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3564. transform_view::iterator::value_type and iterator_category should use const F&

Section: 25.7.9.3 [range.transform.iterator] Status: C++23 Submitter: Tim Song Opened: 2021-06-06 Last modified: 2023-11-22

Priority: 2

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Discussion:

Iterators obtained from a const transform_view invoke the transformation function as const, but the value_type and iterator_category determination uses plain F&, i.e., non-const.

[2021-06-14; Reflector poll]

Set priority to 2 after reflector poll, send to SG9 for design clarification. Should r and as_const(r) guarantee same elements?

[2022-07-08; Reflector poll]

SG9 has decided to proceed with this PR in its 2021-09-13 telecon and not block the issue for a paper on the more general const/non-const problem.

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

[2022-07-15; LWG telecon: move to Ready]

[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready → WP.]

Proposed resolution:

This wording is relative to N4885.

1. Modify 25.7.9.3 [range.transform.iterator] as indicated:

   namespace std::ranges {
     template
       requires view && is_object_v &&
                regular_invocable> &&
                can-reference>>
     template
     class transform_view::iterator {
     private:
       […]
     public:
       using iterator_concept = see below;
       using iterator_category = see below; // not always present
       using value_type =
         remove_cvref_tmaybe-const F>&, 
         range_reference_t<Base>>>;
       using difference_type = range_difference_t<Base>;  
       […]
     };
   }
   

   -1- […]

   -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_traitsBase>>::iterator_category.

   1. (2.1) — If is_lvalue_reference_vmaybe-const F>&, range_reference_t<Base>>> is true, then

      1. (2.1.1) — […]

      2. (2.1.2) — […]

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