What is Friend function in C++?

If a function is defined as a friend function in C++, then the protected and private data of a class can be accessed using the function. By using the keyword friend compiler knows the given function is a friend function. For accessing the data, the declaration of a friend function should be done inside the body of a class starting with the keyword friend.

It has the following characteristics:-

  • It cannot access the member names directly and has to use an object name and dot membership operator with the member name.
  • It is not in the scope of the class to which it has been declared as a friend.
  • It cannot be called using the object as it is not in the scope of that class.
  • It can be invoked like a normal function without using the object.
  • It can be declared either in the private or the public part.


class Box    
        int length;    
        Box(): length(0) { }    
        friend int printLength(Box); //friend function    
int printLength(Box b)    
   b.length += 10;    
    return b.length;    
int main()    
    Box b;    
    cout<<"Length of box: "<< printLength(b)<<endl;    
    return 0;