Include v/s Extend in Ruby

Include is used to importing module code. Ruby will throw an error when we try to access the methods of import module with the class directly because it gets imported as a subclass for the superclass. So, the only way is to access it through the instance of the class. Extend is also used to importing module code but extends import them as class methods. Ruby will throw an error when we try to access methods of import module with the instance of the class because the module gets import to the superclass just as the instance of the extended module. So, the only way is to access it through the class definition. In simple words, the difference between include and extend is that 'include' is for adding methods only to an instance of a class and 'extend' is for adding methods to the class but not to its instance. Example : 

# Ruby program of Include and Extend

# Creating a module contains a method
module Geek
  def geeks
    puts 'GeeksforGeeks!'
  end
end
 
class Lord

  # only Lord can access geek methods
  # with the instance of the class.
  include Geek
end
 
class Star
  
  # only Lord can access geek methods
  # with the class definition.
  extend Geek
end
 
# object access 
Lord.new.geeks

# class access
Star.geeks 

# NoMethodError: undefined  method
# `geeks' for Lord:Class
Lord.geeks 

Output

GeeksforGeeks!
GeeksforGeeks!
main.rb:20:in `': undefined method `geeks' for Lord:Class (NoMethodError)

If we want to import instance methods on a class and its class methods too. We can ‘include’ and ‘extend’ it at the same time. Example : 

# Ruby program to understand include and extend

# Creating a module contains a method
module Geek
  def prints(x)
    puts x
  end
end
 
class GFG

  # by using both include and extend
  # we can access them by both instances
  #  and class name.
  include Geek
  extend Geek
end

# access the prints() in Geek
# module by include in Lord class
GFG.new.prints("Howdy") # object access
 
# access the prints() in Geek 
# module by extend it in Lord class
GFG.prints("GeeksforGeeks!!") # class access

Output

Howdy
GeeksforGeeks!!