Issue 3798: Rvalue reference and iterator_category (original) (raw)

This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++23 status.

3798. Rvalue reference and iterator_category

Section: 24.3.2.3 [iterator.traits] Status: C++23 Submitter: Jiang An Opened: 2022-10-22 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.traits].

View all issues with C++23 status.

Discussion:

Since C++11, a forward iterator may have an rvalue reference as its reference type. I think this is an intentional change made by N3066. However, several components added (or changed) in C++20/23 recognize an iterator as a Cpp17ForwardIterator (by using forward_iterator_tag or a stronger iterator category tag type as the iterator_categorytype) only if the reference type would be an lvalue reference type, which seems a regression.

[2022-11-01; Reflector poll]

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

[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4917.

1. Modify 24.3.2.3 [iterator.traits] as indicated:

   -2- The definitions in this subclause make use of the following exposition-only concepts:

   […]
   template
   concept cpp17-forward-iterator =
   cpp17-input-iterator && constructible_from &&
   is_lvalue_reference_v<iter_reference_t> &&
   same_as<remove_cvref_t<iter_reference_t>,
   typename indirectly_readable_traits::value_type> &&

requires(I i) {
{ i++ } -> convertible_to<const I&>;
{ *i++ } -> same_as<iter_reference_t>;
};
[…] 2. Modify 25.7.9.3 [range.transform.iterator] as indicated:
-2- The member typedef-name iterator_category is defined if and only if _Base_models forward_range. In that case, _iterator_::iterator_category is defined as follows: Let C denote the type iterator_traits<iterator_t<_Base_>>::iterator_category.

1. (2.1) — If is_~~lvalue_~~reference_v<invoke_result_t<_maybe-const_<Const, F>&, range_reference_t<_Base_>>> is true, then

   1. (2.1.1) — if C models derived_from<contiguous_iterator_tag>, iterator_category denotes random_access_iterator_tag;
   2. (2.1.2) — otherwise, iterator_category denotes C.

2. (2.2) — Otherwise, iterator_category denotes input_iterator_tag.

3. Modify 25.7.15.3 [range.join.with.iterator] as indicated:
   -2- The member typedef-name iterator_category is defined if and only if _ref-is-glvalue_is true, and _Base_ and _InnerBase_ each model forward_range. In that case, _iterator_::iterator_category is defined as follows:

4. (2.1) — […]

5. (2.2) — If

   is_lvalue_reference_v<common_reference_t<iter_reference_t<_InnerIter_>,
   iter_reference_t<_PatternIter_>>>

is false, iterator_category denotes input_iterator_tag.
3. (2.3) — […] 4. Modify 25.7.26.3 [range.zip.transform.iterator] as indicated:
-1- The member typedef-name _iterator_::iterator_category is defined if and only if _Base_ models forward_range. In that case, _iterator_::iterator_categoryis defined as follows:

1. (1.1) — If

   invoke_result_t<_maybe-const_<Const, F>&, range_reference_t<_maybe-const_<Const, Views>>...>
   is not an lvalue reference, iterator_category denotes input_iterator_tag.

2. (1.2) — […]

3. Modify 25.7.28.3 [range.adjacent.transform.iterator] as indicated:
   -1- The member typedef-name _iterator_::iterator_category is defined as follows:

4. (1.1) — If invoke_result_t<_maybe-const_<Const, F>&, _REPEAT_(range_reference_t<_Base_>, N)...>is not an lvalue reference, iterator_category denotes input_iterator_tag.

5. (1.2) — […]