CODEWITHSUNDEEP
c++cmacrosundefined-behavioroffsetof
Ta Thanh Dinh
Ta Thanh Dinh

Reputation: 638

Does offsetof require pointer derefence?

I am wondering whether a simple macro offset_of_ requires a pointer dereference of not. For example, a C++ (means that this code will be compiled using a C++ compiler) struct which is declared with packed attribute

struct A {
  int x;
  int y;
} __attribute__(packed);

Suppose that sizeof(int) == 4 in this case, to calculate the offset of y inside A, I wrote a simple macro:

#define offset_of_y (ptrdiff_t)(&(((A*)0)->y))

Can we be sure that the value of this macro is always 4? Is there any pointer dereference from 0 (which may be a UB) when the macro is invoked?

Many thanks for any response.

Upvotes: 4

Views: 193

Answers (1)

HolyBlackCat
HolyBlackCat

Reputation: 96013

Accessing something through a null pointer is UB, period.

(Unless the context is unevaluated, but in this case it IS evaluated.)

You probably want offsetof().

In practice your code probably could work, buf formally it's undefined.

Upvotes: 4

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