C++ multimap operator==

C++ operator== is a non-member overloaded function of multimap. This function is used to check whether the two multimaps are equal or not.

Note: Comparison between multimaps objects depend on a pair wise comparison of the elements. Two mappings are equal if they have the same number of elements and their corresponding elements have the same values. Otherwise they are not equal.

Syntax

template <class Key, class T, class Compare, class Alloc>
  bool operator== ( const multimap<Key,T,Compare,Alloc>& lhs,
                    const multimap<Key,T,Compare,Alloc>& rhs );

Parameter

lhs: First multimap object.

rhs: Second multimap object.

Return value

It returns true if the left side of the multimap object is equal to the right side of the multimap object otherwise false.

Complexity

Complexity will be constant, if the size of lhs and rhs is different.

Otherwise, up to linear in the size (equality comparisons).

Iterator validity

No changes.

Data Races

Containers, lhs and rhs are accessed.

Exception Safety

This function does not throw an exception.

Example 1

Let's see the simple example to check whether the two multimaps are equal or not:

#include <iostream>
#include <map>

using namespace std;

int main() {
   multimap<char, int> m1;
   multimap<char, int> m2;

   if (m1 == m2)
      cout << "Both multimaps are equal." << endl;

   m1.emplace('a', 10);
   
   //after adding element in multimap m1
   if (!(m1 == m2))
      cout << "Both multimaps are not equal." << endl;

   return 0;
}

Output:

Both multimaps are equal.
Both multimaps are not equal.

In the above example, multimap m1 and m2 are empty. Therefore operator== will return true and after adding one element in multimap m1 size of m1 becomes different to size of m2. Therefore, it will return false.

Example 2

Let's see a simple example:

#include <map>  
#include <iostream>  
  
int main ()  
{  
   using namespace std;  
   multimap <int, int> m1, m2, m3;  
   int i;  
   typedef pair <int, int> Int_Pair;  
  
   for (i = 0; i <3; i ++)  
   {  
      m1.insert (Int_Pair (i, i));  
      m2.insert (Int_Pair (i, i * i));  
      m3.insert (Int_Pair (i, i));  
   }  
  
   if (m1 == m2)  
      cout << "The multimaps m1 and m2 are equal." << endl;  
   else  
      cout << "The multimaps m1 and m2 are not equal." << endl;  
  
   if (m1 == m3)  
      cout << "The multimaps m1 and m3 are equal." << endl;  
   else  
      cout << "The multimaps m1 and m3 are not equal." << endl;  
      return 0;
}  

Output:

The multimaps m1 and m2 are not equal.
The multimaps m1 and m3 are equal.

Example 3

Let's see a simple example:

#include <iostream>
#include <map>

using namespace std;

int  main () 
{ 
  multimap < int ,  char >  m1 ; 
  m1 . insert ( make_pair ( 1 ,  'a' ));

  auto  m2  =  m1 ;

  cout  <<  ( m1  ==  m2 )  <<  endl ;

  m2 . insert ( make_pair ( 1 ,  'b' ));
  m2 . insert ( make_pair ( 2 ,  'c' ));

  cout  <<  ( m1  ==  m2 )  <<  endl ;

  return  0 ; 
}

Output:

1
0

In the above example if m1 and m2 are equal then it will return 1 otherwise 0.

Example 4

#include <map>  
#include <iostream>  
using namespace std; 
  
int main ()  
{  
   multimap<int,string> m2;
   typedef multimap<int,string> login; 
   
  m2.insert({2040, "xyz@123"}) ; //stored id and password
   
   string password;
   int id;
   login m1;
   
       cout<<"---------Login----------"<<endl<<endl;
       cout<<"Enter the ID and password: \n";
       cin>> id;      // Get key
       cin>> password;    // Get value
       m1.insert({id, password});   // Put them in multimap

     cout<<"ID and password you have entered: \n";
     for (auto it = m1.begin(); it != m1.end(); it++) {
        cout << (*it).first << "    " << (*it).second << endl;
      }
      cout<<"ID and Password stored in the system :\n";
     for (auto it = m2.begin(); it != m2.end(); it++) {
        cout << (*it).first << "    " << (*it).second << endl;
     }

  
   if (m1 == m2)  
      cout << "\nWelcome to your Page..." << endl;  
   else  
      cout << "\nIncorrect ID or Password..." << endl;  

      return 0;
}

Output:

1).
---------Login----------

Enter the ID and password: 
1020     xyz
ID and password you have entered: 
1020     xyz
ID and Password stored in the system:
2040     xyz@123

Incorrect ID or Password...


2).
---------Login----------

Enter the ID and password: 
2040     xyz@123
ID and password you have entered: 
2040     xyz@123
ID and Password stored in the system:
2040     xyz@123

Welcome to your Page...

In the above example, there are two multimaps m1 and m2. m1 contains stored ID and password and second multimap m2 stores user's entered ID and password. It checks whether the both multimap has same elements or not. If ID and password will match then login is successful otherwise login fails.

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