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.