accessing a protected member of a base class in another subclass

When foo receives a FooBase reference, the compiler doesn’t know whether the argument is a descendant of Foo, so it has to assume it’s not. Foo has access to inherited protected members of other Foo objects, not all other sibling classes.

Consider this code:

class FooSibling: public FooBase { };

FooSibling sib;
Foo f;
f.foo(sib); // calls sib.fooBase()!?

If Foo::foo can call protected members of arbitrary FooBase descendants, then it can call the protected method of FooSibling, which has no direct relationship to Foo. That’s not how protected access is supposed to work.

If Foo needs access to protected members of all FooBase objects, not just those that are also known to be Foo descendants, then Foo needs to be a friend of FooBase:

class FooBase
{
protected:
  void fooBase(void);
  friend class Foo;
};