A null pointer constant is an integral constant expression ([expr.const]) prvalue of integer type that evaluates to zero or a prvalue of type std::nullptr_t. A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of object pointer or function pointer type. Such a conversion is called a null pointer conversion. Two null pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion ([conv.qual]). A null pointer constant of integral type can be converted to a prvalue of type std::nullptr_t. [ Note: The resulting prvalue is not a null pointer value. — end note ]
A prvalue of type “pointer to cv T,” where T is an object type, can be converted to a prvalue of type “pointer to cv void”. The result of converting a “pointer to cv T” to a “pointer to cv void” points to the start of the storage location where the object of type T resides, as if the object is a most derived object ([intro.object]) of type T (that is, not a base class subobject). The null pointer value is converted to the null pointer value of the destination type.
A prvalue of type “pointer to cv D”, where D is a class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class (Clause [class.derived]) of D. If B is an inaccessible (Clause [class.access]) or ambiguous ([class.member.lookup]) base class of D, a program that necessitates this conversion is ill-formed. The result of the conversion is a pointer to the base class subobject of the derived class object. The null pointer value is converted to the null pointer value of the destination type.