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2858. LWG 2472: actually an incompatibility with C++03

Section: 24.5.1 [reverse.iterators] Status: New Submitter: Hubert Tong Opened: 2017-01-28 Last modified: 2020-09-06

Priority: 4

View all other issues in [reverse.iterators].

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Discussion:

Further to LWG 2472⁽ⁱ⁾, the case of reverse_iterator comparisons is a regression introduced by LWG 280⁽ⁱ⁾.

Consider the following program:

#include <utility>
#include <iterator>

using namespace std::rel_ops;

bool f(std::reverse_iterator<int *> it) { return it != it; }

Under C++03, the operator!= in lib.reverse.iterator is more specialized than the operator!= in std::rel_ops.

Following LWG 280⁽ⁱ⁾, neither operator!= candidate is more specialized than the other. The program is observed to fail with libc++.

Online compiler example, see here.

Suggested resolution:

Reintroduce the homogeneous comparison operators from C++03 alongside the new ones.

[2017-03-04, Kona]

Set priority to 4. STL to write a paper deprecating relops Alisdair to provide an example for Annex C.

Proposed resolution:

This wording is relative to N4618.

1. Modify 24.5.1 [reverse.iterators], class template reverse_iterator synopsis, as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator==(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator1, class Iterator2>
     constexpr bool operator<(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator1, class Iterator2>
     constexpr bool operator!=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator1, class Iterator2>
     constexpr bool operator>(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator1, class Iterator2>
     constexpr bool operator>=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator1, class Iterator2>
     constexpr bool operator<=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator==(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   template <class Iterator>
     constexpr bool operator<(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   template <class Iterator>
     constexpr bool operator!=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   template <class Iterator>
     constexpr bool operator>(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   template <class Iterator>
     constexpr bool operator>=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   template <class Iterator>
     constexpr bool operator<=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

2. Modify [reverse.iter.op==] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator==(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator==(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current == y.current.

3. Modify [reverse.iter.op<] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator<(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator<(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current > y.current.

4. Modify [reverse.iter.op!=] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator!=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator!=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current != y.current.

5. Modify [reverse.iter.op>] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator>(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator>(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current < y.current.

6. Modify [reverse.iter.op>=] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator>=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator>=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current <= y.current.

7. Modify [reverse.iter.op<=] as indicated:

   template <class Iterator1, class Iterator2>
     constexpr bool operator<=(
       const reverse_iterator<Iterator1>& x,
       const reverse_iterator<Iterator2>& y);
   template <class Iterator>
     constexpr bool operator<=(
       const reverse_iterator<Iterator>& x,
       const reverse_iterator<Iterator>& y);
   

   -1- Returns: x.current >= y.current.