[mariodiana]

It's sad that late-binding languages like Objective-C never got the love they should have, and instead people have favored strongly (or stronger) typed languages. In Objective-C, you could have your User class take a delegate. The delegate could either handle messages for students or employees. And you could code the base User object to ignore anything that couldn't be handled by its particular delegate.

This is a very flexible way of doing things. Sure, you'll have people complain that this is "slow." But it's only slow in computer standards. By human standards—meaning the person sitting at a desktop or phone UI—it's fast enough that they'll never notice.

[incrudible]

I will complain that it is hard to reason about. You better have a really good reason to do something like this, like third party extensibility as an absolute requirement. It should not be your go to approach for everything.