Why is accessing private variable allowed here?

Question

I have this class declared. I observed that in the method distance(Point b), how is it possible to access the private members of Point - b.x and b.y? If I try to access b.x and b.y in main, it is not allowed.

#include <iostream>
#include <cmath>

using namespace std;

class Point {
private:
    int x, y;
public:
    Point() {
        cout << "Constructor called" << endl;
        x = 0; y = 0;
    }

    ~Point() {
    }

    void set(int a, int b) {
        x = a;
        y = b;
    }

    void offset(int dx, int dy) {
        x += dx;
        y += dy;
    }

    void print() {
        cout << "(" << x << "," << y << ")" << endl;
    }

    // HERE
    double distance(Point b) {
        return (sqrt(pow(x-b.x, 2)+pow(y-b.y, 2)));
    }
};

int main() {
Point p, q;

p.print();
p.offset(4, 3);
p.print();

q.set(10, 2);

cout << "Distance: " << p.distance(q) << endl;

return 0;
}

NOTE: I have compiled and ran the program on ideone.com

If class member functions couldn't access private variables, what could? — David Schwartz
– David Schwartz, Commented Apr 1, 2013 at 6:14
possible duplicate of What are access specifiers? Should I inherit with private, protected or public? — Alok Save
– Alok Save, Commented Apr 1, 2013 at 6:14
I am just confused / did not know that even 'other' objects can do that!! — aakash
– aakash, Commented Apr 1, 2013 at 6:15
@aakash: While it doesn't work this way in every language, for C++ at least, access control is per-class rather than per-object. That is -- access specifiers are about limiting the scope of the code that knows about (and thus maintains invariants about) the internals of objects. Member functions as a whole are responsible for maintaining the invariants for the internals of objects, so what does it matter whether they are looking through this or through some other access path? — Mankarse
– Mankarse, Commented Apr 1, 2013 at 6:22

maditya · Accepted Answer · 2013-04-01 06:24:20Z

4

The concept of access specifiers such as private, public etc. applies to classes, not just objects of classes. If a variable is private in a class, and an object A of that class has a function that takes another object B of the same class, A has access to B's private members since A and B belong to the same class.

Copy constructors rely on this:

#include <iostream>                                                                

using namespace std;                                                            


class A {                                                                       
  public:                                                                       

     A(){val = 1.0;}                                                            

     //copy constructor                                
     A(const A& _other) {                                                       
       val = _other.val; //accessing private member of _other                   
     }                                                                          

    double dist(A _a) {return val - _a.val;} //accessing private member of _other

  private:                                                                      
    double val;                                                                 
};                                                                              


int main() {                                                                    

A a;                                                                            
A b;                                                                            

cout << a.dist(b) << endl;                                                      

}

edited Apr 1, 2013 at 6:24

answered Apr 1, 2013 at 6:18

maditya

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4 Comments

aakash Over a year ago

this makes sense. Probably i thought that access specifiers were for objects.

Cheers and hth. - Alf Over a year ago

well there is a difference between this and e.g. method arguments (and other objects) with respect to access checking. on the this object you can freely access protected members of a base class. that is not so for other objects, so yes, there is a difference wrt. to which object.

maditya Over a year ago

@Alf I'm not understanding ... do you mean that class A's function cannot access the protected member of the base class of an object that is passed as a parameter to that function? I seem to be able to ...

Cheers and hth. - Alf Over a year ago

@maditya: i meant what i wrote, which means that a class A function cannot freely access protected members of a base class object that's passed as parameter. there's no absolute barrier though. the type system of C++ allows access to just about anything if you employ sufficiently intricate low level means (in this case member pointers are sufficient to work around the ordinary access rules).

Suvarna Pattayil · Accepted Answer · 2013-04-01 06:31:51Z

int x, y; are private to the Class Point. It means all the members of Class Point can access,modify it.

The functions and constuctors of Class Point are public i.e Other functions(from some other class etc) [like main] can access them. The idea of OOPS is to keep your data safe, Anyone who wants to modify the values can do it via the public methods of that class. They can't access them directly. This allows you to keep any check for invalid modifications to data [setting invalid values like height = -4; etc]

If u keep int x,y as public [which is not correct OOPs] any function (main etc) can access it and modify the value to something undesirable.

Point p;

p.x = -5 (suppose you only wanted +ve values for x , you can't check if the functions accessing is setting some undesirable value)`

Not a relevant analogy , but still...You have a room in the house which only the family members can access. Any outsider who wishes to keep or remove or do anything to the things in the house has to ask your family members to do it. If u assume even family members are not allowed to access the room [assuming even member functions can't access variables ] then there is no use of keeping anythin in the room [the data cannot be used by anyone]

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