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3391. Problems with counted_iterator/move_iterator::base() const &

Section: 25.5.4 [move.iterators], 25.5.7 [iterators.counted] Status: WP Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2021-02-26

Priority: 2

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Discussion:

It is not possible to use the const & overloads of counted_iterator::base() or move_iterator::base() to get at an underlying move-only iterator in order to compare it against a sentinel.

More concretely, assuming issue LWG 3389 is fixed, this means that

auto v = r | views::take(5);
ranges::begin(v) == ranges::end(v);

is invalid when r is a view whose begin() is a move-only input iterator. The code is invalid because ranges::take_view::sentinel::operator==() must call counted_iterator::base() to compare the underlying iterator against its sentinel, and therefore this operator==() requires that the underlying iterator is copy_constructible.

Suggested resolution:

Make these const & base() overloads return the underlying iterator by const reference. Remove the copy_constructible constraint on these overloads. Perhaps the base() overloads for the iterator wrappers in 26.7 [range.adaptors] could use the same treatment?

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

[2021-01-28; Reflector poll]

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

[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 25.5.4.2 [move.iterator], class template move_iterator synopsis, as indicated:

   namespace std {
     template<class Iterator>
     class move_iterator {
     public:
       using iterator_type = Iterator;
       […]
       constexpr const iterator_type& base() const &;
       constexpr iterator_type base() &&;
       […]
     };
   }
   

2. Modify 25.5.4.5 [move.iter.op.conv] as indicated:

   constexpr const Iterator& base() const &;
   

   -1- Constraints: Iterator satisfies copy_constructible.

   -2- Preconditions: Iterator models copy_constructible.

   -3- Returns: current.

3. Modify 25.5.7.1 [counted.iterator], class template counted_iterator synopsis, as indicated:

   namespace std {
     template<input_or_output_iterator I>
     class counted_iterator {
     public:
       using iterator_type = I;
       […]
       constexpr const I& base() const & requires copy_constructible<I>;
       constexpr I base() &&;
       […]
     };
   }
   

4. Modify 25.5.7.3 [counted.iter.access] as indicated:

   constexpr const I& base() const & requires copy_constructible<I>;
   

   -1- Effects: Equivalent to: return current;