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4001.iota_view should provide empty
Section: 25.6.4.2 [range.iota.view] Status: WP Submitter: Hewill Kang Opened: 2023-10-27 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.iota.view].
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Discussion:
When iota_view's template parameter is not an integer type and does not model advanceable, its size member will not be provided as constraints are not satisfied.
incrementable, this results in its view_interface base being unable to synthesize a valid empty member as iota_view will just be an input_range (demo):
#include <ranges>
#include <vector>
#include <iostream>
int main() {
std::vector<int> v;
auto it = std::back_inserter(v);
auto s = std::ranges::subrange(it, std::unreachable_sentinel);
auto r = std::views::iota(it);
std::cout << s.empty() << "\n"; // 0
std::cout << r.empty() << "\n"; // ill-formed
}
This seems to be an oversight. I don't see a reason why iota_view doesn't provide empty as it does store the start and end like subrange, in which case it's easy to tell if it's empty just by comparing the two.
[2023-11-02; Reflector poll]
Set status to Tentatively Ready after seven votes in favour during reflector poll.
[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4964.
Modify 25.6.4.2 [range.iota.view], class template iota_view synopsis, as indicated:
namespace std::ranges { […] template<weakly_incrementable W, semiregular Bound = unreachable_sentinel_t> requires weakly-equality-comparable-with<W, Bound> && copyable<W> class iota_view : public view_interface<iota_view<W, Bound>> { private: […] W value_ = W(); // exposition only Bound bound_ = Bound(); // exposition only public: […] constexpr iterator begin() const; constexpr auto end() const; constexpr iterator end() const requires same_as<W, Bound>; constexpr bool empty() const; constexpr auto size() const requires see below; }; […] }[…]
constexpr iterator end() const requires same_as<W, Bound>;-14- Effects: Equivalent to:
return iterator{bound_};constexpr bool empty() const;-?- Effects: Equivalent to:
[…]return value_ == bound_;
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