Question

I have seen this complex declaration of sizeof operator somewhere in book which is teasing me:

#define SIZEOF(int)  (int)&((int *)0)[1]

Can any one explain this declaration, whats going on here…?

Answer 1

Here it is with the [1] part ‘expanded’:

(int)(&(*(((int*)0) + 1)))

Now do you see how it works?

The 0 will increase by the sizeof int due to the properties of pointer arithmetic and the final int cast is getting you the value of the resulting address which is the ‘size’.

It’s totally undefined behavior though (the arithmetic on null and possibly the dereference of invalid pointer), fairly pointless.

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The update with the macro parameter doesn’t make much sense. The final cast should probably be std::uintptr_t.