23 Containers library [containers]

23.5 Unordered associative containers [unord]

23.5.7 Class template unordered_multiset [unord.multiset]

23.5.7.1 Overview [unord.multiset.overview]

23.5.7.2 Constructors [unord.multiset.cnstr]

23.5.7.3 Erasure [unord.multiset.erasure]

23.5.7.1 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 ([container.reqmts]), of an allocator-aware container ([container.alloc.reqmts]), and of an unordered associative container ([unord.req]).
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.
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The types iterator and const_iterator meet the constexpr iterator requirements ([iterator.requirements.general]).
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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 = allocator_traits<Allocator>::pointer; using const_pointer = 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 constexpr unordered_multiset(); constexpr 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> constexpr 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()); template<container-compatible-range<value_type> R> constexpr unordered_multiset(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); constexpr unordered_multiset(const unordered_multiset&); constexpr unordered_multiset(unordered_multiset&&); constexpr explicit unordered_multiset(const Allocator&); constexpr unordered_multiset(const unordered_multiset&, const type_identity_t<Allocator>&); constexpr unordered_multiset(unordered_multiset&&, const type_identity_t<Allocator>&); constexpr 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()); constexpr unordered_multiset(size_type n, const allocator_type& a) : unordered_multiset(n, hasher(), key_equal(), a) { } constexpr unordered_multiset(size_type n, const hasher& hf, const allocator_type& a) : unordered_multiset(n, hf, key_equal(), a) { } template<class InputIterator> constexpr 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> constexpr 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) { } template<container-compatible-range<value_type> R> constexpr 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> constexpr 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) { } constexpr unordered_multiset(initializer_list<value_type> il, size_type n, const allocator_type& a) : unordered_multiset(il, n, hasher(), key_equal(), a) { } constexpr 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) { } constexpr ~unordered_multiset(); constexpr unordered_multiset& operator=(const unordered_multiset&); constexpr 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>); constexpr unordered_multiset& operator=(initializer_list<value_type>); constexpr allocator_type get_allocator() const noexcept; // iterators constexpr iterator begin() noexcept; constexpr const_iterator begin() const noexcept; constexpr iterator end() noexcept; constexpr const_iterator end() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; // capacity constexpr bool empty() const noexcept; constexpr size_type size() const noexcept; constexpr size_type max_size() const noexcept; // modifiers template<class... Args> constexpr iterator emplace(Args&&... args); template<class... Args> constexpr iterator emplace_hint(const_iterator position, Args&&... args); constexpr iterator insert(const value_type& obj); constexpr iterator insert(value_type&& obj); constexpr iterator insert(const_iterator hint, const value_type& obj); constexpr iterator insert(const_iterator hint, value_type&& obj); template<class InputIterator> constexpr void insert(InputIterator first, InputIterator last); template<container-compatible-range<value_type> R> constexpr void insert_range(R&& rg); constexpr void insert(initializer_list<value_type>); constexpr node_type extract(const_iterator position); constexpr node_type extract(const key_type& x); template<class K> constexpr node_type extract(K&& x); constexpr iterator insert(node_type&& nh); constexpr iterator insert(const_iterator hint, node_type&& nh); constexpr iterator erase(iterator position) requires (!same_as<iterator, const_iterator>); constexpr iterator erase(const_iterator position); constexpr size_type erase(const key_type& k); template<class K> constexpr size_type erase(K&& x); constexpr iterator erase(const_iterator first, const_iterator last); constexpr void swap(unordered_multiset&) noexcept(allocator_traits<Allocator>::is_always_equal::value && is_nothrow_swappable_v<Hash> && is_nothrow_swappable_v<Pred>); constexpr void clear() noexcept; template<class H2, class P2> constexpr void merge(unordered_multiset<Key, H2, P2, Allocator>& source); template<class H2, class P2> constexpr void merge(unordered_multiset<Key, H2, P2, Allocator>&& source); template<class H2, class P2> constexpr void merge(unordered_set<Key, H2, P2, Allocator>& source); template<class H2, class P2> constexpr void merge(unordered_set<Key, H2, P2, Allocator>&& source); // observers constexpr hasher hash_function() const; constexpr key_equal key_eq() const; // set operations constexpr iterator find(const key_type& k); constexpr const_iterator find(const key_type& k) const; template<class K> constexpr iterator find(const K& k); template<class K> constexpr const_iterator find(const K& k) const; constexpr size_type count(const key_type& k) const; template<class K> constexpr size_type count(const K& k) const; constexpr bool contains(const key_type& k) const; template<class K> constexpr bool contains(const K& k) const; constexpr pair<iterator, iterator> equal_range(const key_type& k); constexpr pair<const_iterator, const_iterator> equal_range(const key_type& k) const; template<class K> constexpr pair<iterator, iterator> equal_range(const K& k); template<class K> constexpr pair<const_iterator, const_iterator> equal_range(const K& k) const; // bucket interface constexpr size_type bucket_count() const noexcept; constexpr size_type max_bucket_count() const noexcept; constexpr size_type bucket_size(size_type n) const; constexpr size_type bucket(const key_type& k) const; template<class K> constexpr size_type bucket(const K& k) const; constexpr local_iterator begin(size_type n); constexpr const_local_iterator begin(size_type n) const; constexpr local_iterator end(size_type n); constexpr const_local_iterator end(size_type n) const; constexpr const_local_iterator cbegin(size_type n) const; constexpr const_local_iterator cend(size_type n) const; // hash policy constexpr float load_factor() const noexcept; constexpr float max_load_factor() const noexcept; constexpr void max_load_factor(float z); constexpr void rehash(size_type n); constexpr 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, typename see below::size_type = see below, Hash = Hash(), Pred = Pred(), Allocator = Allocator()) -> unordered_multiset<iter-value-type<InputIterator>, Hash, Pred, Allocator>; template<ranges::input_range R, class Hash = hash<ranges::range_value_t<R>>, class Pred = equal_to<ranges::range_value_t<R>>, class Allocator = allocator<ranges::range_value_t<R>>> unordered_multiset(from_range_t, R&&, typename see below::size_type = see below, 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 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<ranges::input_range R, class Allocator> unordered_multiset(from_range_t, R&&, typename see below::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 below::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 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>; }
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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.

23.5.7.2 Constructors [unord.multiset.cnstr]

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constexpr unordered_multiset() : unordered_multiset(size_type(see below)) { } constexpr 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.
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template<class InputIterator> constexpr 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()); template<container-compatible-range<value_type> R> constexpr unordered_multiset(from_range_t, R&& rg, size_type n = see below, const hasher& hf = hasher(), const key_equal& eql = key_equal(), const allocator_type& a = allocator_type()); constexpr 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), rg, or il, respectively.
max_load_factor() returns 1.0.
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Complexity: Average case linear, worst case quadratic.

23.5.7.3 Erasure [unord.multiset.erasure]

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template<class K, class H, class P, class A, class Predicate> constexpr 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();