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0 pointskazinator5mo ago0 comments

Unfortunately, ISO C requires offsetof to calculate a constant; moreover, the right operand is a "member designator", which elements[N] isn't.

However, the traditional (and pretty much the only sensible) ways of defining offsetof do make it work.

N3220 draft:

  offsetof(type, member-designator)

[expands] to an integer constant expression that has type size_t, the value of which is the offset in bytes, to the subobject (designated by member-designator), from the beginning of any object of type type. The type and member designator shall be such that given

  static type t;

then the expression &(t. member-designator) evaluates to an address constant. If the specified type name contains a comma not between matching parentheses or if the specified member is a bit-field, the behavior is undefined.

This doesn't mean I won't use it, but just something to be aware of.

It might not be a bad idea to propose to ISO C that offsetof(type, array[n]) should be required to work, and for non-constant n too.

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kazinatorOP5mo ago

In GCC, __builtint_offsetof explicitly supports the extended syntax for the member designator. when the offsetof macro uses __builtin_offsetof, the capability is a documented extension.

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