Inheritance

Syntax#

class SubClass < SuperClass

Refactoring existing classes to use Inheritance

Let’s say we have two classes, Cat and Dog.

class Cat
  def eat
    die unless has_food?
    self.food_amount -= 1
    self.hungry = false
  end
  def sound
    puts "Meow"
  end
end

class Dog
  def eat
    die unless has_food?
    self.food_amount -= 1
    self.hungry = false
  end
  def sound
    puts "Woof"
  end
end

The eat method is exactly the same in these two classes. While this works, it is hard to maintain. The problem will get worse if there are more animals with the same eat method. Inheritance can solve this problem.

class Animal
  def eat
    die unless has_food?
    self.food_amount -= 1
    self.hungry = false
  end
  # No sound method
end

class Cat < Animal
  def sound
    puts "Meow"
  end
end

class Dog < Animal
  def sound
    puts "Woof"
  end
end

We have created a new class, Animal, and moved our eat method to that class. Then, we made Cat and Dog inherit from this new common superclass. This removes the need for repeating code

Multiple Inheritance

Multiple inheritance is a feature that allows one class to inherit from multiple classes(i.e., more than one parent). Ruby does not support multiple inheritance. It only supports single-inheritance (i.e. class can have only one parent), but you can use composition to build more complex classes using Modules.

Subclasses

Inheritance allows classes to define specific behaviour based on an existing class.

class Animal
  def say_hello
    'Meep!'
  end

  def eat
    'Yumm!'
  end
end

class Dog < Animal
  def say_hello
    'Woof!'
  end
end

spot = Dog.new
spot.say_hello # 'Woof!'
spot.eat       # 'Yumm!'

In this example:

Dog Inherits from Animal, making it a Subclass.
Dog gains both the say_hello and eat methods from Animal.
Dog overrides the say_hello method with different functionality.

Mixins

Mixins are a beautiful way to achieve something similar to multiple inheritance. It allows us to inherit or rather include methods defined in a module into a class. These methods can be included as either instance or class methods. The below example depicts this design.

module SampleModule

  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods

    def method_static
      puts "This is a static method"
    end

  end

  def insta_method
    puts "This is an instance method"
  end

end

class SampleClass
  include SampleModule
end

sc = SampleClass.new

sc.insta_method

prints "This is an instance method"

sc.class.method_static

prints "This is a static method"

What is inherited?

Methods are inherited

class A
  def boo; p 'boo' end
end

class B < A; end

b = B.new
b.boo # => 'boo'

Class methods are inherited

class A
  def self.boo; p 'boo' end
end

class B < A; end

p B.boo # => 'boo'

Constants are inherited

class A
  WOO = 1
end

class B < A; end

p B::WOO # => 1

But beware, they can be overridden:

class B
  WOO = WOO + 1
end

p B::WOO # => 2

Instance variables are inherited:

class A
  attr_accessor :ho
  def initialize
    @ho = 'haha'
  end
end

class B < A; end

b = B.new
p b.ho # => 'haha'

Beware, if you override the methods that initialize instance variables without calling super, they will be nil. Continuing from above:

class C < A
  def initialize; end
 end

c = C.new
p c.ho    # => nil

Class instance variables are not inherited:

class A
    @foo = 'foo'
    class << self
        attr_accessor :foo
    end
end

class B < A; end

p B.foo # => nil

# The accessor is inherited, since it is a class method
#
B.foo = 'fob' # possible

Class variables aren’t really inherited

They are shared between the base class and all subclasses as 1 variable:

class A
    @@foo = 0
    def initialize
        @@foo  += 1 
        p @@foo
    end
end

class B < A;end

a = A.new # => 1
b = B.new # => 2

So continuing from above:

class C < A
  def initialize
    @@foo = -10
    p @@foo
  end
end

a = C.new # => -10
b = B.new # => -9