When you’re inheriting, the base class defaults to private, which means that all of the public member functions are private to the user of the new class. Normally, you’ll make the inheritance public so the interface of the base class is also the interface of the derived class. However, you can also use the protected keyword during inheritance.

Protected derivation means “implemented-in-terms-of” to other classes but “is-a” for derived classes and friends. It’s something you don’t use very often, but it’s in the language for completeness.