If you ever get confused about Class Methods, Instance Methods, Class Variables and Instance Variables in Ruby, read this post and you’ll understand it better :)

Class Methods

• Class methods belong to a class in Ruby, and it can be used without instance any class object.
• The definition of class methods has a “self” prefix.

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def self.show_name
end

Instance Methods

• Instance methods belong to any instance of a class in Ruby. To use instance methods, you always need to have an existing instance first. Usually this means you have called new() method.
• Instance methods do not have “self” prefix in their definition.

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def show_name
end

Class Variables

• Class variables belong to a class in Ruby. In other words, each class in Ruby automatically has an object even without any instances. The class variables are constants of the class object.
• Class variables are totally separated with instance variables (obvious though). The value of a class variable doesn’t change when the instance variables having the same name change.
• The name of a class variable has the “@@” prefix.

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class myClass
    @@foo = 0
end

Instance Variables

• Instance variables belong to an instance of a class in Ruby.
• Different instances have their own set of instance variables.
• The name of an instance variable has the “@” prefix.

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class myClass
    def initialize()
        @foo = 0
    end

A quick demo

So now I have a quick demo to show more about the Class/Instance methods and variables.

In this demo, a Fruit class is created. The Fruit class has a class variable @@color, and the Fruit class instances have a instance variables @color.

Fruit class has a class method self.show_color, and an instance method with the same name show_color.

Now check out how these stuff works.

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# fruit.rb

class Fruit
    @@color = 'yellow' # class variable

    def initialize()
        @name = 'apple'
        @color = 'red' # instance variable
    end

    def self.show_color()
        puts "(class method) color: #{@@color}"
    end

    def change()
        change_color()
        puts "try calling instance method"
        show_color()
        puts "try calling class method"
        self.show_color()
        puts "change done"
    end

    def change_color()
        @color = 'green'
    end

    def show_color()
        puts "(instance method) color: #{@color}"
    end
end

The test file:

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# test.rb

$LOAD_PATH.unshift(File.expand_path(File.dirname(__FILE__)))
require 'fruit.rb'

myfruit = Fruit.new
myfruit.show_color
myfruit.change

Fruit.show_color

Executing the test.rb file will generate the following result:

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(instance method) color: red
try calling instance method
(instance method) color: green
try calling class method
(instance method) color: green
change done
(class method) color: yellow

Summary of the demo

• <Class>.new instances an object by calling the initialize method. Instance variables are initialized

• Instance methods only calls the instance methods within itself, even use the self prefix.

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show_color()
self.show_color()

Both call the instance method, and shows the color: green

• The change on instance variables doesn’t affect the class variables

• Class methods can be called without instances

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Fruit.show_color

• Instance methods can’t be called without instances. The following statement will exceptions (NoMethodError).

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myfruit = Fruit.new
myfruit.add

# undefined method `add ... (NoMethodError)