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4245. Operators that interact with counted_iterator and default_sentinel_t should be noexcept

Section: 24.5.7.1 [counted.iterator], 24.5.7.5 [counted.iter.nav], 24.5.7.6 [counted.iter.cmp] Status: WP Submitter: Hewill Kang Opened: 2025-04-18 Last modified: 2025-06-23

Priority: Not Prioritized

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Discussion:

counted_iterator can be compared or subtracted from default_sentinel_t, which only involves simple integer arithmetic and does not have any Preconditions.

In this case, it is reasonable to declare them as noexcept.

[2025-06-12; Reflector poll]

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

[Sofia 2025-06-21; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.5.7.1 [counted.iterator] as indicated:

   namespace std {
     template<input_or_output_iterator I>
     class counted_iterator {
     public:
       […]
       friend constexpr iter_difference_t<I> operator-(
         const counted_iterator& x, default_sentinel_t) noexcept;
       friend constexpr iter_difference_t<I> operator-(
         default_sentinel_t, const counted_iterator& y) noexcept;
       […]
       friend constexpr bool operator==(
         const counted_iterator& x, default_sentinel_t) noexcept;
       […]
     };
     […]
   }
   

2. Modify 24.5.7.5 [counted.iter.nav] as indicated:

   friend constexpr iter_difference_t<I> operator-(
     const counted_iterator& x, default_sentinel_t) noexcept;
   

   -15- Effects: Equivalent to: return -x.length;

   friend constexpr iter_difference_t<I> operator-(
     default_sentinel_t, const counted_iterator& y) noexcept;
   

   -16- Effects: Equivalent to: return y.length;

3. Modify 24.5.7.6 [counted.iter.cmp] as indicated:

   friend constexpr bool operator==(
     const counted_iterator& x, default_sentinel_t) noexcept;
   

   -3- Effects: Equivalent to: return x.length == 0;