LeonidvПовторите. А лучше все-таки разъясните, что происходит в описанном мною случае?

Не вижу "описанного" вами случая.
Вижу только не способность пользоваться гуглом.

http://www.gotw.ca/publications/mill06.htm


There is one additional feature we can get using nonpublic inheritance, and it's the only one that doesn't model IS-IMPLEMENTED-IN-TERMS-OF:

We need "controlled polymorphism" LSP IS-A, but in certain code only.

Public inheritance should always model IS-A as per the Liskov Substitution Principle (LSP).[3] Nonpublic inheritance can express a restricted form of IS-A, even though most people identify IS-A with public inheritance alone. Given class Derived : private Base, from the point of view of outside code, a Derived object IS-NOT-A Base, and so of course can't be used polymorphically as a Base because of the access restrictions imposed by private inheritance. However, inside Derived's own member functions and friends only, a Derived object can indeed be used polymorphically as a Base (you can supply a pointer or reference to a Derived object where a Base object is expected), because members and friends have the necessary access. If instead of private inheritance you use protected inheritance, then the IS-A relationship is additionally visible to further-derived classes, which means subclasses can also make use of the polymorphism.