Memento

Sometimes it is necessary to keep a copy of the internal state of a certain object. However, we can't get the internal state directly, which breaks the encapsulation. Thus the pattern of Memento helps.

As is seen in the figure above, we don't violate the encapsulation. The only problem might be that creating a memento might be expensive in time/storage. Usually the shots of states are maintained so as to recover the original state when necessary.