Call instance method from class method

Question

So I need to call some instance methods from class methods in Objective-C...

Example :

+(id)barWithFoo:(NSFoo *) {
[self foo]; //Raises compiler warning. 
}

-(void)foo {
//cool stuff
}

So my question; StackOverFlow is how do you do such things in Objective-C, I'm new kinda to OOP, so am I mad, or is there a way to do this?

Answer 1

There is not a way to do this. It simply doesn't work with object-orientation.

Classes are kinds of things. That's it. They simply describe what that kind of thing does.

An example might be that you have a 'Dog' class. You would have instance methods that would define how a dog wags its tail or how it eats. You might have a class method to purchase a dog.

My pet dog Fido is an instance of class dog. I can send Fido messages, telling him to wag his tail and eat his food. I cannot, however, ask the class 'Dog' to wag its tail; whose tail would wag? Would it be Fido's, or would it be my neighbor's dog?

When you send a message to the class, you don't have a 'self' variable to work with. There's nothing that could tell itself to wag its own tail. Class messages are mostly used for creating instances of the class or getting at other general information.

Edit: To clarify, that last paragraph is an oversimplification. There is a 'self' variable in class methods as bbum describes - it's the computer's reference to the description of the class. That said, I don't think I've ever had an occasion to use 'self' in a class method.