22 Containers library [containers]

22.5 Unordered associative containers [unord]

22.5.7 Class template unordered_­multiset [unord.multiset]

22.5.7.1 Overview [unord.multiset.overview]

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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.
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An unordered_­multiset meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76).
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.
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Subclause [unord.multiset] 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
    [[nodiscard]] 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;
    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;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  template<class InputIterator,
           class Hash = hash<iter-value-type<InputIterator>>,
           class Pred = equal_to<iter-value-type<InputIterator>>,
           class Allocator = allocator<iter-value-type<InputIterator>>>
    unordered_multiset(InputIterator, InputIterator, see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multiset<iter-value-type<InputIterator>,
                            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<iter-value-type<InputIterator>,
                            hash<iter-value-type<InputIterator>>,
                            equal_to<iter-value-type<InputIterator>>,
                            Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multiset(InputIterator, InputIterator, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multiset<iter-value-type<InputIterator>, Hash,
                            equal_to<iter-value-type<InputIterator>>,
                            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>;

  // 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)));
}
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A size_­type parameter type in an unordered_­multiset deduction guide refers to the size_­type member type of the type deduced by the deduction guide.

22.5.7.2 Constructors [unord.multiset.cnstr]

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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());
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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.
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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());
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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.
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Complexity: Average case linear, worst case quadratic.

22.5.7.3 Erasure [unord.multiset.erasure]

🔗
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);
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Effects: Equivalent to: 
auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();