Why does protected inheritance cause dynamic_cast to fail?

Question

I changed my C++ base class to be protected inheritance and my dynamic_cast(s) stopped working.

Why should changing the inheritance to

Answer 1

You as an outside user cannot access the protected or private members of a class. The same applies to protected or private inheritance. The authors of a class don't want outside users to access protected/private parent classes any more than they want outside users to access protected/private members.

One reason: Suppose the parent class has a non-virtual destructor. Deleting from an instance derived class from a base class pointer would result in undefined behavior. Making the parent class protected/private means you can't do this (see footnote).

Another reason: Suppose the authors of the class in question don't want outside users to have access to the public members of the parent class. One could use public inheritance (is-a) and demote those public interfaces to protected or private, but this would violate the Liskov substitution principle. Protected or private inheritance is not an is-a relationship. Those public methods become protected or private with protected or private inheritance. There's no problem with Liskov substitution because protected/private inheritance is not is-a.

Footnote: There is an ugly way around this: Use C-style casts. Outside users can cast a derived class pointer to a base class pointer, even if the base class is not accessible. To me, that's yet another reason to compile with -Wold-style-cast -Werror.

Answer 2

When you change your inheritance to protected, the relationship between your two classes is hidden from the exterior of the object.

Answer 3

Private (or protected) inheritance is semantically different from public inheritance. It is not a "is-a" relationship but a "implemented in terms of" relationship.

Meaning that you can't use a base class as a handle to a derived object.