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

Section: 25.5.1.6 [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 25.5.1.2 [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).