Standard library header <unordered_set> (C++11)

 

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Language support <cfloat> <climits> <compare> (C++20) <coroutine> (C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint> (C++11) <cstdlib> <exception> <initializer_list> (C++11)    <limits> <new> <source_location> (C++20) <stdfloat> (C++23) <typeinfo> <version> (C++20) Concepts <concepts> (C++20) Diagnostics <cassert> <cerrno> <debugging> (C++26) <stacktrace> (C++23) <stdexcept> <system_error> (C++11) Memory management <memory> <memory_resource> (C++17) <scoped_allocator> (C++11) Metaprogramming <type_traits> (C++11) <ratio> (C++11)

General utilities <any> (C++17) <bitset> <bit> (C++20) <charconv> (C++17) <expected> (C++23) <format> (C++20) <functional> <optional> (C++17) <tuple> (C++11) <typeindex> (C++11) <utility> <variant> (C++17) Containers <array> (C++11) <deque> <flat_map> (C++23) <flat_set> (C++23) <forward_list> (C++11) <inplace_vector> (C++26)    <list> <map> <mdspan> (C++23) <queue> <set> <span> (C++20) <stack> <unordered_map> (C++11) <unordered_set> (C++11) <vector> Iterators <iterator> Ranges <generator> (C++23) <ranges> (C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar> (C++11) <cwchar> <cwctype> <string_view> (C++17) <string> Text processing <clocale> <codecvt> (C++11/17/26*) <locale> <regex> (C++11) <text_encoding> (C++26)    Numerics <cfenv> (C++11) <cmath> <complex> <linalg> (C++26) <numbers> (C++20) <random> (C++11) <valarray> Time <chrono> (C++11) <ctime> C compatibility <ccomplex> (C++11/17/20*) <ciso646> (until C++20) <cstdalign> (C++11/17/20*) <cstdbool> (C++11/17/20*) <ctgmath> (C++11/17/20*)

Input/output <cinttypes> (C++11) <cstdio> <filesystem> (C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print> (C++23) <spanstream> (C++23) <sstream> <streambuf> <strstream> (C++98/26*) <syncstream> (C++20) Concurrency support <atomic> (C++11) <barrier> (C++20) <condition_variable> (C++11) <future> (C++11) <hazard_pointer> (C++26) <latch> (C++20) <mutex> (C++11) <rcu> (C++26) <semaphore> (C++20) <shared_mutex> (C++14) <stdatomic.h> (C++23) <stop_token> (C++20) <thread> (C++11) Execution support <execution> (C++17)

 

This header is part of the containers library.

Includes
<compare> (C++20)	Three-way comparison operator support
<initializer_list> (C++11)	std::initializer_list class template
Classes
unordered_set (C++11)	collection of unique keys, hashed by keys (class template)
unordered_multiset (C++11)	collection of keys, hashed by keys (class template)
Functions
operator==operator!= (C++11)(C++11)(removed in C++20)	compares the values in the unordered_set (function template)
std::swap(std::unordered_set) (C++11)	specializes the std::swap algorithm (function template)
erase_if(std::unordered_set) (C++20)	erases all elements satisfying specific criteria (function template)
operator==operator!= (C++11)(C++11)(removed in C++20)	compares the values in the unordered_multiset (function template)
std::swap(std::unordered_multiset) (C++11)	specializes the std::swap algorithm (function template)
erase_if(std::unordered_multiset) (C++20)	erases all elements satisfying specific criteria (function template)
Range access
begincbegin (C++11)(C++14)	returns an iterator to the beginning of a container or array (function template)
endcend (C++11)(C++14)	returns an iterator to the end of a container or array (function template)
rbegincrbegin (C++14)	returns a reverse iterator to the beginning of a container or array (function template)
rendcrend (C++14)	returns a reverse end iterator for a container or array (function template)
sizessize (C++17)(C++20)	returns the size of a container or array (function template)
empty (C++17)	checks whether the container is empty (function template)
data (C++17)	obtains the pointer to the underlying array (function template)

Synopsis

#include <compare>
#include <initializer_list>
 
namespace std {
  // class template unordered_set
  template<class Key,
           class Hash  = hash<Key>,
           class Pred  = equal_to<Key>,
           class Alloc = allocator<Key>>
  class unordered_set;
 
  // class template unordered_multiset
  template<class Key,
           class Hash  = hash<Key>,
           class Pred  = equal_to<Key>,
           class Alloc = allocator<Key>>
  class unordered_multiset;
 
  template<class Key, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_set<Key, Hash, Pred, Alloc>& a,
                  const unordered_set<Key, Hash, Pred, Alloc>& b);
 
  template<class Key, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_multiset<Key, Hash, Pred, Alloc>& a,
                  const unordered_multiset<Key, Hash, Pred, Alloc>& b);
 
  template<class Key, class Hash, class Pred, class Alloc>
  void swap(unordered_set<Key, Hash, Pred, Alloc>& x,
            unordered_set<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y)));
 
  template<class Key, class Hash, class Pred, class Alloc>
  void swap(unordered_multiset<Key, Hash, Pred, Alloc>& x,
            unordered_multiset<Key, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y)));
 
  // erasure for unordered_set
  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);
 
  // erasure for unordered_multiset
  template<class K, class H, class P, class A, class Predicate>
  typename unordered_multiset<K, H, P, A>::size_type erase_if(
    unordered_multiset<K, H, P, A>& c,
    Predicate pred);
 
  namespace pmr {
    template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>>
    using unordered_set = std::unordered_set<Key, Hash, Pred, polymorphic_allocator<Key>>;
 
    template<class Key, class Hash = hash<Key>, class Pred = equal_to<Key>>
    using unordered_multiset =
      std::unordered_multiset<Key, Hash, Pred, polymorphic_allocator<Key>>;
  }
}

Class template std::unordered_set

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 */;
    using difference_type      = /* implementation-defined */;
 
    using iterator             = /* implementation-defined */;
    using const_iterator       = /* implementation-defined */;
    using local_iterator       = /* implementation-defined */;
    using const_local_iterator = /* implementation-defined */;
    using node_type            = /* unspecified */;
    using insert_return_type   = /*insert-return-type*/<iterator, node_type>;
 
    // 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 InputIter>
    unordered_set(InputIter f,
                  InputIter l,
                  size_type n             = /* see description */,
                  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 description */,
                  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 description */,
                  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 InputIter>
    unordered_set(InputIter f, InputIter l, size_type n, const allocator_type& a)
      : unordered_set(f, l, n, hasher(), key_equal(), a)
    {
    }
    template<class InputIter>
    unordered_set(InputIter f,
                  InputIter 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
    bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;
 
    // 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 InputIter>
    void insert(InputIter first, InputIter 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 InputIter,
           class Hash      = hash</*iter-value-type*/<InputIter>>,
           class Pred      = equal_to</*iter-value-type*/<InputIter>>,
           class Allocator = allocator</*iter-value-type*/<InputIter>>>
  unordered_set(InputIter,
                InputIter,
                typename /* see description */ ::size_type = /* see description */,
                Hash                                       = Hash(),
                Pred                                       = Pred(),
                Allocator                                  = Allocator())
    -> unordered_set</*iter-value-type*/<InputIter>, 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 description */ ::size_type = /* see description */,
                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 description */ ::size_type = /* see description */,
                Hash                                       = Hash(),
                Pred                                       = Pred(),
                Allocator = Allocator()) -> unordered_set<T, Hash, Pred, Allocator>;
 
  template<class InputIter, class Allocator>
  unordered_set(InputIter,
                InputIter,
                typename /* see description */ ::size_type,
                Allocator) -> unordered_set</*iter-value-type*/<InputIter>,
                                            hash</*iter-value-type*/<InputIter>>,
                                            equal_to</*iter-value-type*/<InputIter>>,
                                            Allocator>;
 
  template<class InputIter, class Hash, class Allocator>
  unordered_set(InputIter,
                InputIter,
                typename /* see description */ ::size_type,
                Hash,
                Allocator) -> unordered_set</*iter-value-type*/<InputIter>,
                                            Hash,
                                            equal_to</*iter-value-type*/<InputIter>>,
                                            Allocator>;
 
  template<ranges::input_range R, class Allocator>
  unordered_set(from_range_t, R&&, typename /* see description */ ::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 description */ ::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 description */ ::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 description */ ::size_type,
                Hash,
                Allocator) -> unordered_set<T, Hash, equal_to<T>, Allocator>;
}

Class template std::unordered_multiset

namespace std {
  template<class Key,
           class Hash      = hash<Key>,
           class Pred      = equal_to<Key>,
           class Allocator = allocator<Key>>
  class unordered_multiset
  {
  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 */;
    using difference_type      = /* implementation-defined */;
 
    using iterator             = /* implementation-defined */;
    using const_iterator       = /* implementation-defined */;
    using local_iterator       = /* implementation-defined */;
    using const_local_iterator = /* implementation-defined */;
    using node_type            = /* unspecified */;
 
    // construct/copy/destroy
    unordered_multiset();
    explicit unordered_multiset(size_type n,
                                const hasher& hf        = hasher(),
                                const key_equal& eql    = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIter>
    unordered_multiset(InputIter f,
                       InputIter l,
                       size_type n             = /* see description */,
                       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 description */,
                       const hasher& hf        = hasher(),
                       const key_equal& eql    = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(const unordered_multiset&);
    unordered_multiset(unordered_multiset&&);
    explicit unordered_multiset(const Allocator&);
    unordered_multiset(const unordered_multiset&, const type_identity_t<Allocator>&);
    unordered_multiset(unordered_multiset&&, const type_identity_t<Allocator>&);
    unordered_multiset(initializer_list<value_type> il,
                       size_type n             = /* see description */,
                       const hasher& hf        = hasher(),
                       const key_equal& eql    = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multiset(size_type n, const allocator_type& a)
      : unordered_multiset(n, hasher(), key_equal(), a)
    {
    }
    unordered_multiset(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_multiset(n, hf, key_equal(), a)
    {
    }
    template<class InputIter>
    unordered_multiset(InputIter f, InputIter l, size_type n, const allocator_type& a)
      : unordered_multiset(f, l, n, hasher(), key_equal(), a)
    {
    }
    template<class InputIter>
    unordered_multiset(InputIter f,
                       InputIter l,
                       size_type n,
                       const hasher& hf,
                       const allocator_type& a)
      : unordered_multiset(f, l, n, hf, key_equal(), a)
    {
    }
    template<container-compatible-range<value_type> R>
    unordered_multiset(from_range_t, R&& rg, size_type n, const allocator_type& a)
      : unordered_multiset(from_range, std::forward<R>(rg), n, hasher(), key_equal(), a)
    {
    }
    template<container-compatible-range<value_type> R>
    unordered_multiset(from_range_t,
                       R&& rg,
                       size_type n,
                       const hasher& hf,
                       const allocator_type& a)
      : unordered_multiset(from_range, std::forward<R>(rg), n, hf, key_equal(), a)
    {
    }
    unordered_multiset(initializer_list<value_type> il,
                       size_type n,
                       const allocator_type& a)
      : unordered_multiset(il, n, hasher(), key_equal(), a)
    {
    }
    unordered_multiset(initializer_list<value_type> il,
                       size_type n,
                       const hasher& hf,
                       const allocator_type& a)
      : unordered_multiset(il, n, hf, key_equal(), a)
    {
    }
    ~unordered_multiset();
    unordered_multiset& operator=(const unordered_multiset&);
    unordered_multiset& operator=(unordered_multiset&&) noexcept(
      allocator_traits<Allocator>::is_always_equal::value&&
        is_nothrow_move_assignable_v<Hash>&& is_nothrow_move_assignable_v<Pred>);
    unordered_multiset& 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
    bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;
 
    // modifiers
    template<class... Args>
    iterator emplace(Args&&... args);
    template<class... Args>
    iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class InputIter>
    void insert(InputIter first, InputIter 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);
    iterator 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_multiset&) 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_multiset<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_set<Key, H2, P2, Allocator>& source);
    template<class H2, class P2>
    void merge(unordered_set<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 InputIter,
           class Hash      = hash</*iter-value-type*/<InputIter>>,
           class Pred      = equal_to</*iter-value-type*/<InputIter>>,
           class Allocator = allocator</*iter-value-type*/<InputIter>>>
  unordered_multiset(InputIter,
                     InputIter,
                     /* see description */ ::size_type = /* see description */,
                     Hash                              = Hash(),
                     Pred                              = Pred(),
                     Allocator                         = Allocator())
    -> unordered_multiset</*iter-value-type*/<InputIter>, 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_multiset(from_range_t,
                     R&&,
                     typename /* see description */ ::size_type = /* see description */,
                     Hash                                       = Hash(),
                     Pred                                       = Pred(),
                     Allocator                                  = Allocator())
    -> unordered_multiset<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_multiset(initializer_list<T>,
                     typename /* see description */ ::size_type = /* see description */,
                     Hash                                       = Hash(),
                     Pred                                       = Pred(),
                     Allocator                                  = Allocator())
    -> unordered_multiset<T, Hash, Pred, Allocator>;
 
  template<class InputIter, class Allocator>
  unordered_multiset(InputIter,
                     InputIter,
                     typename /* see description */ ::size_type,
                     Allocator)
    -> unordered_multiset</*iter-value-type*/<InputIter>,
                          hash</*iter-value-type*/<InputIter>>,
                          equal_to</*iter-value-type*/<InputIter>>,
                          Allocator>;
 
  template<class InputIter, class Hash, class Allocator>
  unordered_multiset(InputIter,
                     InputIter,
                     typename /* see description */ ::size_type,
                     Hash,
                     Allocator)
    -> unordered_multiset</*iter-value-type*/<InputIter>,
                          Hash,
                          equal_to</*iter-value-type*/<InputIter>>,
                          Allocator>;
 
  template<ranges::input_range R, class Allocator>
  unordered_multiset(from_range_t,
                     R&&,
                     typename /* see description */ ::size_type,
                     Allocator) -> unordered_multiset<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_multiset(from_range_t, R&&, Allocator)
    -> unordered_multiset<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_multiset(from_range_t,
                     R&&,
                     typename /* see description */ ::size_type,
                     Hash,
                     Allocator) -> unordered_multiset<ranges::range_value_t<R>,
                                                      Hash,
                                                      equal_to<ranges::range_value_t<R>>,
                                                      Allocator>;
 
  template<class T, class Allocator>
  unordered_multiset(initializer_list<T>,
                     typename /* see description */ ::size_type,
                     Allocator) -> unordered_multiset<T, hash<T>, equal_to<T>, Allocator>;
 
  template<class T, class Hash, class Allocator>
  unordered_multiset(initializer_list<T>,
                     typename /* see description */ ::size_type,
                     Hash,
                     Allocator) -> unordered_multiset<T, Hash, equal_to<T>, Allocator>;
}