alt Hacker NewsAnd if you use 0 as the value of NULL pointer, then -1 can't ever point to an object (because adding 1 to it should generate a non-NULL pointer, so that pointer comparisons are not UB).
So yeah, C implementations have to reserve at least two addresses, not just one. By the way, the standard to this day allows NULL, when cast to a pointer type, to be something else than all-bits-zero pattern (and some implementations indeed took this opportunity).
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bobmcnamara • last Friday at 11:22 PM
On a system where -1 points to an object, I don't think comparing that to null(0) would be UB, because null is one past an object.
But adding 1 to a pointer will add sizeof(T) to the underlying value, so you actually need to reserve more than two addresses if you want to distinguish the "past-the-end" pointer for every object from NULL.
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While it's rare to find a platform nowadays that uses something other than a zero bit pattern for NULL as normal pointer type; it's extremely common in C++ for pointer-to-member types: 0 is the first field at the start of a struct (offset 0); and NULL is instead represented with -1.