+ vs -? confused....again

[mrtubby]

ok im a little confused (still learning objective c) on when you use + or -

like i think +(void) vs -(void)

what are the difrfences?

thanks,

mr total noob

[iisword]

+ means class method, - means instance method. Class methods means this

[SomeClass doSomething]

Instance methods require you have an object:

SomeClass *thisClass = [[SomeClass alloc] init];
[thisClass doSomething];

[TheFlyingDutchman]

But that's it? There should be another reason. Please explain better.

[iisword]

Nothing else really. Here's a link: Objective-C: Class vs Instance Methods? - Stack Overflow

[lgehrig1]

Originally Posted by TheFlyingDutchman

But that's it? There should be another reason. Please explain better.

Do you mean like when you'd use one over the other?

It all boils down to object oriented programming, which is a topic you're going to need to review. Google is your friend - my bookshelf is more than 12 years old at this point

In a nutshell, a class method is something that you would not need a specific instantiation of an object in order to invoke. The most useful examples are in NSString - you can call, for example, stringWithFormat to allocate, initialize, and auto-release a string object for you in one fell swoop. They tend to be "meta" methods - you're not doing something to the object, you're doing something about or with the object in a more general sense.

An instance method performs some action on a specific instance (see how that works!) of an object. Using NSString again, the method lowercaseString converts a string to its lowercase form. It doesn't convert every string everywhere in the universe - it only acts on that one specific instance of a string.

You'll find class methods used frequently with singletons - such as the currentDevice method of UIDevice. There's only one device - the one your app is running on! You can't initialize a new UIDevice object because that would be completely silly. Lookup "singleton pattern" on Google.

Because class methods are called without a specific instance attached to them you can't reference any data held by that object. Unless it's held by ALL instances (aka static member), but if you're having trouble with +/- I'd save static members for another day.

Most of the time you'll be creating instance methods. If you have a CKittyCat object and create a method called meow, you'd want that method an instance method (-). Then you could direct an individual kitty cat object to meow for you. I can't think of anything on a CKittyCat that would make sense to have as a class method (alloc, etc. don't count!).

Note: this is a rough sketch only. There's plenty of stuff left out (like an entire discussion of vtables, Obj-C messaging, polymorphism, inheritance, etc.). Hit your bookstores & libraries for object-oriented programming books - it won't really matter if they're Java, C++, C#, Objective-C, Smalltalk, or what have you. Try to find ones that are more overview than language specific, which is why I mention looking at actual paper vs. Amazon searching.