The macro offsetof(type, member-designator) has the same semantics as the corresponding macro in the C standard library header <stddef.h>, but accepts a restricted set of type arguments in this International Standard. Use of the offsetof macro with a type other than a standard-layout class is conditionally-supported.187 The expression offsetof(type, member-designator) is never type-dependent and it is value-dependent if and only if type is dependent. The result of applying the offsetof macro to a static data member or a function member is undefined. No operation invoked by the offsetof macro shall throw an exception and noexcept(offsetof(type, member-designator)) shall be true.
The type ptrdiff_t is an implementation-defined signed integer type that can hold the difference of two subscripts in an array object, as described in [expr.add].
[ Note: It is recommended that implementations choose types for ptrdiff_t and size_t whose integer conversion ranks are no greater than that of signed long int unless a larger size is necessary to contain all the possible values. — end note ]
Note that offsetof is required to work as specified even if unary operator& is overloaded for any of the types involved.