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:

Multimap employee has duplicate keys age.

template < class Key,                                     // multimap::key_type  
           class T,                                                     // multimap::mapped_type  
           class Compare = less,                        // multimap::key_compare  
           class Alloc = allocator >    // multimap::allocator_type  
           > class multimap; 

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:

typedef pair<const Key, T> value_type;

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.

#include <iostream>  
#include <map>  
#include <string>  
  
using namespace std;  
  
int main()  
{  
    multimap m = {  
            {"India","New Delhi"},  
            {"India", "Hyderabad"},  
            {"United Kingdom", "London"},  
            {"United States", "Washington D.C"}  
    };  
      
    cout << "Size of map m: " << m.size() <::iterator it = m.begin(); it != m.end(); ++it)  
    {  
       cout << "  [" << (*it).first << ", " << (*it).second << "]" << endl;  
    }  
  
    return 0;  
}  

Member Functions

Below is the list of all member functions of multimap:

Modifiers

Non-Member Overloaded Functions