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3849.cartesian_product_view::iterator's default constructor is overconstrained
Section: 25.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2023-01-06 Last modified: 2023-11-22
Priority: Not Prioritized
View all other issues in [range.cartesian.iterator].
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Discussion:
Currently, cartesian_product_view::iterator only provides the default constructor when the first range models forward_range, which seems too restrictive since several input iterators like istream_iterator are still default-constructible.
It would be more appropriate to constrain the default constructor only by whether the underlying iterator satisfies default_initializable, as most other range adaptors do. Since cartesian_product_view::iterator contains a tuple member that already has a constrained default constructor, the proposed resolution simply removes the constraint.
[2023-02-01; Reflector poll]
Set status to Tentatively Ready after nine votes in favour during reflector poll.
[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting → WP.]
Proposed resolution:
This wording is relative to N4917.
Modify [ranges.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { public: […] iterator() requires forward_range<maybe-const<Const, First>> = default; […] private: using Parent = maybe-const<Const, cartesian_product_view>; // exposition only Parent* parent_ = nullptr; // exposition only tuple<iterator_t<maybe-const<Const, First>>, iterator_t<maybe-const<Const, Vs>>...> current_; // exposition only […] }; }
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