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

Section: 26.7.7.3 [range.transform.iterator] Status: SG9 Submitter: Tim Song Opened: 2021-06-06 Last modified: 2021-06-14

Priority: 2

View all other issues in [range.transform.iterator].

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?

Proposed resolution:

This wording is relative to N4885.

  1. Modify 26.7.7.3 [range.transform.iterator] as indicated:

    namespace std::ranges {
  template<input_range V, copy_constructible F>
    requires view<V> && is_object_v<F> &&
             regular_invocable<F&, range_reference_t<V>> &&
             can-reference<invoke_result_t<F&, range_reference_t<V>>>
  template<bool Const>
  class transform_view<V, F>::iterator {
  private:
    […]
  public:
    using iterator_concept = see below;
    using iterator_category = see below; // not always present
    using value_type =
      remove_cvref_t<invoke_result_t<maybe-const<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_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) — […]

      2. (2.1.2) — […]

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