declaration

    
template<
    class Key,
    class T,
    class Hash = std::hash<key>,
    class KeyEqual = std::equal_to<key>,
    class Allocator = std::allocator<std::pair<const>>
> class unordered_map;

Key is the type of the keys
T is the type of the mapped values
Hash is the hash function object
KeyEqual is the equality comparison function
Allocator is the allocator used to manage memory

member types

Member type	Definition	Explanation
key_type	Key	Type of the keys.
mapped_type	T	Type of the mapped values.
value_type	std::pair<const>Key, T>	Type of elements stored in the container.
hasher	Hash	The hash function type.
key_equal	KeyEqual	The equality comparison function type.
allocator_type	Allocator	Allocator type.
size_type	Unsigned integer type (usually std::size_t)	Used for sizes and indices.
difference_type	Signed integer type (usually std::ptrdiff_t)	Used for differences between iterators.
reference	value_type&	Reference to an element, allows modification.
const_reference	const value_type&	Const reference to an element, read-only access.
iterator	Iterator to value_type	Traverses and modifies elements.
const_iterator	Const iterator to value_type	Traverses elements without modification.

member functions

element access

mapped_type& at(const Key& key)
Returns a reference to the value corresponding to key. Throws std::out_of_range if key not found.
mapped_type& operator[](const Key& key)
Returns a reference to the value corresponding to key. If key does not exist, a new element is inserted.

iterators

iterator begin()
Returns an iterator to the first element.
iterator end()
Returns an iterator to one past the last element.
const_iterator cbegin() const
Returns a const iterator to the first element.
const_iterator cend() const
Returns a const iterator to one past the last element.

capacity

bool empty() const
Returns true if the container has no elements.
size_type size() const
Returns the number of elements.
size_type max_size() const
Returns the maximum number of elements the container can hold.
void reserve(size_type n)
Reserves space for at least n elements.
size_type bucket_count() const
Returns the number of buckets in the hash table.
float load_factor() const
Returns the average number of elements per bucket.
float max_load_factor() const
Returns the maximum allowed load factor.
void rehash(size_type count)
Sets the number of buckets to at least count.

modifiers

void clear()
Removes all elements from the container.
std::pair<iterator, bool> insert(const value_type& value)
Inserts an element. Returns a pair consisting of an iterator to the element and a bool indicating whether insertion took place.
template<typename> void insert(InputIt first, InputIt last)
Inserts elements from the range [first, last).
void erase(iterator pos)
Removes the element at the given iterator position.
size_type erase(const Key& key)
Removes the element with the specified key. Returns the number of elements removed (0 or 1).
void swap(unordered_map<Key, T>& other)
Exchanges the contents with another unordered_map.