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

I have the following code:

class A 
{
private:
开发者_开发技巧    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?

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.

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).

Short answer: In member methods of class A, all the members of (object/pointer and static member) class A can be accessed.

A(A a1, A a2)
{
    a1.x = 10;
    a2.x = 20;
}

Now from my understanding, you question is how can a object that invoked the constructor call can access other class member variables ?

Now, both the constructor and the arguments a1,a2 are class scoped. So, it can access all it's members irrespective of it's access level. This too will also work in the constructor -

this->x = a1.x; // Notice that "this" members can be accessed too.
                // How ever both the member variables are different and are part of
                // different objects.