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Section: 25.3.4.14 [iterator.concept.contiguous] Status: WP Submitter: Tim Song Opened: 2021-09-28 Last modified: 2022-02-10
Priority: Not Prioritized
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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.
Modify 25.3.4.14 [iterator.concept.contiguous] as indicated:
template<class I> concept contiguous_iterator = random_access_iterator<I> && derived_from<ITER_CONCEPT(I), contiguous_iterator_tag> && is_lvalue_reference_v<iter_reference_t<I>> && same_as<iter_value_t<I>, remove_cvref_t<iter_reference_t<I>>> && requires(const I& i) { { to_address(i) } -> same_as<add_pointer_t<iter_reference_t<I>>>; };-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 I models contiguous_iterator only if
(2.1) — to_address(a) == addressof(*a),
(2.2) — to_address(b) == to_address(a) + D(b - a),
and(2.3) — to_address(c) == to_address(a) + D(c - a)
.,(2.?) — ranges::iter_move(a) has the same type, value category, and effects as std::move(*a), and
(2.?) — if ranges::iter_swap(a, b) is well-formed, it has effects equivalent to ranges::swap(*a, *b).