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 (Table [tab:containers.allocatoraware]). 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 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)));
}
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());
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 function, 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.
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.
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.
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).