Even if "friend class rect" has been commented in square class why it is changing the private variable of rect class?

Question

confused with below program for friend class use, please help me to solve that

#include <iostream>

using namespace std;

class square;

class rect {
    public :
            rect(int w = 0, int h = 0) : width(w), height(h), volume(w*h) {}
            void set_data(square &);
            int get_width(void)
            {
                return width;
            }
            int get_height(void)
            {
                return height;
            }
            int get_volume(void);
    private :
            int width;
            int height;
            int volume;
};

class square {
    public :
            square(int w = 0, int h = 0) : width(w), height(h), volume(w*h) {}
            int width;
            int height;
            int get_volume(void);
**//          friend class rect;**
    private :
            int volume;
};

void rect :: set_data(square &s)
{
    width = s.width; *// the variables of rect class are private and it shud not allow to change as i have commented "//friend class rect;" the sentence in square class.* 
    height = s.height;
}

int rect :: get_volume(void)
{
    return volume;
}

int square :: get_volume(void)
{
    return volume;
}

int main()
{
    rect r(5,10);
    cout<<r.get_volume()<<endl;
    square s(2,2);
    cout<<s.get_volume()<<endl;
    cout<<endl;
    cout<<endl;
    r.set_data(s); *// accessing the private members of the rect class through object of square class*
    cout<<"new width : "<<r.get_width()<<endl;
    cout<<"new height : "<<r.get_height()<<endl;
    cout<<r.get_volume()<<endl;
    return 0;
}

As per friend guide lines if we use friend class then it can able to access and modify the private members of its friend class so even though i have commented "//friend class rect;" in square class why i am seeing that the members of rect class has been changed by square class by "r.set_data(s);" this function In normal condition as per my understanding the private variables of a class can be changed only if it is friend class(so in below output new width and new height should not be changed as I have commented "//friend class rect;" but even though it is commented I am seeing changes of variables of rect class by set_data function so what is the need to use friend class if private members are changed simply by passing other object to any function.

    output of the program :

    50
    4

    new width : 2
    new height : 2
    50

AhmadWabbi · Answer 1 · 2016-06-22T07:47:03.260

set_data is a method of class rect. It copies the public data members of a square into the private data members of rect. Nothing strange about it, and friend has nothing to do here. When you call it, you don't modify the private members of rect by square, you modify them by a public method set_data of the class rect itself. Taking the new values from a square does not mean that square modifies them. When you say "modify private members of rect by square", it means accessing them from a method of the class square, which is not the case here.

Even if "friend class rect" has been commented in square class why it is changing the private variable of rect class?

1 Answers1