Issue 4242: ranges::distance does not work with volatile iterators (original) (raw)

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

Section: 24.4.4.3 [range.iter.op.distance] Status: WP Submitter: Hewill Kang Opened: 2025-04-12 Last modified: 2025-06-23

Priority: Not Prioritized

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Discussion:

After LWG 3664(i), ranges::distance computes the distance between last and firstby 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

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".

[2025-06-12; Reflector poll]

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

"Would prefer auto(first) but the current wording is correct."

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

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

   -3- Effects: Equivalent to:

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