Range-based for loops and observables

In this section, we will implement range-based for loops on a custom type written by us to help you understand how all the things mentioned earlier in this chapter can be put together to write programs that support modern idioms. We will  implement a class that returns a series of numbers within a bound and will implement infrastructure support for the iteration of the values based on range-based for loops. First, we write the  "Iterable/Iterator" (aka "Enumerable/Enumerable") version by leveraging the range-based for loops.  After some tweaks, the implementation will be transformed to Observable/Observer (the key interface of Reactive Programming) patterns: The implementation of Observable/Observer pattern here is just for elucidation purpose and  should not be considered as an Industrial strength implementation...