26 Containers library [containers]

26.5 Unordered associative containers [unord]

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).