Originally Posted by
Elysia
You should study the concept of iterators. However, as for implementation, I can add that since you're probably going to use a vector, you can use its iterators as your class's iterators for MusicCD for iterating tracks. But to understand that, take a look at what iterators are first and how to use and implement them.
The idea is that the enum class should contain the book type, i.e. green or blue. Then, as it's a type, you should composition to bind it to the book class.