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4577. Hardening view_interface::operator[]

Section: 25.5.3 [view.interface] Status: Open Submitter: Hewill Kang Opened: 2026-05-11 Last modified: 2026-06-04

Priority: Not Prioritized

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Discussion:

Hardening view_interface::operator[] to prevent out-of-bounds access is worthwhile for the sized_range cases, and we have already done so for other standard views such as span and string_view's operator[].

[2026-05-29 LWG telecon; status: New → Ready]

[2026-06-03; status: Ready → Open]

Reopen to bump the feature test macro.

[2026-06-03; Jonathan provides new wording]

Proposed resolution:

This wording is relative to N5046.

1. Modify 17.3.2 [version.syn], Header <version> synopsis, as indicated:

   #define __cpp_lib_hardened_vector 202502L // also in <vector>
   #define __cpp_lib_hardened_view_interface 202506YYYYMML // also in <ranges>
   

2. Modify 25.5.3 [view.interface], class template view_interface synopsis, as indicated:

   namespace std::ranges {
     template<class D>
       requires is_class_v<D> && same_as<D, remove_cv_t<D>>
     class view_interface {
       […]
     public:
       […]
       template<random_access_range R = D>
         constexpr decltype(auto) operator[](range_difference_t<R> n) {
           return ranges::begin(derived())[n];
         };
       template<random_access_range R = const D>
         constexpr decltype(auto) operator[](range_difference_t<R> n) const {
           return ranges::begin(derived())[n];
         };
     };
   }
   

3. Modify 25.5.3.2 [view.interface.members] as indicated:

   constexpr decltype(auto) back() requires bidirectional_range<D> && common_range<D>;
   constexpr decltype(auto) back() const
     requires bidirectional_range<const D> && common_range<const D>;
   

   -3- Hardened preconditions: !empty() is true.

   -4- Effects: Equivalent to: return *ranges::prev(ranges::end(derived()));

   
   template<random_access_range R = D>
     constexpr decltype(auto) operator[](range_difference_t<R> n);
   template<random_access_range R = const D>
     constexpr decltype(auto) operator[](range_difference_t<R> n) const;
   

   -?- Hardened preconditions: If D models sized_range, then n >= 0 && n < ranges::distance(derived()) is true.

   -?- Effects: Equivalent to: return ranges::begin(derived())[n];