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cartesian_product_view::iterator::prev
is not quite rightSection: 26.7.33.3 [range.cartesian.iterator] Status: C++23 Submitter: Hewill Kang Opened: 2022-11-08 Last modified: 2023-11-22
Priority: Not Prioritized
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Discussion:
Currently, cartesian_product_view::iterator::prev
has the following Effects:
auto& it = std::get<N>(current_); if (it == ranges::begin(std::get<N>(parent_->bases_))) { it = cartesian-common-arg-end(std::get<N>(parent_->bases_)); if constexpr (N > 0) { prev<N - 1>(); } } --it;
which decrements the underlying iterator one by one using recursion.
However, when N == 0
, it still detects if the first iterator has reached the beginning and assigns it to
the end, which is not only unnecessary, but also causes cartesian-common-arg-end
to be applied to
the first range, making it ill-formed in some cases, for example:
#include <ranges>
int main() {
auto r = std::views::cartesian_product(std::views::iota(0));
r.begin() += 3; // hard error
}
This is because, for the first range, cartesian_product_view::iterator::operator+=
only requires
it to model random_access_range
.
However, when x
is negative, this function will call prev
and indirectly calls
cartesian-common-arg-end
, since the latter constrains its argument to satisfy
cartesian-product-common-arg
, that is, common_range<R> || (sized_range<R> &&
random_access_range<R>)
, which is not the case for the unbounded iota_view
, resulting in a
hard error in prev
's function body.
if constexpr
so that we just decrement the first
iterator and nothing else.
[Kona 2022-11-08; Move to Ready]
[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:
template<size_t N = sizeof...(Vs)> constexpr void prev();-6- Effects: Equivalent to:
auto& it = std::get<N>(current_); if constexpr (N > 0) { if (it == ranges::begin(std::get<N>(parent_->bases_))) { it = cartesian-common-arg-end(std::get<N>(parent_->bases_));if constexpr (N > 0) {prev<N - 1>();}} } --it;