Friend Functions

• We have seen that the private members cannot be accessed from outside the class. That is, a non member function can not have an access to the private data of the class.
• But, there could be a situation where we would like 2 classes to share a particular function.
  For ex:
  Consider a case where 2 classes manager and scientist have been defined, and we would like to use a function income-tax() to operate on the objects of both these classes.
• In such situations, C++ allows the common function to be made friendly with both the classes, there by allowing the function to have access to the private data of these classes.
• Such functions need not be a member of any of these classes.
• To make an outside function "friendly" to a class, we have to declare function as a friend.
• Ex:
  

          
           class ABC
            {
               …….
               ……..
          public:
                 ……..
                 ………
                 friend void xyz();       //declaration
              };

• The function declaration should be preceded by the keyword friend.
• The function definition does not use either the keyword friend or the scope operator ::.
• The functions that are declared with the keyword friend are known as "friend functions".

A friend function processes certain special characteristics

• It is not in the scope of the class to which it has been declared as friend.
• Since it is not in the scope of the class, it cannot be called using the object of that class.
• It can be invoked like a normal function without the help of any object.
• Unlike member functions, it cannot access the member names directly and has to use an object name and dot operator with each member name.
• It can be declared either in the public or the private part of a class without affecting its meaning.
• Usually, it has the objects as arguments.

• The friend function accesses the class variables a and b by using the dot operator and the object passed to it.
• The function call mean(X) passes the object X by value to the friend function.
• Member functions of one class can be friend functions of another class.
• In such cases they are defined using the scope resolution operator as shown below:

• class X
        { 
          …..
          …….
         int fun1();  //member fun of x
        };
  
       class Y
        { 
          ……
          ……
        friend int X :: fun1();      //fun1() of X
        ……….
        };
  

• The function fun1() is a member of class X and a friend of class Y.
• We can also declare all the member functions of one class as the friend functions of another class.
• In such cases, the class is called a friend class.

• Ex:
              class z
              {
               ………..
               friend class x;    //all member functions of x are
                                      //friends to z
              };

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