std::equal with reverse_iterator

Question

Is it legal to use a reverse_iterator with std::equal ?

For example, are any of these legal?

std::equal(v.begin(), v.end(), w.rbegin())

std::equal(v.rbegin(), v.rend(), w.begin())

std::equal(v.rbegin(), v.rend(), w.rbegin())

Answer 1

All are valid, because reverse iterators are , in fact, forward iterators .

"Reverse iterator" is not an iterator category. Remember some iterator categories:

• An iterator that can be dereferenced ( * ) and incremented ( ++ ) is a forward iterator.
• A forward iterator that can also be decremented is a bidirectional iterator.
• A random access iterator is a biderectional iterator that also has + and - operators.

On the other hand, a reverse iterator is a bidirectional iterator or a random access iterator that looks at a collection in reverse. Look at

http://www.cplusplus.com/reference/std/iterator/reverse_iterator/

... especially what it says about iterator_category under the "Member types" heading.