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4242. ranges::distance does not work with volatile iterators

Section: 24.4.4.3 [range.iter.op.distance] Status: New Submitter: Hewill Kang Opened: 2025-04-12 Last modified: 2025-04-13

Priority: Not Prioritized

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Discussion:

After LWG 3664⁽ⁱ⁾, ranges::distance computes the distance between last and first by returning last - static_cast<const decay_t<I>&>(first) when the two are subtractable. However, this will cause a hard error if first is volatile-qualified (demo):

#include <iterator>

int main() {
  int arr[] = {1, 2, 3};
  int* volatile ptr = arr;
  // return std::distance(ptr, arr + 3);      // this is ok
  return std::ranges::distance(ptr, arr + 3); // hard error
}

The resolution changes the Effects of LWG 3664⁽ⁱ⁾ from "cute" to "noncute".

Proposed resolution:

This wording is relative to N5008.

1. Modify 24.4.4.3 [range.iter.op.distance] as indicated:

   template<class I, sized_sentinel_for<decay_t<I>> S>
     constexpr iter_difference_t<decay_t<I>> ranges::distance(I&& first, S last);
   

   -3- Effects: Equivalent to:

   if constexpr (!is_array_v<remove_reference_t<I>>)
     return last - first;
   else
     return last - static_cast<const decay_t<I>&>(first);