memento

Memento

Intent

Without violating encapsulation, capture and externalize an object's internal state so that the object can be restored to this state later.

Also Known As

Token

Applicability

Use the Memento pattern when

• a snapshot of (some portion of) an object's state must be saved so that it can be restored to that state later, and
• a direct interface to obtaining the state would expose implementation details and break the object's encapsulation

Participants

• Memento (SolverState)
  • stores internal state of the Originator object. The memento may store as much or as little of the originator's internal state as necessary as its originator's discretion
  • protects against access by objects other than the originator. Mementos have effectively two interfaces. Caretaker sees a narrow interface to the Memento - it can only pass the memento to other objects. Originator, in contrast, sees a wide interface, one that lets it access all the data necessary to restore itself to its previous state. Ideally, only the originator that produced the memento would be permitted to access the memento's internal state
• Originator (ConstraintSolver)
  • creates a memento containing a snapshot of its current internal state
  • uses the memento to restore its internal state
• Caretaker (undo mechanism)
  • is responsible for the memento's safekeeping
  • never operates on or examines the contents of a memento

Related Patterns

Command: Commands can use mementos to maintain state for undoable operations.
Iterator: Mementos can be used for iteration.