In Objective-C, when should I use class methods and when should I use instance methods?

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.

Answer 2

This is an old post, but since it comes up first in a Google search I thought I'd add to it.

I'm not going to talk about class methods used as factory methods. I'd like to talk about their use in utility methods. You can/should use class methods for utility methods that are independent of state. What does this mean? Well, for instance, if you're formatting a date the same way for all instances, that's a utility method that should be a class method. Think of the utility method like a screw driver. You don't need to make a new instance of the screw driver every time you want to do something with it. The screw driver remains constant. So, for instance, I have a class that includes a private method that generates a string of emDashes used for displaying to the view. This method is not dependent on state and hence will not vary by instance. Think of class utility methods like constants.

+ (NSString *)emDashString {
return @"   \u2014 \u2014 \u2014 \u2014 \u2014 \u2014 \u2014 \u2014 \u2014";

}

You can call this method generically within the class (it's private in my example) like this:

NSString *string = [[self class] emDashString ];

I've deliberately chosen a bit of a trivial example to drive the point home. You would only bother making this a class utility method if you're going to need this string more than once in your class. Notice that instead of referring to the class by name I call it generically with [self class] since this is called internally. If it's exposed and you want to call it from another class then refer to it by the class name as usual.