Issue 3791: join_view::iterator::operator-- may be ill-formed

Title: join_view::iterator::operator-- may be ill-formed
Status: resolved
Section: [range.join.iterator]
Submitter: Hewill Kang

Created on 2022-09-30.00:00:00 last changed 31 months ago

Messages

msg12860 (view)

Date: 2023-03-22.00:00:00

[ 2023-03-22 Resolved by the adoption of P2770R0 in Issaquah. Status changed: New → Resolved. ]

msg12839 (view)

Date: 2022-10-15.00:00:00

[ 2022-10-12; Reflector poll ]

Set priority to 3 after reflector poll.

"Could introduce an as_lvalue lambda (like auto as_lvalue = []<class T>(T&& x) -> T& { return (T&)x; };) and use it throughout."

This wording is relative to N4917.

Modify [range.join.iterator] as indicated:

constexpr iterator& operator--()
  requires ref-is-glvalue && bidirectional_range<Base> &&
            bidirectional_range<range_reference_t<Base>> &&
            common_range<range_reference_t<Base>>;

-13- Effects: Equivalent to:

if (outer_ == ranges::end(parent_->base_)) {
  auto&& inner = *--outer_;
  inner_ = ranges::end(inner*--outer_);
}
while (trueinner_ == ranges::begin(*outer_)) {
  if (auto&& tmp = *outer_; inner_ == ranges::begin(tmp)) {
    auto&& inner = *--outer_;
    inner_ = ranges::end(inner*--outer_);
  } else {
    break;
  }
}
--inner_;
return *this;

msg12838 (view)

Date: 2022-09-30.00:00:00

Currently, join_view::iterator::operator-- has the following Effects:

if (outer_ == ranges::end(parent_->base_))
  inner_ = ranges::end(*--outer_);
while (inner_ == ranges::begin(*outer_))
  inner_ = ranges::end(*--outer_);
--inner_;
return *this;

which uses ranges::end(*--outer_) to get the sentinel of the inner range. However, *--outer_ may return an rvalue reference to a non-borrowed range, in which case calling ranges::end will be ill-formed, for example:

#include <ranges>
#include <vector>

int main() {
  std::vector<std::vector<int>> v;
  auto r = v | std::views::transform([](auto& x) -> auto&& { return std::move(x); })
             | std::views::join;
  auto e = --r.end(); // hard error
}

The proposed resolution uses a temporary reference to bind *--outer_, so that ranges::end is always invoked on an lvalue range, which is consistent with the behavior of join_with_view::iterator::operator--.

History
Date	User	Action	Args
2023-03-23 11:42:08	admin	set	status: new -> resolved
2022-10-12 14:38:10	admin	set	messages: + msg12860
2022-10-01 17:55:26	admin	set	messages: + msg12839
2022-09-30 00:00:00	admin	create