Question

Whats the difference between class methods and Instance methods. Why do we need them separately?
Can somebody please explain?

Class and Instance Methods

• Instances respond to instance methods

 - (id)init;
 - (float)height;
 - (void)walk;

• Classes respond to class methods

 + (id)alloc;
 + (id)person;
 + (Person *)sharedPerson;

Taimur

Answer 1

An instance method is only available on an instance of the class, while a class method does not need an instance but is available on the class.

Class Methods are denoted by a + while instance methods are denoted by a - before their return type.

Let’s take NSObject for example. NSObject has a class method named + (id)alloc. The alloc method is used to allocate an instance of the class. Obviously alloc must be a class method because if it was an instance method, where would you get the “root” instance from?

On the other hand - (id)init is an instance method because it initializes the state of an instance.