[cma]

It isn't a sentinel is it? He said no runtime data, so it sounds like some kind of empty tag for the type system. I'm not sure if it is related to what he is describing, but C++20 also added [[no_unique_address]] for something to do with truly empty members.

[mehrdadn]

Distinguishing two members of a union with... 0 bytes of extra data? Sounds like magic.

Not 100% sure about the terminology. Pick whatever word might fit it better. I just called it a sentinel.

[cma]

I thought he was saying the `end` in range begin/end was now allowed to be some empty signifier to tell things to look for the null pointer (null is a sentinel, but `end` itself is now empty). That may not be right, I don't know enough about the new concepts stuff and range seems to be part of that.

edit: looking at this it maybe covers it:

https://foonathan.net/2020/03/iterator-sentinel/

[mehrdadn]

Yes but I'm saying this is kind of moot when you're writing a struct that needs to store the iterators, because it can't get away with just storing the 'begin' -- you can't guarantee the second iterator you're storing will actually be the end. Meaning this iteration model isn't efficiently composable.

[stkdump]

Why would you need a union?

[mehrdadn]

Write me some code that shows me how to compose iterator-based ranges with just 1 iterator's worth of space in each range.

[stkdump]

It is not so clear to me what you mean. Maybe you can point to a better version (maybe what some other language does), and I can explain how I think the same can be done with C++20 ranges. I believe ranges are strictly more powerful than what other languages provide, and often they can be slimmer/more efficient for the common case.