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3725. reverse_iterator::operator-> should not use prev for non-pointer iterators

Section: 24.5.1.6 [reverse.iter.elem] Status: New Submitter: Hewill Kang Opened: 2022-06-26 Last modified: 2024-06-18

Priority: 3

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Discussion:

When the underlying iterator is not a pointer type, reverse_iterator::operator-> returns prev(current).operator->(). However, prev only works with Cpp17BidirectionalIterator, given that C++20 bidirectional_iterator may just be Cpp17InputIterator, we shouldn't use prev here.

[2022-07-08; Reflector poll]

Set priority to 3 after reflector poll. Suggested to use ranges::prev instead.

[2024-06-18; Jonathan adds a comment]

It's not clear that std::prev requires Cpp17InputIterator, that's the subject of LWG 3197⁽ⁱ⁾.

Proposed resolution:

This wording is relative to N4910.

1. Modify 24.5.1.6 [reverse.iter.elem] as indicated:

   constexpr pointer operator->() const
     requires (is_pointer_v<Iterator> ||
               requires(const Iterator i) { i.operator->(); });
   

   -2- Effects:

   1. (2.1) — If Iterator is a pointer type, equivalent to: return prev(current);

   2. (2.2) — Otherwise, equivalent to: return prev(current).operator->();

      Iterator tmp = current;
      --tmp;
      return tmp.operator->();