A prvalue of type “pointer to member of
B of type
cv
T”, where
B is a class type, can be converted to
a prvalue of type “pointer to member of
D of type
cv
T”, where
D is a complete class derived (
[class.derived])
from
B.If class
D does not contain the original member and
is not a base class of the class containing the original member,
the behavior is undefined
.Otherwise,
the result of the conversion refers to the same member as the pointer to
member before the conversion took place, but it refers to the base class
member as if it were a member of the derived class
.The result refers to
the member in
D's instance of
B.Since the result has
type “pointer to member of
D of type
cv T”,
indirection through it with a
D object is valid
.The result is the same
as if indirecting through the pointer to member of
B with the
B subobject of
D.The null member pointer value is
converted to the null member pointer value of the destination
type
.