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:
Animal, making it a Subclass.
Doggains both the
say_hellomethod with different functionality.
# 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.
# 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
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"
# Refactoring existing classes to use Inheritance
Let's say we have two classes,
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
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
Dog inherit from this new common superclass. This removes the need for repeating code
- class SubClass < SuperClass