26 Containers library [containers]

26.5 Unordered associative containers [unord]

26.5.1 In general [unord.general]

The header <unordered_­map> defines the class templates unordered_­map and unordered_­multimap; the header <unordered_­set> defines the class templates unordered_­set and unordered_­multiset.
The exposition-only alias templates iter_­key_­t, iter_­val_­t, and iter_­to_­alloc_­t defined in [associative.general] may appear in deduction guides for unordered containers.

26.5.2 Header <unordered_­map> synopsis [unord.map.syn]

#include <initializer_list>

namespace std {
  // [unord.map], class template unordered_­map
  template <class Key,
            class T,
            class Hash = hash<Key>,
            class Pred = equal_to<Key>,
            class Alloc = allocator<pair<const Key, T>>>
    class unordered_map;

  // [unord.multimap], class template unordered_­multimap
  template <class Key,
            class T,
            class Hash = hash<Key>,
            class Pred = equal_to<Key>,
            class Alloc = allocator<pair<const Key, T>>>
    class unordered_multimap;

  template <class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template <class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
              unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));

  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& b);
  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& b);
  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);
  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);

  namespace pmr {
    template <class Key,
              class T,
              class Hash = hash<Key>,
              class Pred = equal_to<Key>>
      using unordered_map =
        std::unordered_map<Key, T, Hash, Pred,
                           polymorphic_allocator<pair<const Key, T>>>;
    template <class Key,
              class T,
              class Hash = hash<Key>,
              class Pred = equal_to<Key>>
      using unordered_multimap =
        std::unordered_multimap<Key, T, Hash, Pred,
                                polymorphic_allocator<pair<const Key, T>>>;

  }
}

26.5.3 Header <unordered_­set> synopsis [unord.set.syn]

#include <initializer_list>

namespace std {
  // [unord.set], class template unordered_­set
  template <class Key,
            class Hash = hash<Key>,
            class Pred = equal_to<Key>,
            class Alloc = allocator<Key>>
    class unordered_set;

  // [unord.multiset], 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>
    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)));

  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_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>
    bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& a,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& b);

  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>>;
  }
}

26.5.4 Class template unordered_­map [unord.map]

26.5.4.1 Class template unordered_­map overview [unord.map.overview]

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 type mapped_­type with the keys.
The unordered_­map class supports forward iterators.
An unordered_­map satisfies all of the requirements of a container, of an unordered associative container, and of an allocator-aware container.
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.
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>.
This section only describes operations on unordered_­map that are not described in one of the requirement tables, or for which there is additional semantic information.
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:
    // types:
    using 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]
    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.map.cnstr], construct/copy/destroy
    unordered_map();
    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>
      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());
    unordered_map(const unordered_map&);
    unordered_map(unordered_map&&);
    explicit unordered_map(const Allocator&);
    unordered_map(const unordered_map&, const Allocator&);
    unordered_map(unordered_map&&, const Allocator&);
    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());
    unordered_map(size_type n, const allocator_type& a)
      : unordered_map(n, hasher(), key_equal(), a) { }
    unordered_map(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_map(n, hf, key_equal(), a) { }
    template <class InputIterator>
      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>
      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) { }
    unordered_map(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_map(il, n, hasher(), key_equal(), a) { }
    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) { }
    ~unordered_map();
    unordered_map& operator=(const unordered_map&);
    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>);
    unordered_map& 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;

    // [unord.map.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 P> pair<iterator, bool> insert(P&& obj);
    iterator       insert(const_iterator hint, const value_type& obj);
    iterator       insert(const_iterator hint, value_type&& obj);
    template <class P> iterator insert(const_iterator hint, P&& obj);
    template <class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    template <class... Args>
      pair<iterator, bool> try_emplace(const key_type& k, Args&&... args);
    template <class... Args>
      pair<iterator, bool> try_emplace(key_type&& k, Args&&... args);
    template <class... Args>
      iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
    template <class... Args>
      iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
    template <class M>
      pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);
    template <class M>
      pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);
    template <class M>
      iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
    template <class M>
      iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_map&)
      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_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);

    // observers:
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations:
    iterator       find(const key_type& k);
    const_iterator find(const key_type& k) const;
    size_type      count(const key_type& k) const;
    pair<iterator, iterator>             equal_range(const key_type& k);
    pair<const_iterator, const_iterator> equal_range(const key_type& k) const;

    // [unord.map.elem], element access
    mapped_type& operator[](const key_type& k);
    mapped_type& operator[](key_type&& k);
    mapped_type& at(const key_type& k);
    const mapped_type& at(const key_type& 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;
    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_key_t<InputIterator>>,
           class Pred = equal_to<iter_key_t<InputIterator>>,
           class Allocator = allocator<iter_to_alloc_t<InputIterator>>>
    unordered_map(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_map<iter_key_t<InputIterator>, iter_value_t<InputIterator>, 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<const 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_t<InputIterator>, iter_val_t<InputIterator>,
                       hash<iter_key_t<InputIterator>>, equal_to<iter_key_t<InputIterator>>,
                       Allocator>;

  template<class InputIterator, class Allocator>
    unordered_map(InputIterator, InputIterator, Allocator)
      -> unordered_map<iter_key_t<InputIterator>, iter_val_t<InputIterator>,
                       hash<iter_key_t<InputIterator>>, equal_to<iter_key_t<InputIterator>>,
                       Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_map(InputIterator, InputIterator, typename see below::size_type, Hash, Allocator)
      -> unordered_map<iter_key_t<InputIterator>, iter_val_t<InputIterator>, Hash,
                       equal_to<iter_key_t<InputIterator>>, Allocator>;

  template<class Key, class T, typename Allocator>
    unordered_map(initializer_list<pair<const Key, T>>, typename see below::size_type,
                  Allocator)
      -> unordered_map<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, typename Allocator>
    unordered_map(initializer_list<pair<const 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<const Key, T>>, typename see below::size_type, Hash,
                  Allocator)
      -> unordered_map<Key, T, Hash, equal_to<Key>, Allocator>;

  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& b);
  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_map<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_map<Key, T, Hash, Pred, Alloc>& b);

  // [unord.map.swap], swap
  template <class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x,
              unordered_map<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}
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.

26.5.4.2 unordered_­map constructors [unord.map.cnstr]

unordered_map() : unordered_map(size_type(see below)) { } explicit unordered_map(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Effects: Constructs an empty unordered_­map 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.
Complexity: Constant.
template <class InputIterator> 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()); 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());
Effects: Constructs an empty unordered_­map 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) for the first form, or from the range [il.begin(), il.end()) for the second form.
max_­load_­factor() returns 1.0.
Complexity: Average case linear, worst case quadratic.

26.5.4.3 unordered_­map element access [unord.map.elem]

mapped_type& operator[](const key_type& k);
Effects: Equivalent to: return try_­emplace(k).first->second;
mapped_type& operator[](key_type&& k);
Effects: Equivalent to: return try_­emplace(move(k)).first->second;
mapped_type& at(const key_type& k); const mapped_type& at(const key_type& k) const;
Returns: A reference to x.second, where x is the (unique) element whose key is equivalent to k.
Throws: An exception object of type out_­of_­range if no such element is present.

26.5.4.4 unordered_­map modifiers [unord.map.modifiers]

template <class P> pair<iterator, bool> insert(P&& obj);
Effects: Equivalent to: return emplace(std​::​forward<P>(obj));
Remarks: This signature shall not participate in overload resolution unless is_­constructible_­v<value_­type, P&&> is true.
template <class P> iterator insert(const_iterator hint, P&& obj);
Effects: Equivalent to: return emplace_­hint(hint, std​::​forward<P>(obj));
Remarks: This signature shall not participate in overload resolution unless is_­constructible_­v<value_­type, P&&> is true.
template <class... Args> pair<iterator, bool> try_emplace(const key_type& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, const key_type& k, Args&&... args);
Requires: value_­type shall be EmplaceConstructible into unordered_­map from piecewise_­construct, forward_­as_­tuple(k), forward_­as_­tuple(std​::​forward<Args>(args)...).
Effects: If the map already contains an element whose key is equivalent to k, there is no effect.
Otherwise inserts an object of type value_­type constructed with piecewise_­construct, forward_­as_­tuple(k), forward_­as_­tuple(std​::​forward<Args>(args)...).
Returns: In 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 map element whose key is equivalent to k.
Complexity: The same as emplace and emplace_­hint, respectively.
template <class... Args> pair<iterator, bool> try_emplace(key_type&& k, Args&&... args); template <class... Args> iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args);
Requires: value_­type shall be EmplaceConstructible into unordered_­map from piecewise_­construct, forward_­as_­tuple(std​::​move(k)), forward_­as_­tuple(std​::​forward<Args>(args)...).
Effects: If the map already contains an element whose key is equivalent to k, there is no effect.
Otherwise inserts an object of type value_­type constructed with piecewise_­construct, forward_­as_­tuple(std​::​move(k)), forward_­as_­tuple(std​::​forward<Args>(args)...).
Returns: In 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 map element whose key is equivalent to k.
Complexity: The same as emplace and emplace_­hint, respectively.
template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);
Requires: is_­assignable_­v<mapped_­type&, M&&> shall be true.
value_­type shall be EmplaceConstructible into unordered_­map from k, std​::​forward<M>(obj).
Effects: If the map already contains an element e whose key is equivalent to k, assigns std​::​forward<M>(obj) to e.second.
Otherwise inserts an object of type value_­type constructed with k, std​::​forward<M>(obj).
Returns: In 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 map element whose key is equivalent to k.
Complexity: The same as emplace and emplace_­hint, respectively.
template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj); template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);
Requires: is_­assignable_­v<mapped_­type&, M&&> shall be true.
value_­type shall be EmplaceConstructible into unordered_­map from std​::​move(k), std​::​forward<M>(obj).
Effects: If the map already contains an element e whose key is equivalent to k, assigns std​::​forward<M>(obj) to e.second.
Otherwise inserts an object of type value_­type constructed with std​::​​move(k), std​::​forward<M>(obj).
Returns: In 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 map element whose key is equivalent to k.
Complexity: The same as emplace and emplace_­hint, respectively.

26.5.4.5 unordered_­map swap [unord.map.swap]

template <class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_map<Key, T, Hash, Pred, Alloc>& x, unordered_map<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y)));
Effects: As if by x.swap(y).

26.5.5 Class template unordered_­multimap [unord.multimap]

26.5.5.1 Class template unordered_­multimap overview [unord.multimap.overview]

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.
The unordered_­multimap class supports forward iterators.
An unordered_­multimap satisfies all of the requirements of a container, of an unordered associative container, and of an allocator-aware container.
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.
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>.
This section only describes operations on unordered_­multimap that are not described in one of the requirement tables, or for which there is additional semantic information.
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:
    // types:
    using 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]
    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;

    // [unord.multimap.cnstr], construct/copy/destroy
    unordered_multimap();
    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>
      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());
    unordered_multimap(const unordered_multimap&);
    unordered_multimap(unordered_multimap&&);
    explicit unordered_multimap(const Allocator&);
    unordered_multimap(const unordered_multimap&, const Allocator&);
    unordered_multimap(unordered_multimap&&, const Allocator&);
    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());
    unordered_multimap(size_type n, const allocator_type& a)
      : unordered_multimap(n, hasher(), key_equal(), a) { }
    unordered_multimap(size_type n, const hasher& hf, const allocator_type& a)
      : unordered_multimap(n, hf, key_equal(), a) { }
    template <class InputIterator>
      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>
      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) { }
    unordered_multimap(initializer_list<value_type> il, size_type n, const allocator_type& a)
      : unordered_multimap(il, n, hasher(), key_equal(), a) { }
    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) { }
    ~unordered_multimap();
    unordered_multimap& operator=(const unordered_multimap&);
    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>);
    unordered_multimap& 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;

    // [unord.multimap.modifiers], 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);
    template <class P> iterator insert(P&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template <class P> iterator insert(const_iterator hint, P&& obj);
    template <class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      swap(unordered_multimap&)
      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_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

    // observers:
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations:
    iterator       find(const key_type& k);
    const_iterator find(const key_type& k) const;
    size_type      count(const key_type& k) const;
    pair<iterator, iterator>             equal_range(const key_type& k);
    pair<const_iterator, const_iterator> equal_range(const key_type& 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;
    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_key_t<InputIterator>>,
           class Pred = equal_to<iter_key_t<InputIterator>>,
           class Allocator = allocator<iter_to_alloc_t<InputIterator>>>
    unordered_multimap(InputIterator, InputIterator,
                       typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<iter_key_t<InputIterator>, iter_value_t<InputIterator>, 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<const 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_t<InputIterator>, iter_val_t<InputIterator>,
                            hash<iter_key_t<InputIterator>>,
                            equal_to<iter_key_t<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, Allocator)
      -> unordered_multimap<iter_key_t<InputIterator>, iter_val_t<InputIterator>,
                            hash<iter_key_t<InputIterator>>,
                            equal_to<iter_key_t<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_multimap<iter_key_t<InputIterator>, iter_val_t<InputIterator>, Hash,
                            equal_to<iter_key_t<InputIterator>>, Allocator>;

  template<class Key, class T, typename Allocator>
    unordered_multimap(initializer_list<pair<const Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_multimap<Key, T, hash<Key>, equal_to<Key>, Allocator>;

  template<class Key, class T, typename Allocator>
    unordered_multimap(initializer_list<pair<const 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<const Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multimap<Key, T, Hash, equal_to<Key>, Allocator>;

  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);
  template <class Key, class T, class Hash, class Pred, class Alloc>
    bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& a,
                    const unordered_multimap<Key, T, Hash, Pred, Alloc>& b);

  // [unord.multimap.swap], swap
  template <class Key, class T, class Hash, class Pred, class Alloc>
    void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
              unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
      noexcept(noexcept(x.swap(y)));
}
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.

26.5.5.2 unordered_­multimap constructors [unord.multimap.cnstr]

unordered_multimap() : unordered_multimap(size_type(see below)) { } explicit unordered_multimap(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Effects: Constructs an empty unordered_­multimap 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.
Complexity: Constant.
template <class InputIterator> 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()); 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());
Effects: Constructs an empty unordered_­multimap 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) for the first form, or from the range [il.begin(), il.end()) for the second form.
max_­load_­factor() returns 1.0.
Complexity: Average case linear, worst case quadratic.

26.5.5.3 unordered_­multimap modifiers [unord.multimap.modifiers]

template <class P> iterator insert(P&& obj);
Effects: Equivalent to: return emplace(std​::​forward<P>(obj));
Remarks: This signature shall not participate in overload resolution unless is_­constructible_­v<value_­type, P&&> is true.
template <class P> iterator insert(const_iterator hint, P&& obj);
Effects: Equivalent to: return emplace_­hint(hint, std​::​forward<P>(obj));
Remarks: This signature shall not participate in overload resolution unless is_­constructible_­v<value_­type, P&&> is true.

26.5.5.4 unordered_­multimap swap [unord.multimap.swap]

template <class Key, class T, class Hash, class Pred, class Alloc> void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x, unordered_multimap<Key, T, Hash, Pred, Alloc>& y) noexcept(noexcept(x.swap(y)));
Effects: As if by x.swap(y).

26.5.6 Class template unordered_­set [unord.set]

26.5.6.1 Class template unordered_­set 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.
An unordered_­set satisfies all of the requirements of a container, of an unordered associative container, and of an allocator-aware container.
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.
This section only describes operations on unordered_­set that are not described in one of the requirement tables, or for which there is additional semantic information.
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());
    unordered_set(const unordered_set&);
    unordered_set(unordered_set&&);
    explicit unordered_set(const Allocator&);
    unordered_set(const unordered_set&, const Allocator&);
    unordered_set(unordered_set&&, const 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) { }
    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);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template <class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    insert_return_type insert(node_type&& nh);
    iterator           insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    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;
    size_type      count(const key_type& k) const;
    pair<iterator, iterator>             equal_range(const key_type& k);
    pair<const_iterator, const_iterator> equal_range(const key_type& 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;
    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<typename iterator_traits<InputIterator>::value_type>,
           class Pred = equal_to<typename iterator_traits<InputIterator>::value_type>,
           class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
    unordered_set(InputIterator, InputIterator, typename see below::size_type = see below,
                  Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_set<typename iterator_traits<InputIterator>::value_type,
                       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<typename iterator_traits<InputIterator>::value_type,
                       hash<typename iterator_traits<InputIterator>::value_type>,
                       equal_to<typename iterator_traits<InputIterator>::value_type>,
                       Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_set(InputIterator, InputIterator, typename see below::size_type,
                  Hash, Allocator)
      -> unordered_set<typename iterator_traits<InputIterator>::value_type, Hash,
                       equal_to<typename iterator_traits<InputIterator>::value_type>,
                       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>;

  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_set<Key, Hash, Pred, Alloc>& a,
                    const unordered_set<Key, Hash, Pred, Alloc>& b);

  // [unord.set.swap], swap
  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)));
}
A size_­type parameter type in an unordered_­set deduction guide refers to the size_­type member type of the primary unordered_­set template.

26.5.6.2 unordered_­set constructors [unord.set.cnstr]

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());
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.
Complexity: Constant.
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()); 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());
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) for the first form, or from the range [il.begin(), il.end()) for the second form.
max_­load_­factor() returns 1.0.
Complexity: Average case linear, worst case quadratic.

26.5.6.3 unordered_­set swap [unord.set.swap]

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)));
Effects: As if by x.swap(y).

26.5.7 Class template unordered_­multiset [unord.multiset]

26.5.7.1 Class template unordered_­multiset overview [unord.multiset.overview]

An unordered_­multiset is an unordered associative container that supports equivalent keys (an instance of unordered_­multiset may contain multiple copies of the same key value) and in which each element's key is the element itself.
The unordered_­multiset class supports forward iterators.
An unordered_­multiset satisfies all of the requirements of a container, of an unordered associative container, and of an allocator-aware container.
It provides the operations described in the preceding requirements table for equivalent keys; that is, an unordered_­multiset supports the a_­eq operations in that table, not the a_­uniq operations.
For an unordered_­multiset<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.
This section only describes operations on unordered_­multiset that are not described in one of the requirement tables, or for which there is additional semantic information.
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; // 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;

    // [unord.multiset.cnstr], 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 InputIterator>
      unordered_multiset(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());
    unordered_multiset(const unordered_multiset&);
    unordered_multiset(unordered_multiset&&);
    explicit unordered_multiset(const Allocator&);
    unordered_multiset(const unordered_multiset&, const Allocator&);
    unordered_multiset(unordered_multiset&&, const Allocator&);
    unordered_multiset(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_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 InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const allocator_type& a)
        : unordered_multiset(f, l, n, hasher(), key_equal(), a) { }
    template <class InputIterator>
      unordered_multiset(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a)
      : unordered_multiset(f, l, 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 InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(initializer_list<value_type>);

    node_type extract(const_iterator position);
    node_type extract(const key_type& x);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    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;
    size_type      count(const key_type& k) const;
    pair<iterator, iterator>             equal_range(const key_type& k);
    pair<const_iterator, const_iterator> equal_range(const key_type& 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;
    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<typename iterator_traits<InputIterator>::value_type>,
           class Pred = equal_to<typename iterator_traits<InputIterator>::value_type>,
           class Allocator = allocator<typename iterator_traits<InputIterator>::value_type>>
    unordered_multiset(InputIterator, InputIterator, see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<typename iterator_traits<InputIterator>::value_type,
                            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 below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multiset<typename iterator_traits<InputIterator>::value_type,
                            hash<typename iterator_traits<InputIterator>::value_type>,
                            equal_to<typename iterator_traits<InputIterator>::value_type>,
                            Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multiset<typename iterator_traits<InputIterator>::value_type, Hash,
                            equal_to<typename iterator_traits<InputIterator>::value_type>,
                            Allocator>;

  template<class T, class Allocator>
    unordered_multiset(initializer_list<T>, typename see below::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 below::size_type, Hash, Allocator)
      -> unordered_multiset<T, Hash, equal_to<T>, Allocator>;

  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>
    bool operator!=(const unordered_multiset<Key, Hash, Pred, Alloc>& a,
                    const unordered_multiset<Key, Hash, Pred, Alloc>& b);

  // [unord.multiset.swap], swap
  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)));
}
A size_­type parameter type in an unordered_­multiset deduction guide refers to the size_­type member type of the primary unordered_­multiset template.

26.5.7.2 unordered_­multiset constructors [unord.multiset.cnstr]

unordered_multiset() : unordered_multiset(size_type(see below)) { } explicit unordered_multiset(size_type n, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type());
Effects: Constructs an empty unordered_­multiset 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.
Complexity: Constant.
template <class InputIterator> unordered_multiset(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()); unordered_multiset(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());
Effects: Constructs an empty unordered_­multiset 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) for the first form, or from the range [il.begin(), il.end()) for the second form.
max_­load_­factor() returns 1.0.
Complexity: Average case linear, worst case quadratic.

26.5.7.3 unordered_­multiset swap [unord.multiset.swap]

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)));
Effects: As if by x.swap(y).