subtle C++ inheritance error with protected fields

Question

Below is a subtle example of accessing an instance\'s protected field x. B is a subclass of A so any variable of type B is also of type A. Why can B::foo() access b\'s x fie

Answer 1

Consider:

class A {
protected:
  int x;
};

class C : public A
{
};

class B : public A {
protected:
  unique_ptr a;
public:
  B() : a(new C) // a now points to an instance of "C"
  { }

  void foo() {
    int w = a->x;  // B accessing a protected member of a C? Oops.
  }
};