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4125.move_iterator's default constructor should be constrained
Section: 24.5.4.2 [move.iterator], 24.5.4.4 [move.iter.cons] Status: New Submitter: Hewill Kang Opened: 2024-07-22 Last modified: 2025-02-07
Priority: 3
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Discussion:
Although it is unclear why P2325 did not apply to move_iterator, there is implementation divergence among the current libraries (demo):
#include <istream> #include <iterator> #include <ranges> using R = std::ranges::istream_view<int>; using I = std::ranges::iterator_t<R>; using MI = std::move_iterator<I>; static_assert(std::default_initializable<MI>); // libstdc++ passes, libc++ fails
As libc++ additionally requires that its default constructors satisfy is_constructible_v<Iterator>.
move_iterator only requires the underlying iterator to be input_iterator which may not be default_initializable.
[2024-08-02; Reflector poll]
Set priority to 3 after reflector poll. Six P0 (tentatively ready) votes but one for NAD: "design change, needs paper. Not constraining this seems to be intended by P2325R3. Even if we want to constrain it, why default_initializable rather than just is_constructible_v?" Author of P2325R3 had no such intent and voted P0. Issue submitter used default_initializable to align with forward_iterator requirements, but would not object to implementers using is_constructible_v for backported DRs.
Proposed resolution:
This wording is relative to N4986.
Modify 24.5.4.2 [move.iterator] as indicated:
namespace std {
template<class Iterator>
class move_iterator {
public:
[…]
constexpr move_iterator() requires default_initializable<Iterator> = default;
[…]
private:
Iterator current = Iterator(); // exposition only
};
}
Modify 24.5.4.4 [move.iter.cons] as indicated:
constexpr move_iterator();-1- Effects: Value-initializes
current.
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