With a private modifier, why can the member in other objects be accessed directly?

Question

I have the following code:

class A 
{
private:
    int x;
public:
    A()
    {
        x = 90;
    }
    A(A a1, A a2)
    {
        a1.x = 10;
        a2.x = 20;
    }
    int getX()
    {
        return this->x;
    }
};

I know that code might be weird but I don't understand why a1 and a2 can access private data member x?

Answer 1

Good question. The point is that protection in C++ is class level, not object level. So a method being invoked on one object can access private members of any other instance of the same class.

This makes sense if you see the role of protection being to allow encapsulation to ensure that the writer of a class can build a cohesive class, and not have to protect against external code modifying object contents.

Another thought as to the real "why?". Consider how you write almost any copy constructor; you want access to the original's underlying data structure, not its presented interface.

Answer 2

Any member function of the class as well as the constructors can access the private data. That is the private members of the instance object the method is called on or the private members of other instances.

In this case it's the constructor and it's other instances (namely a1, a2).