How to create default instances of the memento in a Memento Pattern ?

Question

Hi, I use the Memento Pattern to save properties of a multi-instance form, where n forms are created by user inside a parent form. The purpose of the memento is to regain the same number of forms, and their settings, when a user close and later reopen the parent form. The saving of the form mementos are done by a "save" button on the parent form. So I have two challenges..

1. I need a default set of properties to use when opening a new form, so where do I set these default values.. In a default empty constructor of the memento, or somewhere (where?) in the Originator?
2. Next I want the user to be able to change the defaults, hence to make their own default memento. How/where is this saved, and how do I determine if such default memento has been set or not?

Of course I could probably tweak this to work somehow, but I would like to know if there is a general pattern description that solves this, such that I have a firm guideline to follow.

cheers!

Answer 1

The default empty constructor of the memento class should be fine, or you can have a CreateDefault() factory method, e.g., if you want to leave the default constructor to initialize a blank memento as opposed to a pre-set default configuration.

There are a number of different creational patterns you can use for specifying a custom default, including prototype and abstract factory.

Answer 2

Your description does not really match with the Memento patttern. The whole point of Memento is that only instances of the class which is to be restored know anything about the representation of the memento. That is, Memento is about hiding state, rather than allowing clients to set arbitrary states.

As Mark Cidade suggests above, there are other more appropriate patterns to use for your problem.