On mån 2025-02-17 at 10:33, Amit via Std-Discussion wrote:
> C++ language has a big security hole. You can change the values of the
> private member variables directly by getting the pointer to the
> object. So, private member variables are actually not private, they
> are public. Below is the example code:
>
> --------------------------------------------------------------------------------
>
> #include <iostream>
>
> using namespace std;
>
> class MyClass
> {
>
> private:
> int i;
> int j;
>
> public:
> MyClass(int a, int b)
> {
> i = a;
> j = b;
> }
>
> void print_data()
> {
> cout << endl;
> cout << "i = " << i << ", j = " << j;
> }
>
> }; // end of class MyClass
>
> int main(void)
> {
>
> MyClass myobj(1, 4);
>
> myobj.print_data();
>
> MyClass *m = &myobj;
>
> int *i_ptr = (int *)(m);
> int *j_ptr = i_ptr + 1;
>
> *i_ptr = 10;
> *j_ptr = 20;
>
> myobj.print_data();
>
> cout << endl << endl;
>
> return 0;
>
> } // end of function main()
>
> --------------------------------------------------------------------------------
>
> The output is:
>
> i = 1, j = 4
> i = 10, j = 20
>
> So, you see that the values of the private member variables ('i' and
> 'j') were changed directly by using pointers. So, the 'private'
> keyword actually didn't serve its purpose.
>

You are not allowed to do that, the code is invalid.

However, you can change myobj anyway by just assigning new values, like

myobj = MyClass(10, 20);

so not a big deal.