Currently public classes add a new d_ptr, d_ptr is inherited from QObject.

Private classes add a new q_ptr, q_ptr is inherited from QObjectData which is ancestor to QObjectPrivate.

So private classes should inherit QObjectPrivate while public classes in QObject. To access private implementation from public class use Q_D or that dfunc (whatever was the name). In contrast to private access from public, the public access from private is with Q_Q.