Issue 3391: Problems with counted_iterator/move_iterator::base() const & (original) (raw)

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

Section: 24.5.4 [move.iterators], 24.5.7 [iterators.counted] Status: C++23 Submitter: Patrick Palka Opened: 2020-02-07 Last modified: 2023-11-22

Priority: 2

View all other issues in [move.iterators].

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Discussion:

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

More concretely, assuming issue LWG 3389(i) 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 25.7 [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 24.5.4.2 [move.iterator], class template move_iterator synopsis, as indicated:

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

2. Modify 24.5.4.5 [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 24.5.7.1 [counted.iterator], class template counted_iterator synopsis, as indicated:

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

4. Modify 24.5.7.3 [counted.iter.access] as indicated:

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

   -1- Effects: Equivalent to: return current;