[saidinesh5]

Never using inheritance is pushing the dogma to the other extreme imo. The whole prefer composition over inheritance is meant to help people avoid the typical OO over use of inheritance. It doesn't mean Is-A relation doesn't/shouldn't exist. It just means when there is data, prefer using composition to give access to that data.

There will be times when you want to represent Is-A relationship - especially when you want to guarantee a specific interface/set of functions your object will have - irrespective of the under the hood details.

  Notifier n = GetNotifier(backend_name);

Here what you care about is notifier providing a set of functions (sendNotification, updateNotification, removeNotification) - irrespective of what the implementation details are - whether you're using desktop notifications or SMS notifications.

[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.

[cryptonector]

If you have a sufficiently statically typed language then the is-a concern goes away -- certainly in the example you gave, since the compiler/linker knows to look for a `GetNotifier()` that returns a `Notifier`. Now, you might still want to know whether the notifier you got satisfies other traits than just those of `Notifier`, but you do that by using a type that has those traits rather than `Notifier`, and now you have little need for `instanceof` or similar operators. (You still might want to know what kind of thing you got because you might care about semantics that are not expressed in the interface. For example you might care to know whether a logger is "fast" as in local or "slow" as in remote, as that might cause you to log less verbosely to avoid drops or blocking or whatever. But the need for this `instanceof` goes down dramatically if you have interfaces and traits.)

[foobarkey]

Never is a good starting point as a guide but of course there are cases where is-a makes sense too.

Composition generally is not good enough to model all cases without resulting to some AOP or meta programming also, but in that case the is-a or a base class would arguably be the simpler approach, at least it can be reasoned about and debugged, as opposed to some AOP/Meta spaghetti that can only probably be understood from docs