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chunk_view::size
should preserve the signedness of the size of the underlying rangeSection: 26.7.29.2 [range.chunk.view.input], 26.7.29.6 [range.chunk.view.fwd] Status: NAD Submitter: Hewill Kang Opened: 2022-07-15 Last modified: 2022-11-30
Priority: Not Prioritized
View all other issues in [range.chunk.view.input].
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Discussion:
Currently, the Effects of chunk_view::size
simply returns
to-unsigned-like(div-ceil(ranges::distance(base_), n_))
,
where div-ceil
is defined in 26.7.29.2 [range.chunk.view.input] as:
template<class I> constexpr I div-ceil(I num, I denom) { // exposition only I r = num / denom; if (num % denom) ++r; return r; }
There are two problems here. First, for the const version of chunk_view::size
,
the types of ranges::distance(base_)
and n_
are
range_difference_t<const V>
and
range_difference_t<V>
respectively, and the two parameters of
div-ceil
have the same type I
.
Given that the standard does not guarantee that V
and const V
must have
the same difference_type
, this makes the div-ceil
's template
deduction fail when the two are different.
ranges::size
must return an
unsigned type, but here we use to-unsigned-like
to unconditionally
convert the return type of chunk_view::size
to an unsigned type,
which is inconsistent with the behavior of other range adaptors such as
take_view
and drop_view
.
We should try to preserve the characteristics of the range_size_t
of the underlying
range as much as possible.
[2022-08-23; Reflector poll: NAD (would need a paper for LEWG)]
The "range_difference_t<const V> and range_difference_t<V> can be both valid but have different types" part is something I expect Mr. Carter's eventual paper to outlaw. The "preserve signedness" part is LEWG.
[2022-11-30 LWG telecon. Status changed: Tentatively NAD → NAD.]
Proposed resolution:
This wording is relative to N4910.
Modify 26.7.29.2 [range.chunk.view.input] as indicated:
constexpr auto size() requires sized_range<V>; constexpr auto size() const requires sized_range<const V>;-5- Effects: Equivalent to:
returnto-unsigned-like(div-ceil(ranges::sizedistance(base_), static_cast<decltype(ranges::size(base_))>(n_));
Modify 26.7.29.6 [range.chunk.view.fwd] as indicated:
constexpr auto size() requires sized_range<V>; constexpr auto size() const requires sized_range<const V>;-3- Effects: Equivalent to:
returnto-unsigned-like(div-ceil(ranges::sizedistance(base_), static_cast<decltype(ranges::size(base_))>(n_));