Re: Design Pattern Question

Responding to Marshall...

Xllo said something like:

...[snip]...


I would like to explore this design further.
Assuming there is a heirarchy of classes.
Employee from which is derived ManagerEmployee,
HREmployee, HRManagerEmployee, etc.


I'm compelled to offer an off-beat explanation as to why IMO the above (and their inter-relationships) are almost always mistakes when expressed strictly as class hierarchy. What happens when an employee is promoted to Manager...and how could he?

There is a formal mechanism for this known as subclass migration. Basically one deletes the old object and instantiates a new one with the same identity and attributes in the new subclass. However, it tends to be messy and fragile in practice, so it isn't used very much.

Today I think most people prefer to delegate roles and employ the GoF State pattern for exactly this reason. That reduces the migration to dynamic instantiation of a single relationship.


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