C++ Standard Library

C++ STL Library

C++ - <unordered_map>

The C++ <unordered_map> header file defines the unordered_map and unordered_multimap container classes:

unordered_map
unordered_multimap

unordered_map class

An unordered map is an associative container that stores data sequence in key-value pair in no particular order, which allows fast retrieval of an element based on its keys. In a unordered_map, no two mapped values can have the same key. The data type of key and mapped value may differ and can be represented as:

typedef pair<const Key, T> value_type;

Each key in a unordered_map is unique which can be inserted or deleted in the unordered_map but can not be altered. Values associated with keys can be changed. The value in a unordered_map can be directly accessed by specifying corresponding key using the operator [].

Syntax

template < class Key,                                     // key_type
           class T,                                       // mapped_type
           class Hash = hash<Key>,                        // hasher
           class Pred = equal_to<Key>,                    // key_equal
           class Alloc = allocator<pair<const Key,T>>     // allocator_type
           > class unordered_map;

Parameters

`Key`	Type of the key.
`T`	Type of the mapped value.
`Hash`	A unary function that takes two elements of unordered_map as arguments and returns a unique value of type size_t based on it.
`Pred`	A binary predicate that takes two elements of unordered_map as arguments and returns a bool. It is used to determine whether two elements are equivalent or not.
`Alloc`	Type of the allocator object used to define the storage allocation model, default:allocator.

Member Types

Member types	Definition
key_type	T (First template parameter)
value_type	T (First template parameter)
hasher	Hash (Second template parameter), default: hash<key_type>
key_equal	Pred (Third template parameter), default: equal_to<key_type>
allocator_type	Alloc (Fourth template parameter), default: allocator<value_type>
reference	Alloc::reference
const_reference	Alloc::const_reference
pointer	Alloc::pointer, default: value_type*
const_pointer	Alloc::const_pointer, default: value_type*
iterator	a forward iterator to const value_type, convertible to const_iterator
const_iterator	a forward iterator to const value_type
local_iterator	a forward iterator to const value_type, convertible to const_local_iterator
const_local_iterator	a forward iterator to const value_type
difference_type	ptrdiff_t
size_type	size_t

C++ unordered_map - Member Functions

The C++ unordered_map container has a number of member functions which are listed below:

Functions	Description
unordered_map()	Construct a unordered_map object.
~unordered_map()	Destroys container by deallocating container memory.
operator=()	Assign content to a unordered_map.

Capacity

Functions	Description
empty()	Checks whether the unordered_map is empty or not.
size()	Returns the size of the unordered_map.
max_size()	Returns the maximum size of the unordered_map.

Element lookup

Functions	Description
find()	Searches the container for a key and returns the iterator to it if found, else returns the iterator to unordered_map::end.
count()	Returns the number of occurrences of a key in the unordered_map container.
equal_range()	Returns the range of elements in the unordered_map container with a key that matches with the given value.

Element Access

Functions	Description
at()	Access an element of the unordered_map.
operator[]()	Access an element of the unordered_map.

Iterators

Functions	Description
begin()	Returns iterator pointing to the first element of the unordered_map.
end()	Returns iterator pointing to the past-the-last element of the unordered_map.
cbegin()	Returns const_iterator pointing to the first element of the unordered_map.
cend()	Returns const_iterator pointing to the past-the-last element of the unordered_map.

Modifiers

Functions	Description
clear()	Clears all elements of the unordered_map.
erase()	Deletes either a single element or range of elements from a unordered_map.
insert()	Insert elements in the unordered_map.
swap()	Exchanges elements between two unordered_maps.
emplace()	Constructs and inserts a new element in the unordered_map if the key is unique.
emplace_hint()	Constructs and inserts a new element with hint in the unordered_map if the key is unique.

Buckets

Functions	Description
bucket_count()	Returns the number of buckets in the unordered_map.
max_bucket_count()	Returns the maximum number of buckets that the unordered_map can have.
bucket_size()	Returns the number of elements in the specified bucket of the unordered_map.
bucket()	Returns the bucket number of the specified element of the unordered_map.

Hash policy

Functions	Description
load_factor()	Returns current load factor in the unordered_map.
max_load_factor()	Returns or sets the maximum load factor in the unordered_map.
rehash()	Sets number of buckets in the unordered_map.
reserve()	Requests a capacity change in the unordered_map.

Observers

Functions	Description
hash_function()	Get the hash function object used by unordered_map container.
key_eq()	Returns the function which compares keys for equality.
get_allocator()	Return a copy of allocator object associated with the unordered_map.

C++ unordered_map - Non-member Functions

Functions	Description
operator ==	Checks whether two unordered_maps are equal or not.
operator !=	Checks whether two unordered_maps are unequal or not.
swap()	Exchanges elements between two unordered_maps.

unordered_multimap class

An unordered multimap is an associative container that stores data sequence in key-value pair in no particular order, which allows fast retrieval of an element based on its keys. Unlike unordered_map or map, in unordered_multimap two mapped values can have the same key. The data type of key and value may differ and can be represented as: