alt Hacker NewsBut 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.
> 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.