zip_view and adjacent_view are underconstrainedSection: 25.7.25.2 [range.zip.view], 25.7.27.2 [range.adjacent.view], 25.7.33.2 [range.cartesian.view] Status: New Submitter: Hewill Kang Opened: 2022-07-04 Last modified: 2023-08-12
Priority: 3
View all other issues in [range.zip.view].
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Discussion:
Both zip_view::iterator's (25.7.25.3 [range.zip.iterator]) and
adjacent_view::iterator's (25.7.27.3 [range.adjacent.iterator])
operator* have similar Effects: elements:
return tuple-transform([](auto& i) -> decltype(auto) { return *i; }, current_);
where tuple-transform is defined as:
template<class F, class Tuple>
constexpr auto tuple-transform(F&& f, Tuple&& tuple) { // exposition only
return apply([&]<class... Ts>(Ts&&... elements) {
return tuple-or-pair<invoke_result_t<F&, Ts>...>(
invoke(f, std::forward<Ts>(elements))...
);
}, std::forward<Tuple>(tuple));
}
That is, zip_view::iterator will invoke the operator* of each iterator of
Views and return a tuple containing its reference.
reference of iterators is actually the reference type. However,
when the operator* returns a prvalue of non-movable type, tuple-transform will
be ill-formed since there are no suitable constructors for tuple:
#include <ranges>
struct NonMovable {
NonMovable() = default;
NonMovable(NonMovable&&) = delete;
};
auto r = std::views::iota(0, 5)
| std::views::transform([](int) { return NonMovable{}; });
auto z = std::views::zip(r);
auto f = *z.begin(); // hard error
We should constrain the range_reference_t of the underlying range to be move_constructible
when it is not a reference type, which also solves similar issues in zip_view::iterator and
adjacent_view::iterator's operator[] and iter_move.
[2022-08-23; Reflector poll]
Set priority to 3 after reflector poll.
"The constraint should just be move_constructible."
Previous resolution [SUPERSEDED]:
This wording is relative to N4910.
Modify 25.2 [ranges.syn], header
<ranges>synopsis, as indicated:namespace std::ranges { […] // 25.7.25 [range.zip], zip view template<class Ref> concept tuple-constructible-reference = see below; // exposition only template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (tuple-constructible-reference<range_reference_t<Views>> && ...) class zip_view; […] // 25.7.27 [range.adjacent], adjacent view template<forward_range V, size_t N> requires view<V> && (N > 0) && tuple-constructible-reference<range_reference_t<V>> class adjacent_view; } […]Modify 25.7.25.2 [range.zip.view] as indicated:
namespace std::ranges { template<class Ref> concept tuple-constructible-reference = // exposition only is_reference_v<Ref> || move_constructible<Ref>; […] template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (tuple-constructible-reference<range_reference_t<Views>> && ...) class zip_view : public view_interface<zip_view<Views...>> { […] }; […] }Modify 25.7.27.2 [range.adjacent.view] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) && tuple-constructible-reference<range_reference_t<V>> class adjacent_view : public view_interface<adjacent_view<V, N>> { […] }; […] }
[2022-09-25; Hewill provides improved wording]
Previous resolution [SUPERSEDED]:
This wording is relative to N4917.
Modify 25.2 [ranges.syn], header
<ranges>synopsis, as indicated:namespace std::ranges { […] template<class R> concept has-tuplable-ref = // exposition only move_constructible<range_reference_t<R>>; // 25.7.25 [range.zip], zip view template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (has-tuplable-ref<Views> && ...) class zip_view; […] // 25.7.27 [range.adjacent], adjacent view template<forward_range V, size_t N> requires view<V> && (N > 0) && has-tuplable-ref<V> class adjacent_view; […] // 25.7.33 [range.cartesian], cartesian product view template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) && (has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>) class cartesian_product_view; […] }Modify 25.7.25.2 [range.zip.view] as indicated:
namespace std::ranges { […] template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (has-tuplable-ref<Views> && ...) class zip_view : public view_interface<zip_view<Views...>> { […] }; }Modify 25.7.25.3 [range.zip.iterator] as indicated:
namespace std::ranges { […] template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (has-tuplable-ref<Views> && ...) template<bool Const> class zip_view<Views...>::iterator { […] }; }Modify 25.7.25.4 [range.zip.sentinel] as indicated:
namespace std::ranges { template<input_range... Views> requires (view<Views> && ...) && (sizeof...(Views) > 0) && (has-tuplable-ref<Views> && ...) template<bool Const> class zip_view<Views...>::sentinel { […] }; }Modify 25.7.27.2 [range.adjacent.view] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) && has-tuplable-ref<V> class adjacent_view : public view_interface<adjacent_view<V, N>> { […] }; }Modify 25.7.27.3 [range.adjacent.iterator] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) && has-tuplable-ref<V> template<bool Const> class adjacent_view<V, N>::iterator { […] }; }Modify 25.7.27.4 [range.adjacent.sentinel] as indicated:
namespace std::ranges { template<forward_range V, size_t N> requires view<V> && (N > 0) && has-tuplable-ref<V> template<bool Const> class adjacent_view<V, N>::sentinel { […] }; }Modify 25.7.33.2 [range.cartesian.view] as indicated:
namespace std::ranges { […] template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) && (has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>) class cartesian_product_view : public view_interface<cartesian_product_view<First, Vs...>> { […] }; }Modify 25.7.33.3 [range.cartesian.iterator] as indicated:
namespace std::ranges { template<input_range First, forward_range... Vs> requires (view<First> && ... && view<Vs>) && (has-tuplable-ref<First> && ... && has-tuplable-ref<Vs>) template<bool Const> class cartesian_product_view<First, Vs...>::iterator { […] }; }
[2023-08-08; Hewill provides improved wording]
Proposed resolution:
This wording is relative to N4950.
Modify 26.2 25.2 [ranges.syn], header <ranges> synopsis, as indicated:
namespace std::ranges {
[…]
// 25.5.2 [range.utility.helpers], helper concepts
template<class R>
concept range-with-movable-references = see below; // exposition only
// 25.5.3 [view.interface], class template view_interface
template<class D>
requires is_class_v<D> && same_as<D, remove_cv_t<D>>
class view_interface; // freestanding
[…]
// 25.7.24 [range.enumerate], enumerate view
template<input_rangeview V>
requires range-with-movable-references<V>view<View>
class enumerate_view; // freestanding
[…]
// 25.7.25 [range.zip], zip view
template<range-with-movable-referencesinput_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0)
class zip_view; // freestanding
[…]
// 25.7.27 [range.adjacent], adjacent view
template<range-with-movable-referencesforward_range V, size_t N>
requires forward_range<V> && view<V> && (N > 0)
class adjacent_view; // freestanding
[…]
// 25.7.33 [range.cartesian], cartesian product view
template<range-with-movable-referencesinput_range First,
range-with-movable-referencesforward_range... Vs>
requires (view<First> && ... && (forward_range<Vs> && view<Vs>))
class cartesian_product_view; // freestanding
[…]
}
Modify 25.7.25.2 [range.zip.view] as indicated:
namespace std::ranges {
[…]
template<range-with-movable-referencesinput_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0)
class zip_view : public view_interface<zip_view<Views...>> {
[…]
public:
[…]
constexpr auto begin() const requires (range-with-movable-referencesrange<const Views> && ...) {
return iterator<true>(tuple-transform(ranges::begin, views_));
}
[…]
constexpr auto end() const requires (range-with-movable-referencesrange<const Views> && ...) {
if constexpr (!zip-is-common<const Views...>) {
return sentinel<true>(tuple-transform(ranges::end, views_));
} else if constexpr ((random_access_range<const Views> && ...)) {
return begin() + iter_difference_t<iterator<true>>(size());
} else {
return iterator<true>(tuple-transform(ranges::end, views_));
}
}
[…]
};
}
Modify 25.7.25.3 [range.zip.iterator] as indicated:
namespace std::ranges {
[…]
template<range-with-movable-referencesinput_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0)
template<bool Const>
class zip_view<Views...>::iterator {
[…]
};
}
Modify 25.7.25.4 [range.zip.sentinel] as indicated:
namespace std::ranges {
template<range-with-movable-referencesinput_range... Views>
requires (view<Views> && ...) && (sizeof...(Views) > 0)
template<bool Const>
class zip_view<Views...>::sentinel {
[…]
};
}
Modify 25.7.27.2 [range.adjacent.view] as indicated:
namespace std::ranges {
template<range-with-movable-referencesforward_range V, size_t N>
requires forward_range<V> && view<V> && (N > 0)
class adjacent_view : public view_interface<adjacent_view<V, N>> {
[…]
public:
[…]
constexpr auto begin() const requires range-with-movable-referencesrange<const V> {
return iterator<true>(ranges::begin(base_), ranges::end(base_));
}
[…]
constexpr auto end() const requires range-with-movable-referencesrange<const V> {
if constexpr (common_range<const V>) {
return iterator<true>(as-sentinel{}, ranges::begin(base_), ranges::end(base_));
} else {
return sentinel<true>(ranges::end(base_));
}
}
[…]
};
}
Modify 25.7.27.3 [range.adjacent.iterator] as indicated:
namespace std::ranges {
template<range-with-movable-referencesforward_range V, size_t N>
requires forward_range<V> && view<V> && (N > 0)
template<bool Const>
class adjacent_view<V, N>::iterator {
[…]
};
}
Modify 25.7.27.4 [range.adjacent.sentinel] as indicated:
namespace std::ranges {
template<range-with-movable-referencesforward_range V, size_t N>
requires forward_range<V> && view<V> && (N > 0)
template<bool Const>
class adjacent_view<V, N>::sentinel {
[…]
};
}
Modify 25.7.33.2 [range.cartesian.view] as indicated:
namespace std::ranges {
[…]
template<range-with-movable-referencesinput_range First,
range-with-movable-referencesforward_range... Vs>
requires (view<First> && ... && (forward_range<Vs> && view<Vs>))
class cartesian_product_view : public view_interface<cartesian_product_view<First, Vs...>> {
[…]
public:
[…]
constexpr iterator<true> begin() const
requires (range-with-movable-referencesrange<const First> && ... &&
range-with-movable-referencesrange<const Vs>);
[…]
constexpr iterator<true> end() const
requires (range-with-movable-references<const First> && ... &&
range-with-movable-references<const Vs>) &&
cartesian-product-is-common<const First, const Vs...>;
[…]
};
}
[…]constexpr iterator<true> begin() const requires (range-with-movable-referencesrange<const First> && ... && range-with-movable-referencesrange<const Vs>);-3- Effects: Equivalent to:
return iterator<true>(*this, tuple-transform(ranges::begin, bases_));constexpr iterator<false> end() requires ((!simple-view<First> || ... || !simple-view<Vs>) && cartesian-product-is-common<First, Vs...>); constexpr iterator<true> end() const requires (range-with-movable-references<const First> && ... && range-with-movable-references<const Vs>) && cartesian-product-is-common<const First, const Vs...>;-4- Let:
[…]
Modify 25.7.33.3 [range.cartesian.iterator] as indicated:
namespace std::ranges {
[…]
template<range-with-movable-referencesinput_range First, range-with-movable-referencesforward_range... Vs>
requires (view<First> && ... && (forward_range<Vs> && view<Vs>))
class cartesian_product_view<First, Vs...>::iterator {
[…]
};
}