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views::common
may not be a range adaptor objectSection: 25.5.5.1 [common.iterator], 25.5.5.6 [common.iter.cmp] Status: New Submitter: Hewill Kang Opened: 2022-09-18 Last modified: 2022-10-12
Priority: 3
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Discussion:
P2325R3 makes input_or_output_iterator
no longer require default_initializable
,
which means that the template parameter I
of common_iterator
no longer requires
default_initializable
.
common_iterator
itself cannot be default-constructed, it can never be a valid
sentinel even if it can be compared to itself. Furthermore, this also makes views::common
return a
non-range
even if it is well-formed (online example):
#include <ranges>
#include <vector>
int main() {
std::vector<int> v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto cr = r | std::views::common;
static_assert(std::ranges::range<decltype(cr)>); // failed
}
which causes views::common
to be unable to convert a range
into a view
,
making it not a valid range adaptor.
common_iterator
should always be default_initializable
,
which makes it eligible to be a legitimate sentinel.
The proposed resolution provides a default constructor for common_iterator
when I
is
not default_initializable
, in which case constructs the variant
with an alternative type
of S
.
[2022-09-28; Reflector poll]
Set priority to 3 after reflector poll.
"The P/R means that sometimes the variant containers an iterator and sometimes contains a sentinel, depending on whether the iterator is default constructible. Always constructing a sentinel would be more consistent."
Proposed resolution:
This wording is relative to N4917.
Modify 25.5.5.1 [common.iterator], class template common_iterator
synopsis, as indicated:
namespace std { template<input_or_output_iterator I, sentinel_for<I> S> requires (!same_as<I, S> && copyable<I>) class common_iterator { public: constexpr common_iterator() requires default_initializable<I> = default; constexpr common_iterator(); […] }; […] }
Modify 25.5.5.3 [common.iter.const] as indicated:
constexpr common_iterator();-?- Effects: Initializes
v_
as if byv_{in_place_type<S>}
.constexpr common_iterator(I i);-1- Effects: Initializes
v_
as if byv_{in_place_type<I>, std::move(i)}
.