C++ Library - <map>

Introduction to map

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.

In map key values generally used to sort the elements. For map data type of key and value can differ and it is represented as

typedef pair<const Key, T> value_type;

Maps are typically implemented as Binary Search Tree.

Zero sized maps are also valid. In that case map.begin() and map.end() points to same location.

Definition

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

template < class Key,
           class T,
           class Compare = less<Key>,
           class Alloc = allocator<pair<const Key,T> >
           > class map;

Parameters

Key − Type of the key.
T − Type of the mapped values.
Compare − A binary predicate that takes two element keys as arguments 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.

Sr.No.	Member types	Definition
1	key_type	Key (First parameter of the template)
2	mapped_type	T (Second parameter of the template)
3	key_compare	Compare (Third parameter of the template)
4	allocator_type	Alloc (Fourth parameter of the template)
5	value_type	pair<const key_type,mapped_type>
6	value_compare	Nested function class to compare elements
7	reference	allocator_type::reference
8	const_reference	allocator_type::const_reference
9	pointer	allocator_type::pointer
10	const_pointer	allocator_type::const_pointer
11	iterator	bi-directional iterator to `value_type`
12	const_iterator	bi-directional iterator to const `value_type`
13	reverse_iterator	reverse iterator
14	const_reverse_iterator	constant reverse iterator
15	difference_type	ptrdiff_t
16	size_type	size_t

Functions from <map>

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

Constructors

Sr.No.	Method & Description
1	map::map `default constructor` Constructs an empty map with zero elements.
2	map::map `range constructor` Constructs a map with as many elements as in range of `first` to `last`.
3	map::map `copy constructor` Constructs a map with copy of each elements present in existing map.
4	map::map `move constructor` Constructs a map with the contents of other using move semantics.
5	map::map `initializer list constructor` Constructs a map from initialize list.

Destructor

Sr.No.	Method & Description
1	map::~map Destroys map object by deallocating it's memory.

Member functions

Sr.No.	Method & Description
1	map::at Returns a reference to the mapped value associated with key `k`.
2	map::begin Returns a iterator which refers to the first element of the map.
3	map::cbegin Returns a constant iterator which refers to the first element of the map.
4	map::cend Returns a constant iterator which points to `past-the-end` element of the map.
5	map::clear Destroys the map by removing all elements and sets size of map to zero.
6	map::count Returns number of mapped values associated with key `k`.
7	map::crbegin Returns a constant reverse iterator which points to the last element of the container i.
8	map::crend Returns a constant reverse iterator which points to the theoretical element preceding the first element in the container i.
9	map::emplace Extends container by inserting new element.
10	map::emplace_hint `hint version` Inserts a new element in a map using hint as a position for element.
11	map::empty Tests whether map is empty or not.
12	map::end Returns an iterator which points to `past-the-end` element in the map.
13	map::equal_range Returns range of elements that matches specific key.
14	map::erase `position version` Removes single element of the map from `position`.
15	map::erase `position version` Removes single element of the map from `position`.
16	map::erase `key` Removes mapped value associated with key `k`.
17	map::erase `range version` Removes range of element from the the map.
18	map::erase `range version` Removes range of element from the the map.
19	map::find Finds an element associated with key `k`.
20	map::get_allocator Returns an allocator associated with map.
21	map::insert `single element` Extends container by inserting new element in map.
22	map::insert `hint version` Extends container by inserting new element in map.
23	map::insert `range version` Extends container by inserting new elements in the map.
24	map::insert `move hint verstion` Extends map by inserting new element.
25	map::insert `initializer list version` Extends map by inserting new element from initializer list.
26	map::key_comp Returns a function object that compares the keys, which is a copy of this container's constructor argument `comp`.
27	map::lower_bound Returns an iterator pointing to the first element which is not less than key `k`.
28	map::max_size Returns the maximum number of elements can be held by map.
29	map::operator= `copy version` Assign new contents to the map by replacing old ones and modifies size if necessary.
30	map::operator= `move version` Move the contents of one map into another and modifies size if necessary.
31	map::operator= `initializer list version` Copy elements from initializer list to map.
32	map::operator[] `copy version` If key `k` matches an element in the container, then method returns a reference to the element.
33	map::operator[] `move version` If key `k` matches an element in the container, then method returns a reference to the element.
34	map::rbegin Returns a reverse iterator which points to the last element of the map.
35	map::rend Returns a reverse iterator which points to the reverse end of the map i.
36	map::size Returns the number of elements present in the map.
37	map::swap Exchanges the content of map with contents of map x.
38	map::upper_bound Returns an iterator pointing to the first element which is greater than key `k`.
39	map::value_comp Returns a function object that compares objects of type `std::map::value_type`.

Non-member overloaded functions

Sr.No.	Method & Description
1	operator== Tests whether two maps are equal or not.
2	operator!= Tests whether two maps are equal or not.
3	operator< Tests whether first map is less than other or not.
4	map::operator&lt= Tests whether first map is less than or equal to other or not.
5	operator> Tests whether first map is greater than other or not.
6	operator>= Tests whether first map is greater than or equal to other or not.
7	swap() Exchanges the content of map with contents of map x.

Introduction to multimap

Multimap is dictionary like data structure. It is a sequence of (key, value) pair, where multiple values can be associate with equivalent keys. It is often referred as associative array.

In multimap key values generally used to sort the elements. For multimap data type of key and value can differ and it is represented as

typedef pair<const Key, T> value_type;

Multimaps are typically implemented as Binary Search Tree.

Zero sized multimaps are also valid. In that case multimap.begin() and multimap.end() points to same location.

Definition

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

template < class Key,
           class T,
           class Compare = less<Key>,
           class Alloc = allocator<pair<const Key,T> >
           > class multimap;