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### 6.10 Offsetof

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>`.