offsetof

Ответ от Panic Doctor[гуру]
6.50 Offsetof
==
GCC implements for both C and C++ a syntactic extension to implement the
'offsetof' macro.
primary:
"__builtin_offsetof" "(" typename "," offsetof_member_designator ")"
offsetof_member_designator:
identifier
offsetof_member_designator "." identifier
offsetof_member_designator "[" expr "]"
This extension is sufficient such that
#define offsetof(TYPE, MEMBER) __builtin_offsetof (TYPE, MEMBER)
is a suitable definition of the 'offsetof' macro. In C++, TYPE may be
dependent. In either case, MEMBER may consist of a single identifier,
or a sequence of member accesses and array references.
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