Created on 2023-08-23.00:00:00 last changed 8 months ago
Proposed resolution:
This wording is relative to N4958.
Modify [range.utility.conv.to] as indicated:
template<class C, input_range R, class... Args> requires (!view<C>) constexpr C to(R&& r, Args&&... args);-1- Mandates: C is a cv-unqualified class type.
-2- Returns: An object of type C constructed from the elements of r in the following manner:
(2.1) — If C does not satisfy input_range or convertible_to<range_reference_t<R>, range_value_t<C>> is true:
[…]
(2.2) — Otherwise, if input_range<range_reference_t<R>> is true:
to<C>(ref_view(r) | views::transform([](auto&& elem) { return to<range_value_t<C>>(std::forward<decltype(elem)>(elem)); }), std::forward<Args>(args)...);(2.3) — Otherwise, the program is ill-formed.
[ Tokyo 2024-03-23; Status changed: Voting → WP. ]
[ Kona 2023-11-07; move to Ready ]
[ 2023-11-03; Reflector poll ]
Set priority to 3 after reflector poll.
"Should be std::forward<R>(r)
instead?"
When r is a nested range, ranges::to constructs the object recursively through r | views::transform(...).
However, the expression is ill-formed when the type of lvalue r does not model viewable_range (demo):
#include <ranges>
#include <vector>
#include <list>
int main() {
std::vector<std::vector<int>> v;
auto r = std::views::all(std::move(v));
auto l = std::ranges::to<std::list<std::list<int>>>(r); // hard error in MSVC-STL and libc++
}
History | |||
---|---|---|---|
Date | User | Action | Args |
2024-04-02 10:29:12 | admin | set | messages: + msg14034 |
2024-04-02 10:29:12 | admin | set | status: voting -> wp |
2024-03-18 09:32:04 | admin | set | status: ready -> voting |
2023-11-07 22:39:32 | admin | set | messages: + msg13829 |
2023-11-07 22:39:32 | admin | set | status: new -> ready |
2023-11-03 18:08:28 | admin | set | messages: + msg13807 |
2023-09-24 10:08:01 | admin | set | messages: + msg13736 |
2023-08-23 00:00:00 | admin | create |