Semantic requirements of sized_range is circular
Tim Song

Created on 2021-06-03.00:00:00 last changed 2 days ago


Date: 2021-06-14.14:09:34

Proposed resolution:

This wording is relative to N4885.

  1. Modify [range.sized] as indicated:

    template<class T>
      concept sized_range =
        range<T> &&
        requires(T& t) { ranges::size(t); };

    -2- Given an lvalue t of type remove_reference_t<T>, T models sized_range only if

    1. (2.1) — ranges::size(t) is amortized 𝒪(1), does not modify t, and is equal to ranges::distance(ranges::begin(t), ranges::end(t)t), and

    2. (2.2) — if iterator_t<T> models forward_iterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t).

Date: 2021-06-15.00:00:00

[ 2021-06-14; Reflector poll ]

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

Date: 2021-06-03.00:00:00

[range.sized] p2.1 requires that for a sized_range t, ranges::size(t) is equal to ranges::distance(t). But for a sized_range, ranges::distance(t) is simply ranges::size(t) cast to the difference type.

Presumably what we meant is that distance(begin, end) — the actual distance you get from traversing the range — is equal to what ranges::size produces, and not merely that casting from the size type to difference type is value-preserving. Otherwise, sized_range would be more or less meaningless.

Date User Action Args
2021-06-14 14:09:34adminsetmessages: + msg11928
2021-06-14 14:09:34adminsetstatus: new -> ready
2021-06-05 20:28:04adminsetmessages: + msg11866
2021-06-03 00:00:00admincreate