Friend Function & Friend Class in C++

In this tutorials we will study and understand the concept & Applications of Friend Function & Friend Class in C++

Friend Function & Friend Class in C++

One of the important concepts of OOP is data hiding, i.e., a nonmember function cannot access an object’s private or protected data. But, sometimes this restriction may force programmer to write long and complex codes. So, there is mechanism built in C++ programming to access private or protected data from non-member functions. This is done using a friend function or/and a friend class.

Friend Functions in C++

A friend function of a class is defined outside that class’ scope but it has the right to access all private and protected members of the class. Even though the prototypes for friend functions appear in the class definition, friends are not member functions. A friend can be a function, function template, or member function, or a class or class template, in which case the entire class and all of its members are friends.

Declaration of friend function in C++

class class_name
{
... .. ...
friend return_type function_name(argument/s);
... .. ...
}

Declaration & Definition of friend function in C++

class className
{
    ... .. ...
    friend return_type functionName(argument/s);
    ... .. ...
}

return_type functionName(argument/s)
{
    ... .. ...
    // Private and protected data of className can be accessed from
    // this function because it is a friend function of className.
    ... .. ...
}

Example Program of Friend Function in C++

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#include <iostream>
using namespace std;
 
class Box {
   double width;
public:
   friend void printWidth( Box box );
   void setWidth( double wid );
};
// Member function definition
void Box::setWidth( double wid ) {
   width = wid;
}
// Note: printWidth() is not a member function of any class.
void printWidth( Box box ) {
   /* Because printWidth() is a friend of Box, it can
   directly access any member of this class */
   cout << "Width of box : " << box.width <<endl;
}
// Main function for the program
int main( ) {
   Box box;
 
   // set box width with member function
   box.setWidth(10.0);
   // Use friend function to print the wdith.
   printWidth( box );
   return 0;
}

Friend Class in C++

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).

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#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;
		}
};

int main()
{
	MyClass my_class;
	SecondClass sec_class;
	my_class.printMember();
	sec_class.change( my_class, 5 );
	my_class.printMember();
	
	return 0;
}

In this program we are using the SecondClass object to access the MyClass object’s datamember and change its value. This is possible because SecondClass is a friend of MyClass

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