In order to be able to transform a normal iterator into a reverse iterator, it must at least have support for bidirectional iteration. This requirement is fulfilled by any iterator of the bidirectional category or higher.

A reverse iterator kind of contains a normal iterator and mimics its interface completely, but it rewires the increment operation to a decrement operation.

The next detail is about the begin and end iterator positions. Let's have a look at the following diagram, which shows a standard numeric sequence kept in an iterable range. If the sequence goes from 1 to 5, then the begin iterator has to point to the element 1, and the end iterator must point one element past 5:

When defining reverse iterators, the rbegin iterator must point to 5, and the rend iterator must point to the element before 1. Turn the book upside down, and see that it completely makes sense.

If we want our own custom container classes to support reverse iteration, we do not need to implement all these details ourselves; we can just wrap the normal iterators into reverse iterators by using the std::make_reverse_iterator helper function, and it does all the operator rewiring and offset corrections for us.