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### 6.51 Support for `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.