This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of NAD status.
Section: 26.7.26.1 [range.adjacent.overview], 26.7.27.1 [range.adjacent.transform.overview] Status: NAD Submitter: Hewill Kang Opened: 2022-07-13 Last modified: 2022-11-30
Priority: Not Prioritized
View all issues with NAD status.
Discussion:
views::adjacent is very similar to views::slide, except that the window size N is given at compile time.
Since the case where N is 0 does not make sense for slide_view, LWG 3711 and LWG 3712 added preconditions to the constructor and removed the default constructor, respectively. But for views::adjacent, we can still specify N to be 0. According to the description of 26.7.26.1 [range.adjacent.overview], it will return views::empty<tuple<>> as in the case of views::zip applied to an empty pack. And for views::adjacent_transform<0>(E, F), it will return views::zip_transform(F) and eventually return empty_view for some type. This doesn't seem reasonable to me. The reason why views::zip can return views::empty<tuple<>> is that the parameter pack can indeed be empty, so this still makes some sense. However, there is no meaningful sense for the word "adjacent" when N is 0. I don't see any observable value in allowing views::adjacent<0>, we should disable it for consistency with views::slide.[2022-08-23; Reflector poll: NAD]
views::zip() is exactly as meaningful as
views::adjacent<0>(E) - it's just the edge case.
[2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD.]
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.26.1 [range.adjacent.overview] as indicated:
-2- The name views::adjacent<N> denotes a range adaptor object (26.7.2 [range.adaptor.object]). Given a subexpression E and a constant expression N, the expression views::adjacent<N>(E) is expression-equivalent to:
(2.1) —
((void)E, auto(views::empty<tuple<>>))Iif N is equal to 0, views::adjacent<N>(E) is ill-formed.(2.2) — O
otherwise, adjacent_view<views::all_t<decltype((E))>, N>(E).
Modify 26.7.27.1 [range.adjacent.transform.overview] as indicated:
-2- The name views::adjacent_transform<N> denotes a range adaptor object (26.7.2 [range.adaptor.object]). Given subexpressions E and F and a constant expression N:
(2.1) — If N is equal to 0, views::adjacent_transform<N>(E, F) is ill-formed
expression-equivalent to ((void)E, views::zip_transform(F)), except that the evaluations of E and F are indeterminately sequenced.(2.2) — Otherwise, the expression views::adjacent_transform<N>(E, F) is expression-equivalent to adjacent_transform_view<views::all_t<decltype((E))>, decay_t<decltype((F))>, N>(E, F).