Prevent instances of the same class from calling public functions (C++) [duplicate]

Question

Possible Duplicate:
Why do objects of the same class have access to each other’s private data?

Something I have never understood with trying to keep encapsulation:

Say I have a class called GameObject, and a derived class called Human. GameObject has a private variable position. I have multiple instances of Human, I want each human to be able to call SetPos() and set it's position as it wants. I do not however want one human to have the power to set up the position of another human. This is my problem.

If I have SetPos public or protected, each human can alter each others positions, if SetPos() is private, a human cannot even set its own position ( I need this, might be a weak example but I hope you understand).

Can anyone offer a solution?

Thanks.

Answer 1

if SetPos() is private, a human cannot even set its own position

Actually, it can, if SetPos is defined on Human. A private method can only be called from inside the class, but it does not protect instances of the same class from each other:

class Human {
  private:
    void set_pos(int i) { std::cout << "moving to " << i << std::endl; }
  public:
    void set_pos_on_other(Human &other, int i) const { other.set_pos(i); }
};

int main()
{
    Human alice, bob;
    bob.set_pos_on_other(alice, 10);
}

If SetPos is defined on GameObject and it has to be called by a Human, even if only on itself, then it needs to be protected or public.

To solve the problem of objects calling each other's private methods, you simply need to program carefully and stick to your invariants. C++ offers no special syntax for this. Whenever a Human method gets passed a reference or pointer to another Human, it can call that Human's private methods all it wants.