Difference Between Class and Class Instance Variables in Ruby

1. Difference in Terms of Availability to Class and Instance Methods in Ruby
2. Difference in Terms of Inheritance in Ruby

Let’s learn in this tutorial about the difference between the class variable and the class instance variable in Ruby.

Difference in Terms of Availability to Class and Instance Methods in Ruby

Class instance variables are available to only class methods, while class variables are available to both class methods and instance methods.

Example Code:

class Parent
  @variable1 = 'This is a class instance variable'
  @@variable2 = 'This is a class variable'

  def self.class_method
    p @variable1
    p @@variable2
  end

  def instance_method
    p @variable1
    p @@variable2
  end
end

Parent.class_method

object1 = Parent.new
object1.instance_method

Output:

"This is a class instance variable"
"This is a class variable"
nil
"This is a class variable"

The first 2-lines of the output above were gotten from Parent.class_method while the last 2-lines were produced by object1.instance_method. As we can see, @variable1 was not available in the instance_method, and as a result, a nil was produced.

Difference in Terms of Inheritance in Ruby

Class instance variables can not be passed to a child class.

Example Code:

class Parent
  @variable1 = 'This is a class instance variable'
  @@variable2 = 'This is a class variable'

  def self.class_method
    p @variable1
    p @@variable2
  end

  def instance_method
    p @variable1
    p @@variable2
  end
end

class Child < Parent
end

Child.class_method

object2 = Child.new
object2.instance_method

Output:

nil
"This is a class variable"
nil
"This is a class variable"