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4558. drop_view and drop_while_view should be const-iterable for input-only range

Section: 25.7.12.2 [range.drop.view], 25.7.13.2 [range.drop.while.view] Status: New Submitter: Hewill Kang Opened: 2026-03-25 Last modified: 2026-03-26

Priority: Not Prioritized

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Discussion:

After P3725R3, filter_view can be const-iterable as long as the underlying range is input-only range, implying that LEWG do believe that providing const-iterable properties for views does have some value.

Its turn out that drop_view and drop_while_view are also const-iterable in the same situation, because they do not internally need to cache the begin call on the first iteration.

Proposed resolution:

This wording is relative to N5032.

1. Modify 25.7.12.2 [range.drop.view] as indicated:

   namespace std::ranges {
     template<view V>
     class drop_view : public view_interface<drop_view<V>> {
     public:
       […]
       constexpr auto begin()
         requires (!(simple-view<V> &&
                     ((random_access_range<const V> && sized_range<const V>) ||
                      (input_range<const V> && !forward_range<const V>))));
       constexpr auto begin() const
         requires (random_access_range<const V> && sized_range<const V>) ||
                  (input_range<const V> && !forward_range<const V>);
       […]
     };
     […]
   }
   

   constexpr auto begin()
      requires (!(simple-view<V> &&
                  ((random_access_range<const V> && sized_range<const V>) ||
                   (input_range<const V> && !forward_range<const V>))));
   constexpr auto begin() const
     requires (random_access_range<const V> && sized_range<const V>) ||
              (input_range<const V> && !forward_range<const V>);
   

   -3- Returns: ranges::next(ranges::begin(base_), count_, ranges::end(base_)).

   -4- Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_view models forward_range, the first overload caches the result within the drop_view for use on subsequent calls.

2. Modify 25.7.13.2 [range.drop.while.view] as indicated:

   namespace std::ranges {
     template<view V, class Pred>
       requires input_range<V> && is_object_v<Pred> &&
                indirect_unary_predicate<const Pred, iterator_t<V>>
     class drop_while_view : public view_interface<drop_while_view<V, Pred>> {
     public:
       […]
   
       constexpr auto begin()
         requires (!(simple-view<V> &&
                     input_range<const V> && !forward_range<const V>));
       constexpr auto begin() const
         requires (input_range<const V> && !forward_range<const V> &&
                   indirect_unary_predicate<const Pred, iterator_t<const V>>);
   
       constexpr auto end() requires (!simple-view<V>) { return ranges::end(base_); }
       constexpr auto end() const requires range<const V> { return ranges::end(base_); }
   
       […]
     };
     […]
   }
   

   […]

   constexpr auto begin()
     requires (!(simple-view<V> &&
                 input_range<const V> && !forward_range<const V>));
   constexpr auto begin() const
     requires (input_range<const V> && !forward_range<const V> &&
               indirect_unary_predicate<const Pred, iterator_t<const V>>);
   
   

   -3-Preconditions: pred_.has_value() is true.

   -4-Returns: ranges::find_if_not(base_, cref(*pred_)).

   -5-Remarks: In order to provide the amortized constant-time complexity required by the range concept when drop_while_view models forward_range, the first overloadcall caches the result within the drop_while_view for use on subsequent calls.