Somewhat related but C++20 ranges do not replace iterators and instead build a higher level of abstraction upon iterators. Ranges are implemented in terms of iterator pairs.
P.S. (e) Is this sentinel-based approach even composable? If your iterator needs to use other iterators underneath, what do you do? You have to store a 'full' iterator anyway... or now you waste even more space and time storing a discriminated union. The fact that a range represents the whole sequence in one object instead of two makes composition intuitive and trivial.
Not 100% sure about the terminology. Pick whatever word might fit it better. I just called it a sentinel.
edit: looking at this it maybe covers it:
class DirectoryChildren
{
void *dir;
void *buffer;
size_t buffer_size;
size_t buffer_offset;
...
public:
bool next(FileInfo *info);
};
If you had to make an iterator for this, what would you do? You'd basically need to either (a) create iterators that at least duplicate the entire state of DirectoryChildren on the stack, or (b) make iterators allocate on the heap. Both of these suck, because (1) iterators are passed around and copied all over the place on the assumption that these are fundamentally cheap operations, which is an assumption that easily breaks (like here), and (2) the iterators fundamentally do not have any meaning or utility as separate entities from the range itself.