People use the Manager IS-A Employee concept, when in reality, it is instead
a role. Ie, Bob is an Employee who has a Manager role. He is not a TYPE OF
Employee, he is simply an Employee with a Manager role. Jane is an Employee
with no role, or maybe just Grunt role.
Correct. If you read about archetypes in OO design, you'll see four of them:
- moment-interval (time/transaction-related)
- role
- description
- party, place, thing
The key here is the separation of role from party (person or organization) because a party may have multiple roles at a time or over time. Consider what happens when someone is promoted: if you use inheritance to model them (manager is-a user) then you have to change the object's type in order to promote them. That should raise a red flag. If you separate the role out, it should be clear that the person remains the same but their role changes, i.e., promotion is just a matter of changing their 'role' attribute.
Sean A Corfield -- http://www.corfield.org/blog/
"If you're not annoying somebody, you're not really alive." -- Margaret Atwood
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