1
CMSC 433 – Programming Language
Technologies and Paradigms
Spring 2007
Memento Pattern
Apr. 17, 2007
2
What is it?
• Suppose you would like to save the internal state
of an object so you can restore it later.
• Ideally, it should be possible to save and restore
this state without making the object itself take care
of this task, and without violating encapsulation.
• This is the purpose of the Memento pattern.
3
Preliminary Discussion
• Objects frequently expose only some of their internal state
using public methods, but you would still like to be able to
save the entire state of an object because you might need to
restore it later.
• In some cases, you could obtain enough information from
the public interfaces (such as the drawing position of
graphical object s) to save and restor e that data.
• In other cases, the color, shading, angle and connection
relationship to other graphical ob jects need to be save d and
this information is not readily available.
• This sort of information saving and restoration is common
in systems that need to support Undo commands.
4
Preliminary Discussion (contd…)
• If all of the information describing an object is
available in public variables, it is not that difficult
to save them in some external store.
• However, making these data public makes the
entire system vulnerable to change by external
program code, when we usually expect data inside
an object to be private and encapsulated from the
outside world.
