend() const noexcept; // capacity [[nodiscard]] bool empty() const noexcept; size_type size() const noexcept; size_type max_size() const noexcept; // [unord.set.modifiers], modifiers template<class... Args> pair<iterator, bool> emplace(Args&&... args); template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args); pair<iterator, bool> insert(const value_type& obj); pair<iterator, bool> insert(value_type&& obj); template<class K> pair<iterator, bool> insert(K&& obj); iterator insert(const_iterator hint, const value_type& obj); iterator insert(const_iterator hint, value_type&& obj); template<class K> iterator insert(const_iterator hint, K&& obj); template<class InputIterator> void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> void insert_range(R&& rg); void insert(initializer_list<value_type>); node_type extract(const_iterator position); node_type extract(const key_type& x); template<class K> node_type extract(K&& x); insert_return_type insert(node_type&& nh); iterator insert(const_iterator hint, node_type&& nh); iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); iterator erase(const_iterator position); size_type erase(const key_type& k); template<class K> size_type erase(K&& x); iterator erase(const_iterator first, const_iterator last); void swap(unordered_set&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); void clear() noexcept; template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_set<Key, H2, P2, Allocator>&& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); // observers hasher hash_function() const; key_equal key_eq() const; // set operations iterator find(const key_type& k); const_iterator find(const key_type& k) const; template<class K> iterator find(const K& k); template<class K> const_iterator find(const K& k) const; size_type count(const key_type& k) const; template<class K> size_type count(const K& k) const; bool contains(const key_type& k) const; template<class K> bool contains(const K& k) const; pair<iterator, iterator> equal_range(const key_type& k); pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> pair<iterator, iterator> equal_range(const K& k); template<class K> pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface size_type bucket_count() const noexcept; size_type max_bucket_count() const noexcept; size_type bucket_size(size_type n) const; size_type bucket(const key_type& k) const; template<class K> size_type bucket(const K& k) const; local_iterator begin(size_type n); const_local_iterator begin(size_type n) const; local_iterator end(size_type n); const_local_iterator end(size_type n) const; const_local_iterator cbegin(size_type n) const; const_local_iterator cend(size_type n) const; // hash policy float load_factor() const noexcept; float max_load_factor() const noexcept; void max_load_factor(float z); void rehash(size_type n); 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 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 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 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 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>; }

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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.

24.5.6.2 Constructors [unord.set.cnstr]

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unordered_set() : unordered_set(size_type(see below)) { }
explicit unordered_set(size_type n,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());

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Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

For the default constructor, the number of buckets is implementation-defined.

max_load_factor() returns 1.0.

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Complexity: Constant.

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template<class InputIterator>
  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<value_type> R>
  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());
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());

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Effects: Constructs an empty unordered_set using the specified hash function, key equality predicate, and allocator, and using at least n buckets.

If n is not provided, the number of buckets is implementation-defined.

Then inserts elements from the range [f, l), rg, or il, respectively.

max_load_factor() returns 1.0.

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Complexity: Average case linear, worst case quadratic.

24.5.6.3 Erasure [unord.set.erasure]

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

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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();

24.5.6.4 Modifiers [unord.set.modifiers]

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

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Constraints: The qualified-ids Hash::is_transparent and Pred::is_transparent are valid and denote types.

For the second overload, is_convertible_v<K&&, const_iterator> and is_convertible_v<K&&, iterator> are both false.

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Preconditions: value_type is Cpp17EmplaceConstructible into unordered_set from std::forward<K>
(obj).

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Effects: If the set already contains an element that is equivalent to obj, there is no effect.

Otherwise, let h be hash_function()(obj).

Constructs an object u of type value_type with std::forward<K>(obj).

If hash_function()(u) != h || contains(u) is true, the behavior is undefined.

Inserts u into *this.

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Returns: For the first overload, the bool component of the returned pair is true if and only if the insertion took place.

The returned iterator points to the set element that is equivalent to obj.

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Complexity: Average case constant, worst case linear.

24 Containers library [containers]

24.5 Unordered associative containers [unord]

24.5.6 Class template unordered_set [unord.set]

24.5.6.1 Overview [unord.set.overview]

24.5.6.2 Constructors [unord.set.cnstr]

24.5.6.3 Erasure [unord.set.erasure]

24.5.6.4 Modifiers [unord.set.modifiers]