Ruby Mixin

Introduction

Though Ruby is an object oriented language, it doesnot support multiple inheritance. So, only single inheritance is possible in Ruby. To support inheritance, mixin comes into play. Mixin is set of code wrapped up in a module that a class can include or extend. To use mixins mainly two keywords are used, include and extend. It is the concept of inheriting everything defined in a module into a class that includes or extends module including methods(private, protected, public), attributes (attr_accessor :name) and constants. In addition, a class can have multiple mixins.

Examples

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module Month
   def weeks_in_month
      puts "You have four weeks in a month"
   end
   def months_in_year
      puts "You have 12 months in a year"
   end
end
class Year
include Month
   def no_of_weeks x
      puts "There are #{x} number of weeks in a year"
   end
end
y1 = Year.new
y1.weeks_in_month
y1.months_in_year
y1.no_of_weeks 4
Output:
You have four weeks in a month
You have 12 months in a year
There are 4 number of weeks in a year

include vs extend

If include keyword is used the methods within module acts as instance method in current class. But using extend keyword the methods act as class method. So, while accessing instance method we need to instantiate the class whereas to use class method we can just access through the class itself. Here is an example that shows how the class and instance methods can be called.

Class methods VS Instance methods

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class SayHello
  def self.from_the_class
    "Hello, from a class method"
  end
  def from_an_instance
    "Hello, from an instance method"
  end
end
Result:
SayHello.from_the_class
=> "Hello, from a class method"
SayHello.from_an_instance
=> undefined method `from_an_instance' for SayHello:Class
hello = SayHello.new
hello.from_the_class
=> undefined method `from_the_class' for #<SayHello:0x0000557920dac930>
hello.from_an_instance
=> "Hello, from an instance method"

Summary

Ruby mixin is the easiest way to add functionality to classes. It is more powerful than inheritance in terms of usage. So, it is the best way to inherit features in Ruby.