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3702. Should zip_transform_view::iterator remove operator<?

Section: 25.7.26.3 [range.zip.transform.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-05-21 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 status.

Discussion:

After LWG 3692⁽ⁱ⁾, zip_view::iterator only provides operator<=>. Since the comparison of zip_transform_view::iterator uses zip_view::iterator's operator<=>, it is possible to remove zip_transform_view::iterator's operator<, >, <=, >= and just detect if ziperator's operator<=> is available.

Since the ziperator's operator<=> is valid only when zip_view is a random_access_range, we don't need to additionally constrain the ziperator to be three_way_comparable.

[2022-06-21; Reflector poll]

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

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:

   namespace std::ranges {
   […]
   template<copy_constructible F, input_range... Views>
   requires (view<Views> && ...) && (sizeof...(Views) > 0) && is_object_v<F> &&
            regular_invocable<F&, range_reference_t<Views>...> &&
            can-reference<invoke_result_t<F&, range_reference_t<Views>...>>
   template<bool Const>
   class zip_transform_view<F, Views...>::iterator {
     using Parent = maybe-const<Const, zip_transform_view>;        // exposition only
     using Base = maybe-const<Const, InnerView>;                   // exposition only
     Parent* parent_ = nullptr;                                    // exposition only
     ziperator<Const> inner_;                                      // exposition only
   
     constexpr iterator(Parent& parent, ziperator<Const> inner);   // exposition only
   public:
     […]
     friend constexpr bool operator==(const iterator& x, const iterator& y)
       requires equality_comparable<ziperator<Const>>;
     
     friend constexpr bool operator<(const iterator& x, const iterator& y)
       requires random_access_range<Base>;
     friend constexpr bool operator>(const iterator& x, const iterator& y)
       requires random_access_range<Base>;
     friend constexpr bool operator<=(const iterator& x, const iterator& y)
       requires random_access_range<Base>;
     friend constexpr bool operator>=(const iterator& x, const iterator& y)
       requires random_access_range<Base>;
     friend constexpr auto operator<=>(const iterator& x, const iterator& y)
       requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>;
     […]
   };
   […]
   }
   

   […]

   friend constexpr bool operator==(const iterator& x, const iterator& y)
     requires equality_comparable<ziperator<Const>>;
   friend constexpr bool operator<(const iterator& x, const iterator& y)
     requires random_access_range<Base>;
   friend constexpr bool operator>(const iterator& x, const iterator& y)
     requires random_access_range<Base>;
   friend constexpr bool operator<=(const iterator& x, const iterator& y)
     requires random_access_range<Base>;
   friend constexpr bool operator>=(const iterator& x, const iterator& y)
     requires random_access_range<Base>;
   friend constexpr auto operator<=>(const iterator& x, const iterator& y)
     requires random_access_range<Base>&& three_way_comparable<ziperator<Const>>;
   

   -14- Let op be the operator.

   -15- Effects: Equivalent to: return x.inner_ op y.inner_;