Cannot access protected member of base class in derived class

Question

I have the following code:

struct A {
protected:
    A() {}

    A* a;
};

struct B : A {
protected:
    B() { b.a = &b; }

    A b;
};

Answer 1

It seems like a big limitation of the C++ language. How would you solve problem like this:

class Node
{
public:
 void Save();
protected:
 virtual void SaveState(int type) = 0;
};

class BinaryNode : public Node
{
protected:
 Node *left;
 Node *right;

 virtual void SaveState(int type) override
 {
    left->SaveState(type);
    right->SaveState(type);
 }
};

In this example I do not want to make method SaveState visible outside Node hierarchy. Only method Save should be public.