Instance and class methods

Hi Vane,

The difference between class methods and instances methods can be characterized with this example:

Pretend I have Class, Cat, that has the following method definitions:

class Cat

  attr_reader :name
  def initialize(name)
    @name = name
  end

  def self.catNamedCharles
     self.new("Charles")
  end

  def meow
    puts "Meow"
  end
end

cat = Cat.new("Dylan")
cat2 = Cat.catNamedCharles

puts cat.name # => "Dylan"
puts cat2.name #=> "Charles"

cat.meow #=> "Meow"
cat2.meow #=> "Meow"

I define three methods for my Cat class, the initialize method, a class method, self.catNamedCharles, and an instance method, meow.

Firstly notice how I prefix the class method with the word "self" in its definition. This tells the Ruby interpreter that it is a class method and should be called with the class. All this class method is doing is calling the classes "new" method and presetting the name to "Charles". When I instantiate cat2, I am simply assigning the return value of the Cat's class method, which in this case returns an instance of the class.

An instance method on the other hand is a method that an instance can call. Both cat and cat2 are instances of the Cat class, and therefore inherit the method, meow. I can call meow on either instance and for both cat objects they will puts "Meow".

In short, a Class method is a method you directly call on the class, prefixing it's method definition with the word self, while an instance method is a method you call on an individual instance of a class.