instance and class variables in rails controller

Question

I\'m new to rails and ruby. I was studying the concept of class and instance variables. I understood the difference but when I tried it out using the controller in rails it got

Answer 1

Classes are also objects in Ruby, so they can have their own instance variables which are called class instance variables.

• @@world is a class variable
• @insworld is a class instance variable
• #index is an instance method

When you try to access @insworld in #index, Ruby searches for the instance variable in the A object (meaning A.new) because #index is an instance method.

But you defined @insworld as a class instance variable which means it is defined in the class object itself (meaning A).

The following code demonstrates:

class Hi
  @@a = 1 # class variable
  @b  = 2 # class instance variable

  def initialize
    @c = 3 # instance variable
  end

  def test # instance method, works on objects of class Hi
    puts @@a # => 1
    puts @b  # => nil, there is no instance variable @b
    puts @c  # => 3 # we defined this instance variable in the initializer
  end
end

Hi.class_variables        # => @@a
Hi.instance_variables     # => @b
Hi.new.instance_variables # => @c
# Hi is an object of class Class
# Hi.new is an object of class Hi

Keep in mind that all instance variables return nil if they don't exist.