When we looked at the Command design pattern, we noted that recording a list of every single change theoretically allows you to roll back the system to any point in time—after all, you’ve kept a record of all the modifications.
Sometimes, though, you don’t really care about playing back the state of the system, but you do care about being able to roll back the system to a particular state, if need be.
This is precisely what the Memento pattern does: it stores the state of the system and returns it as a dedicated, read-only object with no behavior of its own. This “token,” if you will, ...
