C++ Friend function and Friend class

In the last C++ programming tutorial we looked at inheritance. In this C++ programming tutorial we will take a look at C++ friendship.

Friend Functions

A C++ friend functions are special functions which can access the private members of a class. They are considered to be a loophole in the Object Oriented Programming concepts, but logical use of them can make them useful in certain cases. For instance: when it is not possible to implement some function, without making private members accessible in them. This situation arises mostly in case of operator overloading.
In the following example, the friend function print is a member of class TWO and accesses the private data members a and b of class ONE.

#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);
}

Friend functions have the following properties:

• 1) Friend of the class can be member of some other class.
• 2) Friend of one class can be friend of another class or all the classes in one program, such a friend is known as GLOBAL FRIEND.
• 3) Friend can access the private or protected members of the class in which they are declared to be friend, but they can use the members for a specific object.
• 4) Friends are non-members hence do not get “this” pointer.
• 5) Friends, can be friend of more than one class, hence they can be used for message passing between the classes.
• 6) Friend can be declared anywhere (in public, protected or private section) in the class.

Friend Class

A class can also be declared to be the friend of some other class. When we create a friend class then all the member functions of the friend class also become the friend of the other class. This requires the condition that the friend becoming class must be first declared or defined (forward declaration).

#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();
}

Note:we declared friend class SecondClass; in the class MyClass, so we can access Secret in the class SecondClass.
Another property of friendships is that they are not transitive: The friend of a friend is not considered to be a friend unless explicitly specified.
That is all for this tutorial.

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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