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Friend Function and Friend Class

Friend Functions and Classes
In C++ , one cannot access private/protected members of a class from another function outside the class or from another class. This rule does not apply to friend functions and classes. This can be accomplished by declaring functions and classes with friend keyword.

Objective

• Understanding Friend Functions and Classes
• To know the need for Friend Functions and Classes
• To know the advantages and disadvantages of using Friend Functions and Classes

What is a friend function?
A function which is not a member of the class can access the private and protected members of the class if it is declared a friend of that class. This friend function must be declared within the class with the keyword friend preceding it.

Need for friend functions
The friend function can be used to access the private members of a class as if its a member function of the class.

Advantage of friend functions
Instead of calling member functions like objectName.function( ) , friend functions can be called as function( ) , so it is improves readability of the code.

Disadvantages of friend functions
Dynamic binding is very difficult with friend functions , because non member functions can not be declared virtual .

Example Program for friend functions

#include <iostream>
using namespace std;

//Must be known to TWO
//before declaration of ONE.
class ONE;

class TWO
{
    public:
      void print(ONE& x);
};

class ONE
{
      int a, b;
      friend void TWO::print(ONE& x);
    public:
      ONE() : a(1), b(2) { }
};

void TWO::print(ONE& x)
{
      cout << "a is " << x.a << endl;
      cout << "b is " << x.b << endl;
}

int main()
{
      ONE xobj;
      TWO yobj;
      yobj.print(xobj);
}

Output:

a is 1
b is 2

What is a friend class?

A friend class is a class which can access the private and protected members of another class. The friend class must be defined first , then within the definition of the other class , class should be made as a friend to the class by using the keyword  friend.

Need for Friend class
The friend class can be used to access the private memebers of a class with which friendship is made.

Advantage of Friend class
Sharing of private members between classes is possible by using friend class.

Example Program

#include <iostream>
using namespace std;

class MyClass
{
        // Declare a friend class
        friend class SecondClass;

        public:
        MyClass() : Secret(0){}
        void printMember()
        {
                cout << Secret << endl;
        }
        private:
        int Secret;
};

class SecondClass
{
        public:
                void change( MyClass& yourclass, int x )
                {
                        yourclass.Secret = x;
                }
};

void main()
{
        MyClass my_class;
        SecondClass sec_class;
        my_class.printMember();
        sec_class.change( my_class, 5 );
        my_class.printMember();
}

Output:
05