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

All the compilers that I tested complained about several things, and specifically the protected constructor would be a problem even if the assignment statement were removed.

You don't get to access the protected members of any instance of a type you derive from. This issue is clarified in the examples of 11.4p1.

class B {
protected:
  int i;
  static int j;
};

class D1 : public B {
};

class D2 : public B {
  void mem(B*, D1*);
};

void D2::mem(B* pb, D1* p1) {
  pb->i = 1; // ill-formed
  p1->i = 2; // ill-formed
  // ...
}