When exactly does a method have side effects?

Question

As I always understood it, any change to the programs state (or anything to do with IO) is a side effect. It does not matter, whether the change occurs in a global variable or in a private field of the object the method is called on. It follows that all methods which do not return anything either do nothing at all or have a side effect.
My confusion comes from one of our university's instructors (who is still a student and thus not omniscient yet;) ) telling me setters don't have side effects.

Answer 1

Your instructor is mistaken. With apologies to the SO editors for not pasting the entire article here, this is what Wikipedia has to say:

http://en.wikipedia.org/wiki/Side_effect_(computer_science)

Money Quote #1:

In computer science, a function or expression is said to have a side effect if, in addition to producing a value, it also modifies some state or has an observable interaction with calling functions or the outside world.

Money Quote #2:

In the presence of side effects, a program's behavior depends on past history; that is, the order of evaluation matters.

Non-NOP Setters always satisfy that criteria.