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.