Call instance method from class method

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.

If you want to call an instance method you need an instance to call the method on.

self is a reference to the instance that is being messaged. Thus self in a class method is the class and self in an instance method is an instance of the class.

So, naively, you might do:

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

-(void)foo {
//cool stuff
}

Of course, that'd leak memory in a non-GC application (since the instance isn't being released or autoreleased). Instead of explaining why, I will point you to the Objective-C intro documentation. It is some awesome stuff, covering both Objective-C and general object oriented programming patterns.