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

Why can B::foo() access b's x field, but not a's x field?

A protected member can only be accessed by other members of the same class (or derived classes).

b->x points to a protected member of an instance of class B (through inheritance), so B::foo() can access it.

a->x points to a protected member of an instance of class A, so B::foo() cannot access it.