C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 24:
[24.6] What are the access rules with private and protected inheritance?

Take these classes as examples:

class B                    { /*...*/ };
class D_priv : private   B { /*...*/ };
class D_prot : protected B { /*...*/ };
class D_publ : public    B { /*...*/ };
class UserClass            { B b; /*...*/ };
None of the derived classes can access anything that is private in B. In D_priv, the public and protected parts of B are private. In D_prot, the public and protected parts of B are protected. In D_publ, the public parts of B are public and the protected parts of B are protected (D_publ is-a-kind-of-a B). class UserClass can access only the public parts of B, which "seals off" UserClass from B.

To make a public member of B public in D_priv or D_prot, state the name of the member with a B:: prefix. E.g., to make member B::f(int,float) public in D_prot, you would say:

class D_prot : protected B {
  using B::f;  // Note: Not using B::f(int,float)