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movable-box
as member should use default-initialization instead of copy-initializationSection: 26.6.5.2 [range.repeat.view], 26.7.31.2 [range.chunk.by.view] Status: C++23 Submitter: Hewill Kang Opened: 2022-10-20 Last modified: 2023-11-22
Priority: Not Prioritized
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Discussion:
Currently, the member variable value_
of repeat_view
is initialized with the following expression:
movable-box<W> value_ = W();
which will result in the following unexpected error in libstdc++:
#include <ranges>
int main() {
std::ranges::repeat_view<int> r; // error: could not convert '0' from 'int' to 'std::ranges::__detail::__box<int>'
}
This is because the single-argument constructors of the simple version of movable-box
in libstdc++
are declared as explicit
, which is different from the conditional explicit
declared by the
optional
's constructor, resulting in no visible conversion between those two types.
movable-box
does not provide a single-argument constructor,
so this form of initialization will also produce a hard error.
This may be a bug of existing implementations, but we don't need such "copy-initialization", the default-constructed
movable-box
already value-initializes the underlying type.
[2022-11-01; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify 26.6.5.2 [range.repeat.view] as indicated:
namespace std::ranges { template<move_constructible W, semiregular Bound = unreachable_sentinel_t> requires (is_object_v<W> && same_as<W, remove_cv_t<W>> && (is-integer-like<Bound> || same_as<Bound, unreachable_sentinel_t>)) class repeat_view : public view_interface<repeat_view<W, Bound>> { private: // 26.6.5.3 [range.repeat.iterator], class repeat_view::iterator struct iterator; // exposition only movable-box<W> value_= W(); // exposition only, see 26.7.3 [range.move.wrap] Bound bound_ = Bound(); // exposition only […] }; […] }
Modify 26.7.31.2 [range.chunk.by.view] as indicated:
namespace std::ranges { template<forward_range V, indirect_binary_predicate<iterator_t<V>, iterator_t<V>> Pred> requires view<V> && is_object_v<Pred> class chunk_by_view : public view_interface<chunk_by_view<V, Pred>> { V base_ = V(); // exposition only movable-box<Pred> pred_= Pred(); // exposition only // 26.7.31.3 [range.chunk.by.iter], class chunk_by_view::iterator class iterator; // exposition only […] }; […] }