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3394. ranges::basic_istream_view::iterator has an empty iterator_traits

Section: 25.6.6.3 [range.istream.iterator] Status: NAD Submitter: Patrick Palka Opened: 2020-02-09 Last modified: 2020-11-09

Priority: 2

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Discussion:

Every instantiation of ranges::basic_istream_view::iterator has an empty iterator_traits, i.e. the type

iterator_traits<ranges::basic_istream_view<Val, CharT, Traits>::iterator>

has no members.

This happens because basic_istream_view::iterator neither models cpp17-iterator (since this concept requires copyability, which this iterator is by design not) nor does it define all four of the member types difference_type, value_type, reference, and iterator_category (it is missing reference). Therefore by 24.3.2.3 [iterator.traits] p3, this iterator's specialization of iterator_traits will be empty if generated from the iterator_traits primary template.

Since basic_istream_view::iterator is indeed an iterator, its iterator_traits should certainly not be empty. The simplest solution here is to define the member type reference in the definition of basic_istream_view::iterator, which will enable its iterator_traits specialization to be appropriately populated from the primary template.

[2020-02-10; Jonathan comments]

Jonathan and Casey have concerns about the proposed resolution. Casey: The wording makes it look as if this iterator is supposed to be an cpp17-input-iterator.

See also LWG 3283⁽ⁱ⁾ and 3289⁽ⁱ⁾.

[2020-02 Prioritized as P2 Monday morning in Prague]

This wording is relative to N4849.

1. Modify 25.6.6.3 [range.istream.iterator], class template basic_istream_view::iterator synopsis, as indicated:

   namespace std::ranges {
     template<class Val, class CharT, class Traits>
     class basic_istream_view<Val, CharT, Traits>::iterator { // exposition only
     public:
       using iterator_category = input_iterator_tag;
       using difference_type = ptrdiff_t;
       using value_type = Val;
       using reference = Val&;
   
       iterator() = default;
       […]
     };
   }
   

[2020-02-13, Prague]

LWG decided for NAD: The ranges::basic_istream_view::iterator is a move-only type and thus cannot meet the Cpp17 requirements (even output_iterator_tag), as such it should not specialize iterator_traits, to avoid misleading results when it is passed to new algorithms.

A related issue, LWG 3397⁽ⁱ⁾, is supposed to take care for a problem with the definition of the iterator_category member type of this template.

[2020-11-09 Status changed: Tentatively NAD → NAD.]

Proposed resolution: