Issue 3607: contiguous_iterator should not be allowed to have custom iter_move and iter_swap behavior (original) (raw)

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3607. contiguous_iterator should not be allowed to have custom iter_move and iter_swap behavior

Section: 24.3.4.14 [iterator.concept.contiguous] Status: C++23 Submitter: Tim Song Opened: 2021-09-28 Last modified: 2023-11-22

Priority: Not Prioritized

View all other issues in [iterator.concept.contiguous].

View all issues with C++23 status.

Discussion:

The iter_move and iter_swap customization points were introduced primarily for proxy iterators. Whatever their application to non-proxy iterators in general, they should not be allowed to have custom behavior for contiguous iterators — this new iterator category was introduced in large part to permit better optimizations, and allowing custom iter_move/iter_swap prevents such optimizations for a wide variety of algorithms that are specified to call them.

[2021-10-14; Reflector poll]

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

[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready → WP.]

Proposed resolution:

This wording is relative to N4892.

  1. Modify 24.3.4.14 [iterator.concept.contiguous] as indicated:

    template
    concept contiguous_iterator =
    random_access_iterator &&
    derived_from<_ITERCONCEPT_(I), contiguous_iterator_tag> &&
    is_lvalue_reference_v<iter_reference_t> &&
    same_as<iter_value_t, remove_cvref_t<iter_reference_t>> &&
    requires(const I& i) {
    { to_address(i) } -> same_as<add_pointer_t<iter_reference_t>>;

};

-2- Let a and b be dereferenceable iterators and c be a non-dereferenceable iterator of type I such that b is reachable from a and c is reachable from b, and let D be iter_difference_t<I>. The type Imodels contiguous_iterator only if

  1. (2.1) — to_address(a) == addressof(*a),
  2. (2.2) — to_address(b) == to_address(a) + D(b - a), and
  3. (2.3) — to_address(c) == to_address(a) + D(c - a).,
  4. (2.?) — ranges::iter_move(a) has the same type, value category, and effects as std::move(*a), and
  5. (2.?) — if ranges::iter_swap(a, b) is well-formed, it has effects equivalent to ranges::swap(*a, *b).