4 ms·

> Each such allocation shall yield a pointer to an object disjoint from any other object. Phrasing could be slightly more clear to prevent someone from making

> Each such allocation shall yield a pointer to an object disjoint from any other object. Phrasing could be slightly more clear to prevent someone from making the argument that -1 is disjoint from all objects as it does not point to an object

Joker_vD 1y ago
And 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).
- ynik 1y ago
  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. -- 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.
  - Joker_vD 1y ago
    > 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. Well, yes and no. A 4-byte int can not reside at -4, but a char could be; but no object can reside at -1. So implementations need to take care that one-past-the-end addresses never equal to whatever happens to serve as nullptr but this requirement only makes address -1 completely unavailable for the C-native objects.
- bobmcnamara 1y ago
  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.

Joker_vD 1y ago

And 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).

ynik 1y ago
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. -- 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.
- Joker_vD 1y ago
  > 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. Well, yes and no. A 4-byte int can not reside at -4, but a char could be; but no object can reside at -1. So implementations need to take care that one-past-the-end addresses never equal to whatever happens to serve as nullptr but this requirement only makes address -1 completely unavailable for the C-native objects.
bobmcnamara 1y ago
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.