G++ uses a syntactic extension to implement the offsetof macro.
In particular:
__offsetof__ (expression)
is equivalent to the parenthesized expression, except that the
expression is considered an integral constant expression even if it
contains certain operators that are not normally permitted in an
integral constant expression. Users should never use
__offsetof__ directly; the only valid use of
__offsetof__ is to implement the offsetof macro in
<stddef.h>.