FAQ 37.02 What is the difference between private inheritance and protected inheritance?

In private inheritance, the relationship with the base class is a private decision; only members and friends of the privately derived class can exploit the relationship with the private base class. In protected inheritance, the relationship with the base class is a protected decision, so members and friends of the protected derived class and members and friends of classes derived from the protected derived class can exploit the protected inheritance relationship, but normal users cannot.

Protected inheritance is less restrictive than private inheritance and therefore introduces more coupling between the derived class and the base class. With protected inheritance, if the relationship between the protected base class and the derived class is changed (or if the protected operations of the protected base class change), the effects may reach beyond the protected derived class and its friends to classes derived from the protected derived class, classes derived from those derived classes, and so on.

This is a for-better-for-worse situation; derived classes have more coupling, but they also have the ability to exploit the relationship between the derived class and the base class.