Why can't I access private members of the outer class from the inner class when nesting classes?

Question

class X
{
    int Xi;

    class Y
    {
        int Yi;

        void func()
        {
            X x;
            x.Xi = 5;
        }
    };

    void func()
    {
        Y y;
        y.Yi = 5;
    //  ^^^^ 'X::Y::Yi': cannot access private member declared in class 'X::Y'
    }
};

I was learning about the Memento pattern and in the book I read, it was stated that one of the ways of achieving the pattern is by writing the Memento class inside the Originator class so that only the Originator can have access to the private members of the Memento class. When I tried to apply this method I got an error telling me that the private member is inaccessible. I know that I can use the keyword friend and that will give me access to the private members. Also I know that I can access the outer class's private members from the inner class. But why can't the inner class access the private members of the inner class?

I can do this in java for example:

public class X {

    int Xi;

    public class Y
    {
        private int Yi;

        public void func()
        {
            X x = new X();
            x.Xi = 5;
        }
    }

    public void func()
    {
        Y y = new Y();
        y.Yi = 5;
    }
}

Why is it not doable in C++?

Answer 1

Despite the title of your question, what you are trying to do on the line y.Yi = 5; is access a private member of the inner class from the body of the outer class. This you cannot do, because the Yi member is private - so it can only be accessed from inside its class.

On the other hand, the linex.Xi = 5; does indeed access a private member of the outer class from the inner class; this, you are allowed to do, because your inner Y class is part of the outer X class.

One way to get round this is to declare the X::func() function a friend of class Y; however, you will then need to provide a 'prototype' of that function before you make that declaration, and you would thus need to make the actual definition of the function outside the class body (it must come after the class Y declaration, as it uses an object of that class):

class X {
private: // Even without this line, members are private by default!
    int Xi;
    void func(); // Just a declaration (prototype) - wwwe still need the definition
    class Y {
    private:
        int Yi;
        void func() {
            X x;
            x.Xi = 5;
        }
        friend void X::func();
    };
};

void X::func() { // This is the actual definition of the function.
    Y y;
    y.Yi = 5;
}