C++ Library - <unordered_map>


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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 constructor

Constructs an empty unordered_map with zero elements.

2 unordered_map::unordered_map copy constructor

Constructs an unordered_map with copy of each elements present in existing unordered_map.

3 unordered_map::unordered_map move constructor

Constructs an unordered_map with the contents of other using move semantics.

4 unordered_map::unordered_map range constructor

Constructs 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 iterator

Returns an iterator which refers to the first element of the map.

3 unordered_map::begin bucket iterator

Returns 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 iterator

Returns a constant iterator which refers to the first element of the unordered_map.

8 unordered_map::cbegin bucket iterator

Returns a constant iterator pointing to the first element in one of its buckets.

9 unordered_map::cend container iterator

Returns a constant iterator which points to past-the-end element of the unordered_map.

10 unordered_map::cend bucket iterator

Returns 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 iterator

Returns an iterator which points to past-the-end element in the unordered_map.

17 unordered_map::end bucket iterator

Returns 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 version

Removes single element of the unordered_map from position.

20 unordered_map::erase key version

Removes mapped value associated with key k.

21 unordered_map::erase range version

Removes 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 version

Extends container by inserting new element in unordered_map.

27 unordered_map::insert hint version

Extends conta iner by inserting new element in unordered_map.

28 unordered_map::insert move and hint version

Extends unordered_map by inserting new element.

29 unordered_map::insert range version

Extends container by inserting new elements in the unordered_map.

30 unordered_map::insert initializer_list version

Extends 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 version

Returns the current maximum load factor for the unordered_map container.

35 unordered_map::max_load_factor set version

Assigns 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 version

Assigns new contents to the unordered_map by replacing old ones and modifies size if necessary.

38 unordered_map::operator= move version

Move the contents of one unordered_map into another and modifies size if necessary.

39 unordered_map::operator= initializer_list version

Copy 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 version

If 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 constructor

Constructs an unordered_multimap with copy of each elements present in existing unordered_multimap.

3 unordered_multimap::unordered_multimap() move constructor

Constructs 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 constructor

Constructs 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 iterator

Returns an iterator which refers to the first element of the unordered_mulitmap.

2 unordered_multimap::begin() bucket iterator

Returns 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 iterator

Returns a constant iterator which refers to the first element of the unordered_multimap.

7 unordered_multimap::cbegin() bucket iterator

Returns a constant iterator pointing to the first element in one of its buckets.

8 unordered_multimap::cend() container iterator

Returns a constant iterator which points to past-the-end element of the unordered_multimap.

9 unordered_multimap::cend() bucket iterator

Returns 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 iterator

Returns an iterator which points to past-the-end element in the unordered_multimap.

16 unordered_multimap::end() bucket iterator

Returns 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 version

Removes single element of the unordered_multimap from position.

19 unordered_multimap::erase() key version

Removes mapped value associated with key k.

20 unordered_multimap::erase() range version

Removes 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 version

Extends container by inserting new element in unordered_multimap.

25 unordered_multimap::insert() move version

Extends unordered_multimap by inserting new element.

26 unordered_multimap::insert() hint version

Extends container by inserting new element in unordered_multimap.

27 unordered_multimap::insert() hint move version

Extends container by inserting new element in unordered_multimap by using move semantics.

28 unordered_multimap::insert() range version

Extends container by inserting new elements in the unordered_multimap.

29 unordered_multimap::insert() initializer_list version

Extends 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 version

Returns the current maximum load factor for the unordered_multimap container.

34 unordered_multimap::max_load_factor() set version

Assigns 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 version

Assigns new contents to the unordered_multimap by replacing old ones and modifies size if necessary.

37 unordered_multimap::operator=() move version

Move the contents of one unordered_multimap into another and modifies size if necessary.

38 unordered_multimap::operator=() initializer_list version

Copy 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.



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