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

Class method signatures are prefixed with +, instance methods with - so in your header file declarations would look something like this:

-(void)setAllThings:(NSArray*)things; //instance method
+(void)setAllClassThings:(NSArray*)things; //class method

And of course the same rules apply when you define the methods in the .m file.

Answer 2

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 3

Instance methods do things with instances of a class:

NSString *myString;
myString = [[[NSString alloc] initWithString:@"Hello, world."] autorelease];
NSLog (@"myString's length: %u", [myString length]); // instance method

Class methods can do class-specific things without relying on an object instance, often returning an instance of the class, or some other class-specific result:

NSLog (@"%@", [NSString stringWithString:@"Hello, world."]); // class method

I think it may be rare to see class methods that do not return something.

Answer 4

You don't need to implement both. Either option is available to you as you design your class.

An instance method can operate on an instance of the class. This can get or set a property, or cause behavior you only want that instance to perform. You need to actually have an instance to use it. These can either use or change the state of the instance.

// Notional instance methods
myHouse.color = blueColor;

[myCar accelerate];
speed = myCar.speed;

A class method operates on the notion that the class exists. It can be used to create an instance, or perform a calculation that doesn't depend on having an instance. You might have a class for custom math helper, that essentially contains functions.

// Notional class method uses
myString = [NSString stringWithFormat:@"&f", floatToConvert];

myResult = [MyMathHelper MyFunctionWithInput:myInput];