4035. single_view should provide empty

Section: 25.6.3.2 [range.single.view] Status: WP Submitter: Hewill Kang Opened: 2023-12-31 Last modified: 2024-04-02

Priority: Not Prioritized

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Discussion:

Although single_view::empty can be synthesized through view_interface, it seems more worthwhile to provide a static empty for it which eliminates the need to pass in an object parameter, guarantees noexcept-ness, and is consistent with the design of empty_view (demo):

#include <ranges>
auto empty = std::views::empty<int>;
static_assert(noexcept(empty.empty()));
static_assert(noexcept(empty.size()));
auto single = std::views::single(0);
static_assert(noexcept(single.empty())); // fire
static_assert(noexcept(single.size()));

[2024-03-12; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[Tokyo 2024-03-23; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4971.

1. Modify 25.6.3.2 [range.single.view] as indicated:

   namespace std::ranges {
     template<move_constructible T>
       requires is_object_v<T>
     class single_view : public view_interface<single_view<T>> {
       […]
     public:
       […]
       constexpr T* begin() noexcept;
       constexpr const T* begin() const noexcept;
       constexpr T* end() noexcept;
       constexpr const T* end() const noexcept;
       static constexpr bool empty() noexcept;
       static constexpr size_t size() noexcept;
       constexpr T* data() noexcept;
       constexpr const T* data() const noexcept;
     };
     […]
   }
   

   […]

   constexpr T* end() noexcept;
   constexpr const T* end() const noexcept;
   

   -5- Effects: Equivalent to: return data() + 1;

   static constexpr bool empty() noexcept;
   

   -?- Effects: Equivalent to: return false;