A null pointer constant ([conv.ptr]) can be converted to a pointer to member type; the result is the null member pointer value of that type and is distinguishable from any pointer to member not created from a null pointer constant. Such a conversion is called a null member pointer conversion. Two null member pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to member of cv-qualified type is a single conversion, and not the sequence of a pointer to member conversion followed by a qualification conversion ([conv.qual]).
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 derived class (Clause [class.derived]) of B. If B is an inaccessible (Clause [class.access]), ambiguous ([class.member.lookup]), or virtual ([class.mi]) base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed. 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.59
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