[bedobi]

I see no need for inheritance there, that can and should be done using interfaces

eg in the contrived example I gave, any of all three of the User, Student and Employee can implement the interface (and if needed Student could simply delegate to its internal User while Employee could "override" by providing its own, different implementation)

[tsimionescu]

What difference do you see between implementing an interface and inheriting from a class, that makes one good and the other bad?

I'm asking beyond the arbitrary distinctions that some languages like Java or C# bake in.

[cryptonector]

Inheriting from a class creates dispatch problems, and there's instance variables/fields/whatever to deal with. Never mind multiple inheritance, as that gets hairy real fast. With interfaces there is no hierarchy, and all there is is a type and a data, and you either pass them around together, you monomorphize to avoid the need to pass two pointers instead of one, or you have the data point to its type in a generic way. With class hierarchies you have to have operations to cast data up and down the hierarchy, meaning trade one dispatch table for another.

[bedobi]

this! with interfaces, you get all the benefits and none of the negatives

[taylodl]

Interfaces manifest the object's responsibility. Functions accepting objects as parameters should work with interfaces, not type instances. That way the responsibilities and capabilities are clear to the user of the interface and the implementor.

As far as how the implementor may fulfill its interface obligation, it may use inheritance, if it truly has an IS-A or subtype relationship with the base object.