Question

What is the difference between class and instance methods in Objective-C and when should I use each of them?

Answer 1

Using the tired old Car analogy…

Think of a Class like it is a factory that makes Instances of the class. For example, you might have a Car class and you might declare a method like:

+ carWithColor: (NSColor *) aColor;

And that method would then create a new Car instance, set the color, and return it:

 + carWithColor: (NSColor *) aColor
 {
     Car *aCar = [[[self alloc] init] autorelease];
     [aCar paintWithColor: aColor];
     return aCar;
 }

Now, that Car class would then declare an instance method that allows the car to be painted. Why an instance method? Because every car can have a different color (and the color of the car would likely be stored in an instance variable).

- (void) paintWithColor: (NSColor *) aColor
{
    ... do your paint stuff here ...
}

This is explained in the Objects, Classes, and Messaging section of the Objective-C documentation.