void g() {
  Derived d;
  Base* bp = &d;                // standard conversion:
                                // Derived* to Base*
  ...
  B*  p = bp->vf4();            // calls Derived​::​pf() and converts the
                                // result to B*
  Derived*  dp = &d;
  D*  q = dp->vf4();            // calls Derived​::​pf() and does not
                                // convert the result to B*
  ...
}