Friend classes in C++

Friend classes in C++:

Just as we have the possibility to define a friend function, we can also define a class as the friend of another one, granting that second-class access to the protected and private members of the first one. 

// friend class in C++

Program Code:

#include <iostream> 

using namespace std; 

class CSquare; 

class CRectangle 

{ 

int width, height; 

public: 

int area () 

{

return (width * height);

} 

void convert (CSquare a); 

};

class CSquare 

{ 

private: 

int side; 

public: 

void set_side (int a) 

{

side=a;

} 

friend class CRectangle; 

};

void CRectangle::convert (CSquare a) 

{ 

width = a.side; 

height = a.side; 

} 

int main() 

{ 

CSquare sqr; 

CRectangle rect; 

sqr.set_side(4);

rect.convert(sqr); 

cout << rect.area(); 

return 0; 

}

In this example, we have declared CRectangle as a friend of CSquare so that CRectangle member functions could have access to the protected and private members of CSquare, more concretely to CSquare :: side, which describes the side width of the square.