3717. common_view::end should improve random_access_range case

Section: 25.7.20.2 [range.common.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-06-15 Last modified: 2023-11-22

Priority: 3

View all other issues in [range.common.view].

View all issues with C++23 status.

Discussion:

This issue is part of NB comment US 47-109 26 [ranges] Resolve open issues

In view of the fact that random access iterators are only required to work with its difference type, P2393R1 improves the wording of take_view, which first convert the integer type to difference type and then operate with the iterator. However, the paper omits the handling of random access iterators in common_view::end, which directly operates on the return types of ranges::begin and ranges::size. We should improve this, too.

[2022-07-06; Reflector poll]

Set priority to 3 after reflector poll.

Previous resolution [SUPERSEDED]:

This wording is relative to N4910.

1. Modify 25.7.20.2 [range.common.view] as indicated:

   namespace std::ranges {
     template<view V>
       requires (!common_range<V> && copyable<iterator_t<V>>)
     class common_view : public view_interface<common_view<V>> {
     private:
       V base_ = V();  // exposition only
     public:
       […]
       constexpr auto begin() {
         if constexpr (random_access_range<V> && sized_range<V>)
           return ranges::begin(base_);
         else
           return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
       }
   
       constexpr auto begin() const requires range<const V> {
         if constexpr (random_access_range<const V> && sized_range<const V>)
           return ranges::begin(base_);
         else
           return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
       }
       
       constexpr auto end() {
         if constexpr (random_access_range<V> && sized_range<V>)
           return ranges::begin(base_) + range_difference_t<V>(size())ranges::size(base_);
         else
           return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
       }
   
       constexpr auto end() const requires range<const V> {
         if constexpr (random_access_range<const V> && sized_range<const V>)
           return ranges::begin(base_) + range_difference_t<const V>(size())ranges::size(base_);
         else
           return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
       }
   
       constexpr auto size() requires sized_range<V> {
         return ranges::size(base_);
       }
       constexpr auto size() const requires sized_range<const V> {
         return ranges::size(base_);
       }
     };
     […]
   }
   

[Kona 2022-11-08; Discussed at joint LWG/SG9 session. Move to Open]

[2022-11-09 Tim updates wording per LWG discussion]

[Kona 2022-11-10; Move to Immediate]

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Immediate → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 25.7.20.2 [range.common.view] as indicated:

   namespace std::ranges {
     template<view V>
       requires (!common_range<V> && copyable<iterator_t<V>>)
     class common_view : public view_interface<common_view<V>> {
     private:
       V base_ = V();  // exposition only
     public:
       […]
       constexpr auto begin() {
         if constexpr (random_access_range<V> && sized_range<V>)
           return ranges::begin(base_);
         else
           return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::begin(base_));
       }
   
       constexpr auto begin() const requires range<const V> {
         if constexpr (random_access_range<const V> && sized_range<const V>)
           return ranges::begin(base_);
         else
           return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::begin(base_));
       }
       
       constexpr auto end() {
         if constexpr (random_access_range<V> && sized_range<V>)
           return ranges::begin(base_) + ranges::sizedistance(base_);
         else
           return common_iterator<iterator_t<V>, sentinel_t<V>>(ranges::end(base_));
       }
   
       constexpr auto end() const requires range<const V> {
         if constexpr (random_access_range<const V> && sized_range<const V>)
           return ranges::begin(base_) + ranges::sizedistance(base_);
         else
           return common_iterator<iterator_t<const V>, sentinel_t<const V>>(ranges::end(base_));
       }
   
       constexpr auto size() requires sized_range<V> {
         return ranges::size(base_);
       }
       constexpr auto size() const requires sized_range<const V> {
         return ranges::size(base_);
       }
     };
     […]
   }