Friend function acts as a friend of the class. It can access the private and protected members of the class. The friend function is not a member of the class, but it must be listed in the class definition. The non-member function cannot access the private data of the class. Sometimes, it is necessary for the non-member function to access the data. The friend function is a non-member function and has the ability to access the private data of the class.

To make an outside function friendly to the class, we need to declare the function as a friend of the class as shown below:

1. class sample  
2. {  
3.    // data members;  
4.  public:  
5. friend void abc(void);  
6. };  

Following are the characteristics of a friend function:

• The friend function is not in the scope of the class in which it has been declared.
• Since it is not in the scope of the class, so it cannot be called by using the object of the class. Therefore, friend function can be invoked like a normal function.
• A friend function cannot access the private members directly, it has to use an object name and dot operator with each member name.
• Friend function uses objects as arguments.

Let’s understand this through an example:

1. #include <iostream>  
2. using namespace std;  
3. class Addition  
4. {  
5.  int a=5;  
6.  int b=6;  
7.  public:  
8.  friend int add(Addition a1)  
9.  {  
10.      return(a1.a+a1.b);  
11.  }  
12. };  
13. int main()  
14. {  
15. int result;  
16. Addition a1;  
17.  result=add(a1);  
18.  cout<<result;  
19. return 0;  
20. }  

Output:

11