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### 2347. `reverse_iterator::operator[]` calls const version of `current[]`

Section: 23.5.1.6 [reverse.iter.elem] Status: NAD Submitter: Timo Bingmann Opened: 2013-11-11 Last modified: 2021-06-06

Priority: 2

View all other issues in [reverse.iter.elem].

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Discussion:

Currently `reverse_iterator::operator[]()` returns "`current[-n-1]`" and has an "unspecified" return type.

When `Iterator` is a mutable random access iterator, the expression "`current[-n-1]`" calls "`Iterator::operator[] const`", which returns a const reference. This const reference cannot be converted back to a mutable reference.

This issue is related to the "unspecified" return value of `reverse_iterator::operator[]`, see defect 386.

The -1 is due to "current" pointing one item beyond the `reverse_iterator`'s real current value.

The current libstdc++ implementation reads "`*(current + n)`" for `reverse_iterator::operator[]`.

This copied `current`, advances (backwards) via `operator+` and dereferences. It bypasses the issues due to `reverse_iterator::operator[]` being const by copying the iterator.

[2014-02-13 Issaquah : close as NAD]

Proposed resolution:

This wording is relative to N3797.

1. Edit 23.5.1.2 [reverse.iterator], class template `reverse_iterator` synopsis, as indicated:

   >     namespace std {
   >       template <class Iterator>
   >       class reverse_iterator : public
   >       iterator<typename iterator_traits<Iterator>::iterator_category,
   >         typename iterator_traits<Iterator>::value_type,
   >         typename iterator_traits<Iterator>::difference_type,
   >         typename iterator_traits<Iterator>::pointer,
   >         typename iterator_traits<Iterator>::reference> {
   >       public:
   >         […]
   >         unspecifiedreference operator[](difference_type n) const;
   >         […]
   >       };
   >       […]
   >     }

2. Edit [reverse.iter.opindex] as indicated:

   >     unspecifiedreference operator[](
   >       typename reverse_iterator<Iterator>::difference_type n) const;

   > > -1- Returns: `current[-n-1]*(current + n)`.