Your design above with the private type being returned could possibly be useful. Perhaps outsiders to the class will be prevented from instantiating the nested-type without using the class itself to generate the nested type. This could be contextually useful and certain (interface) functionality could be grouped in a manner that is more convenient to use than by calling methods on the original class.

I like that C++ allows us to express this situation, even if it's a bit of an unusual design pattern.