FAQ 37.07 Should a pointer be cast from a private or protected derived class to its base class?

No.

Within a member function or friend function of a private or protected derived class, the relationship to the base class is known and the upward pointer or reference conversion takes place automatically without a cast.

In normal user code, the relationship to a private or protected base class is inaccessible and the conversion is illegal. Users should not perform a cast because private or protected inheritance is a nonpublic decision of the derived class. The cast will subtly break at some future date if/when the private or protected derived class chooses to change or remove the private/protected inheritance relationship.

The conclusion is that only a class and its friends have the right to convert a pointer to the class's nonpublic base class. The member functions and friend functions of the privately/protectedly derived class don't need a cast because the relationship with the base class is directly accessible to them.

Here's an even simpler conclusion don't use pointer casts unless there is an overriding reason to do so!