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Section: 99 [reverse.iter.op-=] Status: CD1 Submitter: Matt Austern Opened: 2002-10-23 Last modified: 2016-02-10
Priority: Not Prioritized
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In 99 [reverse.iter.op-=], reverse_iterator<>::operator is specified as having a return type of reverse_iterator::reference, which is the same as iterator_traits<Iterator>::reference. (Where Iterator is the underlying iterator type.)
The trouble is that Iterator's own operator doesn't necessarily have a return type of iterator_traits<Iterator>::reference. Its return type is merely required to be convertible to Iterator's value type. The return type specified for reverse_iterator's operator would thus appear to be impossible.
With the resolution of issue 299, the type of a[n] will continue to be required (for random access iterators) to be convertible to the value type, and also a[n] = t will be a valid expression. Implementations of reverse_iterator will likely need to return a proxy from operator to meet these requirements. As mentioned in the comment from Dave Abrahams, the simplest way to specify that reverse_iterator meet this requirement to just mandate it and leave the return type of operator unspecified.
In 22.214.171.124 [reverse.iter.requirements] change:
reference operator(difference_type n) const;
unspecified operator(difference_type n) const; // see 126.96.36.199 [random.access.iterators]
[ Comments from Dave Abrahams: IMO we should resolve 386 by just saying that the return type of reverse_iterator's operator is unspecified, allowing the random access iterator requirements to impose an appropriate return type. If we accept 299's proposed resolution (and I think we should), the return type will be readable and writable, which is about as good as we can do. ]