What is the difference between iterators and generators? What is the relationship between them? What happens when an iterator reaches the end?

An iterator is an object that allows sequential traversal of the elements in a collection, typically implementing methods to get the next element and to check if the traversal is complete. A generator is a special type of iterator that is created using a function capable of pausing its execution and returning values one at a time (for example, using the yield keyword in Python).

The connection between them is that a generator provides a convenient way to create iterators without manually implementing the entire iterator interface.

When an iterator reaches the end, it usually signals this in a specific way: in Python, by raising a StopIteration exception; in other languages, by returning a value indicating completion (such as null or false). This allows loops and other constructs to understand that there are no more elements.