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Source: http://www.vincehuston.org/dp/memento.html

Intent

• Without violating encapsulation, capture and externalize an object's internal state so that the object can be returned to this state later.
• A magic cookie that encapsulates a "check point" capability.
• Promote undo or rollback to full object status.

Problem

Need to restore an object back to its previous state (e.g. "undo" or "rollback" operations).

Example

The Memento captures and externalizes an object's internal state so that the object can later be restored to that state. This pattern is common among do-it-yourself mechanics repairing drum brakes on their cars. The drums are removed from both sides, exposing both the right and left brakes. Only one side is disassembled and the other serves as a Memento of how the brake parts fit together. Only after the job has been completed on one side is the other side disassembled. When the second side is disassembled, the first side acts as the Memento.