C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 19:
[19.6] Why can't my derived class access private things from my base class?

To protect you from future changes to the base class.

Derived classes do not get access to private members of a base class. This effectively "seals off" the derived class from any changes made to the private members of the base class.