Sometimes, it is valuable to iterate over a range the other way around, not forward but backward. The range-based for loop, as well as all STL algorithms usually iterate over the given ranges by incrementing iterators, although iterating backward requires decrementing them. Of course, it is possible to wrap iterators into a layer that transforms an increment call effectively into a decrement call. This sounds like a lot of boilerplate code for every type on which we would like to support that.

The STL provides a helpful reverse-iterator adapter, which helps us set up such iterators.