• A friend function of a class is defined outside that class's scope but it has the right to access all private and protected members
• Friend declaration can go in either the public, private, or protected section of a class

//Rectangle.h
#ifndef RECTANGLE_H
#define RECTANGLE_H
#include "Util.h"

class Rectangle
{
	private:
		double width;
		double length;
	public:
		//constructor
		Rectangle(double w, double l);

		//accessor
		double getWidth() const;
		double getLength() const;
		double getArea() const;

		//mutator
		void setWidth(double w);
		void setLength(double l);

		//friend
		friend void display(const Rectangle &r);
};
#endif
			

//Rectangle.cpp
#include "Rectangle.h"

Rectangle::Rectangle(double w, double l):width(w), length(l){}

double Rectangle::getWidth() const
{
	return width;
}

double Rectangle::getLength() const
{
	return length;
}

double Rectangle::getArea() const
{
	return width*length;
}

void Rectangle::setWidth(double w)
{
	width = w;
}

void Rectangle::setLength(double l)
{
	length = l;
}
			

//Util.h
#ifndef UTIL_H
#define UTIL_H
#include <iostream>

//forward declaration
class Rectangle;

void display(const Rectangle &r);
#endif
			

//Util.cpp
#include <iostream>
#include "Rectangle.h"
#include "Util.h"

void display(const Rectangle &r)
{
	std::cout<<"Width: "<<r.width<<std::endl;
	std::cout<<"Length: "<<r.length<<std::endl;
}

			

#include <iostream>
#include "Rectangle.h"
#include "Util.h"

int main(int argc, char *argv[])
{
	Rectangle *r = new Rectangle(5, 10);

	display(*r);

	return 0;
}
			

//Square.h
#ifndef SQUARE_H
#define SQUARE_H
#include "Rectangle.h"

class Square
{
	private:
		int edge;
	public:
		Square(int e):edge(e){}
		int getEdge() const;
		friend void Rectangle::setEdge(const Square &s);
};
#endif
			

//Square.cpp
#include "Square.h"

int Square::getEdge() const {return edge;}
			

//Rectangle.h
#ifndef RECTANGLE_H
#define RECTANGLE_H
class Square;

class Rectangle
{
	private:
		double width;
		double length;
	public:
		//constructor
		Rectangle(double w, double l);

		//accessor
		double getArea() const;

		//mutator
		void setEdge(const Square &s);
};
#endif
			

//Rectangle.cpp
#include "Rectangle.h"
#include "Square.h"

Rectangle::Rectangle(double w, double l):width(w), length(l){}

double Rectangle::getArea() const
{
	return width*length;
}

void Rectangle::setEdge(const Square &s)
{
	width = s.edge;
	length = s.edge;
}
			

//main.cpp
#include <iostream>
#include "Rectangle.h"
#include "Square.h"

int main(int argc, char *argv[])
{
	Square s(10);

	Rectangle r(0, 0);
	r.setEdge(s);

	std::cout<<"Area: "<<r.getArea()<<std::endl;

	return 0;
}
			

//Square.h
#ifndef SQUARE_H
#define SQUARE_H

class Square
{
	private:
		int edge;
	public:
		friend class Cube;
};
#endif
			

//Cube.h
#ifndef CUBE_H
#define CUBE_H
#include "Square.h"

class Cube
{
	private: 
		Square s;
	public:
		int getArea() const
		{
			return s.edge*s.edge;
		}

		void setEdge(int e)
		{
			s.edge = e;
		}
};
#endif
			

//main.cpp
#include <iostream>
#include "Cube.h"

int main(int argc, char *argv[])
{
	Cube c;

	c.setEdge(10);

	std::cout<<"Area: "<<c.getArea()<<std::endl;

	return 0;
}
			

Other class have direct access to protected and private members of the class