C++ Library -

Introduction to unordered_map

Unordered map is dictionary like data structure. It is a sequence of (key, value) pair, where only single value is associated with each unique key. It is often referred as associative array. It enables fast retrieval of individual elements based on their keys.
It also implements the direct access operator(subscript operator[]) which allows for direct access of the mapped value using its key value as argument.
Unordered map does not sort its element in any particular order with respect to either their key or mapped values, instead organizes into buckets depending on their hash values to allow for fast access to individual elements directly by their key values.
Unordered map performs better than map while accessing individual elements by their keys. But for range iteration their performance is considerably low.

Definition

Below is definition of std::unordered_map from <unordered_map> header file

template < class Key,
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator< pair<const Key,T> >
           > class unordered_map;

Parameters

• Key − Type of the key.
• T − Type of the mapped values.
• Hash − A unary function object type which takes an object of type key type as argument and returns a unique value of type size_t based on it.
• Pred − A binary predicate that which two arguments of the key type and returns a bool.
• Alloc − Type of the allocator object.
T may be substituted by any other data type including user-defined type.

Member types

Following member types can be used as parameters or return type by member functions.

S.N.	Member types	Definition
1	key_type	Key (First parameter of the template)
2	mapped_type	T (Second parameter of the template)
3	value_type	pair<const key_type,mapped_type>
4	hasher	The third template parameter (defaults to: hash<key_type>)
5	key_equal	The fourth template parameter (defaults to: equal_to<key_type>)
6	allocator_type	Alloc (Fifth parameter of the template)
7	reference	value_type&
8	const_reference	const value_type&
9	pointer	allocator_traits<Alloc>::pointer
10	const_pointer	allocator_traits<Alloc>::const_pointer
11	iterator	A forward iterator to value_type value_type
12	const_iterator	A forward iterator to const value_type value_type
13	local_iterator	A forward iterator to value_type
14	const_local_iterator	A forward iterator to const value_type
15	difference_type	ptrdiff_t
16	size_type	size_t

Functions from <unordered_map>

Below is list of all methods from <unordered_map> header.

Constructors

S.N.	Method & Description
1	unordered_map::unordered_map default constructorConstructs an empty unordered_map with zero elements.
2	unordered_map::unordered_map copy constructorConstructs an unordered_map with copy of each elements present in existing unordered_map.
3	unordered_map::unordered_map move constructorConstructs an unordered_map with the contents of other using move semantics.
4	unordered_map::unordered_map range constructorConstructs an unordered_map with as many elements as in range of first to last.
5	unordered_map::unordered_map initializer_list constructor Constructs an unordered_map from initialize list.

Destructor

S.N.	Method & Description
1	unordered_map::~unordered_map Destroys unordered_map object by deallocating it's memory.

Member functions

S.N.	Method & Description
1	unordered_map::at Returns a reference to the mapped value associated with key k.
2	unordered_map::begin container iteratorReturns an iterator which refers to the first element of the map.
3	unordered_map::begin bucket iteratorReturns an iterator pointing to the first element in one of its buckets.
4	unordered_map::bucket Returns the bucket number where element with key k is located.
5	unordered_map::bucket_count Returns the number of buckets in unordered_map container.
6	unordered_map::bucket_size Returns the number of elements presents in the nth bucket.
7	unordered_map::cbegin container iteratorReturns a constant iterator which refers to the first element of the unordered_map.
8	unordered_map::cbegin bucket iteratorReturns a constant iterator pointing to the first element in one of its buckets.
9	unordered_map::cend container iteratorReturns a constant iterator which points to past-the-end element of the unordered_map.
10	unordered_map::cend bucket iteratorReturns a constant iterator which points to past-the-end element in one of its buckets.
11	unordered_map::clear Destroys the unordered_map by removing all elements and sets the size of unordered_map to zero.
12	unordered_map::count Returns the number of mapped values associated with key k.
13	unordered_map::emplace Extends container by inserting new element.
14	unordered_map::emplace_hint Inserts a new element in unordered_map using hint as a position for element.
15	unordered_map::empty Tests whether unordered_map is empty or not.
16	unordered_map::end container iteratorReturns an iterator which points to past-the-end element in the unordered_map.
17	unordered_map::end bucket iteratorReturns an iterator which points to past-the-end element in one of its buckets.
18	unordered_map::equal Returns range of elements that matches specific key.
19	unordered_map::erase position versionRemoves single element of the unordered_map from position.
20	unordered_map::erase key versionRemoves mapped value associated with key k.
21	unordered_map::erase range versionRemoves range of element from the the unordered_map.
22	unordered_map::find Finds an element associated with key k.
23	unordered_map::get_allocator Returns an allocator associated with unordered_map.
24	unordered_map::hash_function Calculates the hash function object used by the unordered_map container.
25	unordered_map::insert Extends container by inserting new element in unordered_map.
26	unordered_map::insert move versionExtends container by inserting new element in unordered_map.
27	unordered_map::insert hint versionExtends conta iner by inserting new element in unordered_map.
28	unordered_map::insert move and hint versionExtends unordered_map by inserting new element.
29	unordered_map::insert range versionExtends container by inserting new elements in the unordered_map.
30	unordered_map::insert initializer_list versionExtends map by inserting new element from initializer list.
31	unordered_map::key_eq Returns the function that compares keys for equality.
32	unordered_map::load_factor Returns the current load factor of the unordered_map container.
33	unordered_map::max_bucket_count Returns the maximum number of buckets that the unordered_map container can have.
34	unordered_map::max_load_factor get versionReturns the current maximum load factor for the unordered_map container.
35	unordered_map::max_load_factor set versionAssigns new load factor for the unordered_map container.
36	unordered_map::max_size Returns the maximum number of elements can be held by unordered_map.
37	unordered_map::operator= copy versionAssigns new contents to the unordered_map by replacing old ones and modifies size if necessary.
38	unordered_map::operator= move versionMove the contents of one unordered_map into another and modifies size if necessary.
39	unordered_map::operator= initializer_list versionCopy elements from initializer list to unordered_map.
40	unordered_map::operator[] If key k matches an element in the container, then method returns a reference to the element.
41	unordered_map::operator[] move versionIf key k matches an element in the container, then method returns a reference to the element.
42	unordered_map::rehash Sets the number of buckets in the container to n or more.
43	unordered_map::reserve Sets the number of buckets in the container to the most appropriate to contain at least n elements.
44	unordered_map::size Returns the number of elements present in the unordered_map.
45	unordered_map::swap Exchanges the content of first unordered_map with another.

Non-member overloaded functions

S.N.	Method & Description
1	unordered_map::operator== Tests whether two unordered_maps are equal or not.
2	unordered_map::operator!= Tests whether two unordered_maps are equal or not.
3	unordered_map::swap Exchanges the content of first unordered_map with another.

Introduction to unordered_multimap

Unordered_multimap is dictionary like data structure. It is a sequence of (key, value) pair, where different elements can have equivalent keys. Elements with equivalent keys are grouped together in the same bucket and in such a way that an equal_range iterator can iterate through all of them.
Unordered_multimap does not sort its element in any particular order with respect to either their key or mapped values, instead organizes into buckets depending on their hash values to allow for fast access to individual elements directly by their key values.

Definition

Below is definition of std::unordered_multimap from <unordered_map> header file

template < class Key, 
           class T,
           class Hash = hash<Key>,
           class Pred = equal_to<Key>,
           class Alloc = allocator< pair<const Key,T> >
           > class unordered_multimap;

Parameters

• Key − Type of the key.
• T − Type of the mapped values.
• Hash − A unary function object type which takes an object of type key type as argument and returns a unique value of type size_t based on it.
• Pred − A binary predicate that which two arguments of the key type and returns a bool.
• Alloc − Type of the allocator object.
  T may be substituted by any other data type including user-defined type.

Member types

Following member types can be used as parameters or return type by member functions.

S.N.	Member types	Definition
1	key_type	Key (First parameter of the template)
2	mapped_type	T (Second parameter of the template)
3	value_type	pair<const key_type,mapped_type>
4	hasher	The third template parameter (defaults to: hash<key_type>)
5	key_equal	The fourth template parameter (defaults to: equal_to<key_type>)
6	allocator_type	Alloc (Fifth parameter of the template)
7	reference	value_type&
8	const_reference	const value_type&
9	pointer	allocator_traits<Alloc>::pointer
10	const_pointer	allocator_traits<Alloc>::const_pointer
11	iterator	A forward iterator to value_type value_type
12	const_iterator	A forward iterator to const value_type value_type
13	local_iterator	A forward iterator to value_type
14	const_local_iterator	A forward iterator to const value_type
15	difference_type	ptrdiff_t
16	size_type	size_t

Functions from <unordered_multimap>

Below is list of all methods from <unordered_map> header.

Constructors

S.N.	Method & Description
1	unordered_multimap::unordered_multimap() default constructor Constructs an empty unordered_multimap with zero elements.
2	unordered_multimap::unordered_multimap() copy constructorConstructs an unordered_multimap with copy of each elements present in existing unordered_multimap.
3	unordered_multimap::unordered_multimap() move constructorConstructs an unordered_multimap with the contents of other using move semantics.
4	unordered_multimap::unordered_multimap() range constructor Constructs an unordered_multimap with as many elements as in range of first to last.
5	unordered_multimap::unordered_multimap() initializer_list constructorConstructs an unordered_multimap from initialize list.

Destructor

S.N.	Method & Description
1	unordered_multimap::~unordered_multimap() Destroys unordered_multimap object by deallocating it's memory.

Member functions

S.N.	Method & Description
1	unordered_multimap::begin() container iteratorReturns an iterator which refers to the first element of the unordered_mulitmap.
2	unordered_multimap::begin() bucket iteratorReturns an iterator pointing to the first element in one of its buckets.
3	unordered_multimap::bucket() Returns the bucket number where element with key k is located.
4	unordered_multimap::bucket_count() Returns the number of buckets present in unordered_multimap container.
5	unordered_multimap::bucket_size() Returns the number of elements presents in the nth bucket.
6	unordered_multimap::cbegin() container iteratorReturns a constant iterator which refers to the first element of the unordered_multimap.
7	unordered_multimap::cbegin() bucket iteratorReturns a constant iterator pointing to the first element in one of its buckets.
8	unordered_multimap::cend() container iteratorReturns a constant iterator which points to past-the-end element of the unordered_multimap.
9	unordered_multimap::cend() bucket iteratorReturns a constant iterator which points to past-the-end element in one of its buckets.
10	unordered_multimap::clear() Destroys the unordered_multimap by removing all elements and sets the size of unordered_multimap to zero.
11	unordered_multimap::count() Returns the number of mapped values associated with key k.
12	unordered_multimap::emplace() Extends container by inserting new element.
13	unordered_multimap::emplace_hint() Inserts a new element in a unordered_multimap using hint as a position for element.
14	unordered_multimap::empty() Tests whether unordered_multimap is empty or not.
15	unordered_multimap::end() container iteratorReturns an iterator which points to past-the-end element in the unordered_multimap.
16	unordered_multimap::end() bucket iteratorReturns an iterator which points to past-the-end element in one of its buckets.
17	unordered_multimap::equal_range() Returns range of elements that matches specific key.
18	unordered_multimap::erase() position versionRemoves single element of the unordered_multimap from position.
19	unordered_multimap::erase() key versionRemoves mapped value associated with key k.
20	unordered_multimap::erase() range versionRemoves range of element from the the unordered_multimap.
21	unordered_multimap::find() Finds an element associated with key k.
22	unordered_multimap::get_allocator() Returns an allocator associated with unordered_multimap.
23	unordered_multimap::hash_function() Calculates the hash function object used by the unordered_multimap container.
24	unordered_multimap::insert() value versionExtends container by inserting new element in unordered_multimap.
25	unordered_multimap::insert() move versionExtends unordered_multimap by inserting new element.
26	unordered_multimap::insert() hint versionExtends container by inserting new element in unordered_multimap.
27	unordered_multimap::insert() hint move versionExtends container by inserting new element in unordered_multimap by using move semantics.
28	unordered_multimap::insert() range versionExtends container by inserting new elements in the unordered_multimap.
29	unordered_multimap::insert() initializer_list versionExtends unordered_multimap by inserting new element from initializer list.
30	unordered_multimap::key_eq() Returns the function that compares keys for equality.
31	unordered_multimap::load_factor() Returns the current load factor of the unordered_multimap container.
32	unordered_multimap::max_bucket_count() Returns the maximum number of buckets that the unordered_multimap container can have.
33	unordered_multimap::max_load_factor() get versionReturns the current maximum load factor for the unordered_multimap container.
34	unordered_multimap::max_load_factor() set versionAssigns new load factor for the unordered_multimap container.
35	unordered_multimap::max_size() Returns the maximum number of elements can be held by unordered_multimap.
36	unordered_multimap::operator=() copy versionAssigns new contents to the unordered_multimap by replacing old ones and modifies size if necessary.
37	unordered_multimap::operator=() move versionMove the contents of one unordered_multimap into another and modifies size if necessary.
38	unordered_multimap::operator=() initializer_list versionCopy elements from initializer list to unordered_multimap.
39	unordered_multimap::rehash() Sets the number of buckets in the container to n or more.
40	unordered_multimap::reserve() Sets the number of buckets in the container to the most appropriate to contain at least n elements.
41	unordered_multimap::size() Returns the number of elements present in the unordered_multimap.
42	unordered_multimap::swap() Exchanges the content of first unordered_multimap with another.

Non-member overloaded functions

S.N.	Method & Description
1	unordered_multimap::operator==() Tests whether two unordered_multimaps are equal or not.
2	unordered_multimap::operator!=() Tests whether two unordered_multimaps are equal or not.
3	unordered_multimap::swap() Exchanges the content of first unordered_multimap with another.