C++ friend class and functions

Friend Class

A friend class can access private and protected members of other classes in which it is declared as friend. It is sometimes useful to allow a particular class to access private members of other classes.

Consider a scenario in which you forget your hall ticket to your examination in such a situation having a friend who has access to your private drawer can be really helpful similarly a friend class is very useful.

Syntax for friend Class


class ClassNmae {
private:
    int key;
    Node* next;
    /* Other members of Node Class */
 
    // Now class  LinkedList can
    // access private members of Node
    friend class FriendClassName;
};

Like friend class, a friend function can be given a special grant to access private and protected members.

A friend function can either be a global function or a member function of a different class.

Syntax for friend function


// When Friend Function is a member of a class
class className1 {
private:
    int key;
    Node* next;
 
    /* Other members of Node Class */
    friend int className2::FreindFuncName();
    // Only FreindFuncName() of className2 can access the private members
    // can access internal members
};


//When the friend function is a global function
class A {
    int a;
 
public:
    A() { a = 0; }
 
    // global friend function
    friend void showA(A&);// friend function showA() is global.
};

Points to remember

The use of friends should be limited. If we declare too many friends of a class, it lessens the value of encapsulation of separate classes in object-oriented programming.
Friendship is not mutual. If class A is a friend of B, then B doesn’t become a friend of A.
Friendship is not inherited.
The concept of friends is not there in Java.

Friendship and Inheritance

In C++, when we derive a new class from a base class, friendship is not inherited. If a base class has a friend function, then the function doesn’t become a friend of the derived class.

Demonstration of how a friend of base class is not friend of a derived class


#include 
using namespace std;
  
class X
{
  protected:
    int a;
  public:
    X() { a = 0;}
  friend void show();
};
  
class Y: public X
{
  public:
    Y() : b (0) {}
  private:
    int c;
  
};
  
void show()
{
  Y b;
  cout << "The default value of A::x = " << b.a;
  
  // Can't access private member "c" declared in class 'B'
  cout << "The default value of B::y = " << b.c;
}
  
int main()
{
  show();
  getchar();
  return 0;
}

We will get an error message because show() which is a friend of base class X tries to access private data of derived class Y.

A simple C++ program to demonstrate friend function of another class


#include 

class Y;

class X{
public:
	void showB(B&);
};

class Y {
private:
	int b;

public:
	Y() { y= 0; }
	friend void A::showB(B& x); // Friend function
};

void X::showY(Y& x)
{
	// Since showB() is friend of B, it can
	// access private members of B
	std::cout << "Y::y = " << x.y;
}

int main()
{
	X a;
	Y x;
	a.showY(x);
	return 0;
}


Y::y = 0