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C++ multimap

Multimaps are part of the C++ STL (Standard Template Library). Multimaps are the associative containers like map that stores sorted key-value pair, but unlike maps which store only unique keys, multimap can have duplicate keys. By default it uses < operator to compare the keys.

For example: A multimap of Employees where employee age is the key and name is the value can be represented as:

Keys Values
23 Nikita
28 Robin
25 Deep
25 Aman

Multimap employee has duplicate keys age.

Syntax

Parameter

key: The key data type to be stored in the multimap.

type: The data type of value to be stored in the multimap.

compare: A comparison class that takes two arguments of the same type bool and returns a value. This argument is optional and the binary predicate less<"key"> is the default value.

alloc: Type of the allocator object. This argument is optional and the default value is allocator .

Creating a multimap

Multimaps can easily be created using the following statement:

The above form will use to create a multimap with key of type Key_type and value of type value_type. One important thing is that key of a multimap and corresponding values are always inserted as a pair, you cannot insert only key or just a value in a multimap.

Example

Output:

Size of map m: 4
Elements in m: 
  [India, New Delhi]
  [India, Hyderabad]
  [United Kingdom, London]
  [United States, Washington D.C]

Member Functions

Below is the list of all member functions of multimap:

Constructor/Destructor

Functions Description
constructor Construct multimap
destructor Multimap destructor
operator= Copy elements of the multimap to another multimap.

Iterators

Functions Description
begin Returns an iterator pointing to the first element in the multimap.
cbegin Returns a const_iterator pointing to the first element in the multimap.
end Returns an iterator pointing to the past-end.
cend Returns a constant iterator pointing to the past-end.
rbegin Returns a reverse iterator pointing to the end.
rend Returns a reverse iterator pointing to the beginning.
crbegin Returns a constant reverse iterator pointing to the end.
crend Returns a constant reverse iterator pointing to the beginning.

Capacity

Functions Description
empty Return true if multimap is empty.
size Returns the number of elements in the multimap.
max_size Returns the maximum size of the multimap.

Modifiers

Functions Description
insert Insert element in the multimap.
erase Erase elements from the multimap.
swap Exchange the content of the multimap.
clear Delete all the elements of the multimap.
emplace Construct and insert the new elements into the multimap.
emplace_hint Construct and insert new elements into the multimap by hint.

Observers

Functions Description
key_comp Return a copy of key comparison object.
value_comp Return a copy of value comparison object.

Operations

Functions Description
find Search for an element with given key.
count Gets the number of elements matching with given key.
lower_bound Returns an iterator to lower bound.
upper_bound Returns an iterator to upper bound.
equal_range() Returns the range of elements matches with given key.

Allocator

Functions Description
get_allocator Returns an allocator object that is used to construct the multimap.

Non-Member Overloaded Functions

Functions Description
operator== Checks whether the two multimaps are equal or not.
operator!= Checks whether the two multimaps are equal or not.
operator< Checks whether the first multimap is less than other or not.
operator<= Checks whether the first multimap is less than or equal to other or not.
operator> Checks whether the first multimap is greater than other or not.
operator>= Checks whether the first multimap is greater than equal to other or not.
swap() Exchanges the element of two multimaps.
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