C++ Friend function and Friend class

#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);
}

#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();
}

There are currently 23 responses to “C++ Friend function and Friend class”

Why not let us know what you think by adding your own comment!

1. karan on March 15th, 2010:

   hi , can u give me the difference between the friend class and friend function….

2. admin on March 15th, 2010:

   The tutorial is clear on this, but here you go, I found this definition on another site:

   A friend function is used for accessing the non-public members of a class. A class can allow non-member functions and other classes to access its own private data, by making them friends. Thus, a friend function is an ordinary function or a member of another class. A friend class has full access of private data members of another class without being member of that class.

3. shrihari on June 28th, 2010:

   can two functions be declared friend to one class?

4. admin on June 29th, 2010:

   @shrihari Sure you can declare two functions friend of one class, take a look at an example:

   #include<iostream>
   using namespace std;

   class AClass {
   private:
   int a,b;
   public:
   void test() {
   a=10;
   b=20;
   }
   // Declare the two functions friends.
   friend int add(AClass input);
   friend int minus(AClass input);
   };

   // Function one
   int add(AClass input) {
   return int(input.a + input.b);
   }

   // Function two
   int minus(AClass input) {
   return int(input.b – input.a);
   }

   void main(void){
   AClass output;
   // Initialize output
   output.test();
   cout << add(output) << endl;
   cout << minus(output) << endl;
   }

   I hope this answers your question.

5. shrihari on June 30th, 2010:

   Thank u. It works!!

6. hemed on May 18th, 2011:

   why the argument or parameter of a friend function must be of the type of Aclass
   eg from the code above
   // Declare the two functions friends.
   friend int add(AClass input);
   friend int minus(AClass input);

   am confused with this please help me

7. silu on June 13th, 2011:

   friend function can access the private members of class. Is it possible, so that friend function should not access the private members of a class? Is any concept deals with it.

8. Yash Bawne on September 24th, 2011:

   We consider that friend function access the private data member of the class.But I did’nt get actually ,because member function of the class also use private data directly.
   Plese clarify that…

9. veena bopche on November 9th, 2011:

   can we make more than one friend functions within a class?

10. tek on December 2nd, 2011:

    C++ :Can we access private function of class from another class(not friend or inherited) without using public function?I mean Class A has private function A1.Can we access A1 from another class B directly using pointer.You cannot use public function in class A to return address of private function A1.class B is not friend or child of class A.
    -Tek

11. SHAGUN on January 17th, 2012:

    worth!!!!!!!!!!!

12. Reshma on February 8th, 2012:

    wat is the output for the program friend class…

13. elite leena on February 24th, 2012:

    @reshma
    the output is
    0
    and
    5

14. Sateesh Kumar on September 1st, 2012:

    This is helpful to all who need this.

15. bhanawrlal on September 10th, 2012:

    this is not helpfull for me i want to see syntax of friend function

16. deepali on October 6th, 2012:

    Well Explained, good concept !!

17. Anuj on November 6th, 2012:

    Hey!
    Can i pass an object to friend function? (of the same class whose friend the function is)

    can a member function call this friend function? (if it does accept a class argument)

18. Shailesh on November 20th, 2012:

    Characteristics of friend function

    1. The friend function can access private and protected data members.
    2. Friend function cannot call with the help of Object of that class, it is call by using normal ‘C’ function.
    3. Generally, friend function can take Object as a argument.
    4. Friend function is not in scope of class.

19. Awet(success) on December 21st, 2012:

    wow it is fantastic so continue…….

20. syra on January 7th, 2013:

    nyc explanation!!!

21. radhika obhan on January 7th, 2013:

    can u show a program in which we can make use of friendly function using two classes to calculate area of two objects using any fake formula

22. VinC on March 6th, 2013:

    class square
    {
    int side;

    square(int a)
    {
    side = a;
    }
    friend int area(square sqObj);
    };

    class rect
    {
    int len, bre;
    rect (int a, int b)
    {
    len = a;
    bre = b;
    }

    friend int area1(rect recObj);
    }

    int area (square sqObj)
    {
    return(sqObj.side*sqObj.side);
    }

    int area1 (rect rectObj)
    {
    return(rectObj.len*rectObj.bre);
    }

    void main()
    {
    square sq(5);
    rect rec(3,5);
    cout << area(sq);
    cout << area1(rec);

    }

23. Dharmendra Kumar on April 26th, 2013:

    A friend function can define also inside the class. then what is the make difference b/w if i defining a friend function inside a class and outside a class.

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