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.
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.
Getters and setters are just syntactic sugar for get_ and set_ methods. They can absolutely have side effects (though it's probably a bad idea to start tweaking lots of fields when all the caller wanted was to increment a counter or something).
First of all: I am aware of the language agnostic tag! "runako" answered the question quite correctly. But often you want to apply your knowledge to the real world so I think I would be nice to also provide an answer that addresses this problem in a more pragmatic way.
When dealing with real world languages like c++,c# or java then even a nop function has actual side effects which can cause code to be executed!
Just think about static-constructors. Even though the specs don't always specify the time a static constructor is ran for a class, most of the time it will be the point in time where a method or member of the class is first accessed.
Example in C#:
class NotSoObvious
{
static NotSoObvious()
{
CauseSomeSideEffects();
}
// calling this can cause the constructor to run first!
public static void DoNothing()
{
return;
}
}
What's more is that even a method that isn't being called at all can cause side effects! Think of reflection (the ability of a program to query information about its own structure). When a method is present but not being called it can still be detected by reflection.
A method with no calls to it surely has a side effect on a program that outputs the number of methods inside!
It all boils down to this: If you want to know about the actual side effects of a method, you first have to determine what you even consider to be a "side effect".
来源：https://stackoverflow.com/questions/810289/when-exactly-does-a-method-have-side-effects