Ruby Sub-Classes/Inheritance, Include, And Extend

Overview

Ruby Objects, Modules, and Classes

  • In Ruby, an object is a collection of (zero or more) instance variables. It also has a class (see below) and possibly a lazily-created singleton class to hold object-specified methods.
  • A module is an object containing a collection of (zero or more) constants, class variables, instance methods, and included modules. You can include a module in another module and you can extend most objects with a module. Since Ruby 2, you can also prepend a module to a module.
    # Parts of a module
    CONSTANT = "I'm a constant"
    @@class_var = "I'm a class variable"
    @class_inst_var = "I'm a class instance variable" # in a class/module definition
    def self.method; "I'm a class method"; end
    class << self
      def another_method; "I'm a class method too"; end
    end
    def method
      @inst_var = "I'm an instance variable" # inside an instance method
      "I'm an instance method"
    end
  • A class is sub-class of module.
    • Each class has a parent class called a super-class. The child class is called a sub-class. The class inherits the behaviors of the super-class. New classes are sub-classes of the Object class unless you specify otherwise.
    • Classes can typically be instantiated via the new method.
    • Classes are not valid parameters for include or extend.
  • A “def method” adds a method to the “currently open” class or module. A “def object.method” adds a method to the singleton class for the object.
  • When you include a module (let’s call it M1) in another module (let’s call it M2), M1’s constants and instance methods become visible in M2 (as constants and instance methods), and M1 will appear in M2’s included_modules list. M1’s class methods are not added to M2 (but see Including Class Methods below).
  • When you extend an object with a module, the module’s instance methods are added to the object via an automatically-generated anonymous super-class of the singleton class (one for each extending module). In the case where the extended object is a module, the added methods are class methods, not instance methods. The object is unaffected by the module’s constants or class methods.

Confirming The Effects Of include And extend In Modules

The following program can be used to see the affect of using include and extend in modules (and classes):

module Inner
    INNER = "Inner constant"
    def self.inner_cm; "Inner class method"; end
    def inner_im; "Inner instance method"; end
end

module Outer
    include Inner;
    OUTER = "Outer constant"
    def self.outer_cm; "Outer class method"; end
    def outer_im; "Outer instance method"; end
end

module Extension
    EXT = "Extension constant"
    def self.ext_cm; "Extension class method"; end
    def ext_im; "Extension instance method"; end
end

class MyClass; include Outer; extend Extension; end

puts "Constants: " +
    (MyClass.constants(true) - Object.constants(true)).inspect
puts "Class methods: " + (MyClass.methods - Object.methods).inspect
puts "Instance methods: " +
  (MyClass.instance_methods - Object.instance_methods).inspect

The output is as follows:

Constants: [:OUTER, :INNER]
Class methods: [:ext_im]
Instance methods: [:outer_im, :inner_im]

Method Resolution Order

The following program can be used to show the class/module hierarchy and order of method resolution for sub-classing (inheritance), include, and extend:

module Mod1; def m; puts "Mod 1"; super; end; end
module Mod2; def m; puts "Mod 2"; super; end; end
module Mod3; def m; puts "Mod 3"; super; end; end
module Mod4; def m; puts "Mod 4"; super; end; end
module Mod5; def m; puts "Mod 5"; super; end; end
module Mod6; def m; puts "Mod 6"; super; end; end
class Base; def m; puts "Base"; end; end
class Sub < Base
    include Mod1, Mod2; include Mod3
    def m; puts "Sub"; super; end
end
o = Sub.new.extend(Mod4, Mod5).extend Mod6
puts "Sub ancestors: " + o.class.ancestors.inspect
o.m

Regrettably, the include and extend methods process their parameters from last to first, so you need to know that method resolution order is not simply last-to-first encountered when called with multiple modules. The output is as follows:

Sub ancestors: [Sub, Mod3, Mod1, Mod2, Base, Object, Kernel, BasicObject]
Mod 6
Mod 4
Mod 5
Sub
Mod 3
Mod 1
Mod 2
Base

Pictorially, it looks like this (with the number in parentheses indicating the search order):
Ruby extend/include/Sub-class Method Resolution Order

Including Class Methods

It is also possible to add class methods as part of an include or to add instance methods as part of an extend using the included or extended callbacks, respectively:

module Inc_Me
  def inst_m; end
  module ClassMethods; def class_m1; end; end
  def self.included (base)
    base.class_exec do
      extend ClassMethods     # method 1 - extend with named sub-module
      Module.new do           # method 2 - extend with anonymous module
        def class_m2; end
      end.tap { |mod| extend mod }
      def self.class_m3; end  # method 3 - add directly to the class
    end
  end
end

module Ext_Me
  def class_m; end            # instance method here, class there
  module InstanceMethods; def inst_m1; end; end
  def self.extended (base)
    base.class_exec do
      include InstanceMethods # method 1
      Module.new do           # method 2
        def inst_m2; end
      end.tap { |mod| include mod }
      def inst_m3; end        # method 3
    end
  end
end

module M1; include Inc_Me; end
puts "M1 class methods: " + (M1.methods - Object.methods).inspect
puts "M1 instance methods: " +
  (M1.instance_methods - Object.instance_methods).inspect
puts "M1 included modules: " + M1.included_modules.inspect, ''

module M2; extend Ext_Me; end
puts "M2 class methods: " + (M2.methods - Object.methods).inspect
puts "M2 instance methods: " +
  (M2.instance_methods - Object.instance_methods).inspect
puts "M2 included modules: " + M2.included_modules.inspect

which produces:

M1 class methods: [:class_m3, :class_m2, :class_m1]
M1 instance methods: [:inst_m]
M1 included modules: [Inc_Me]

M2 class methods: [:class_m]
M2 instance methods: [:inst_m3, :inst_m2, :inst_m1]
M2 included modules: [#<Module:0x00000000cbd108>, Ext_Me::InstanceMethods]

It is better to use the include-with-extend method (as in module Inc_Me) than the extend-with-include method (as in module Ext_Me), as the primary module name gets included in the included_modules list.

It is also better to extend a sub-class (methods 1 or 2) rather than adding the class methods directly (method 3), since the extended modules are each added to a separate, invisible super-class instead of to the including module itself. The benefit here is that the behaviors can be chained using super if desired, as shown by this code:

module Inc1
  module ClassMethods; def m1; puts "Inc1 m1"; super rescue nil; end; end
  def self.included (base)
    base.class_exec do
      extend ClassMethods
      Module.new do
        def m2; puts "Inc1 m2"; super rescue nil; end
      end.tap { |mod| extend mod }
      def self.m3; puts "Inc1 m3"; super rescue nil; end
    end
  end
end

module Inc2
  module ClassMethods; def m1; puts "Inc2 m1"; super rescue nil; end; end
  def self.included (base)
    base.class_exec do
      extend ClassMethods
      Module.new do
        def m2; puts "Inc2 m2"; super rescue nil; end
      end.tap { |mod| extend mod }
      def self.m3; puts "Inc2 m3"; super rescue nil; end
    end
  end
end

module M; include Inc2, Inc1; end
M.m1; M.m2; M.m3

which produces:

Inc2 m1
Inc1 m1
Inc2 m2
Inc1 m2
Inc2 m3

The included Callback And Nested Includes

If your module includes other modules, the included callbacks for the other modules (if present) will be called when they are included in your module, but not when your module is included elsewhere. This code shows the problem:

module M1
  CONST1 = 'M1 constant'
  module ClassMethods; def cm1; 'M1 class method'; end; end
  def im1; 'M1 instance method'; end
  def self.included (base)
    puts "#{self} included in #{base}"
    base.class_exec { extend ClassMethods }
  end
end

module M2
  include M1
  def self.included (base); puts "#{self} included in #{base}"; end
end

module M3; include M2; end

puts "M2 class methods: " + (M2.methods - Object.methods).inspect
puts M3::CONST1
puts "M3 class methods: " + (M3.methods - Object.methods).inspect
puts "M3 instance methods: " +
  (M3.instance_methods - Object.instance_methods).inspect

which produces:

M1 included in M2
M2 included in M3
M2 class methods: [:included, :cm1]
M1 constant
M3 class methods: []
M3 instance methods: [:im1]

The including module’s included callback should therefore call the included callback for any included modules if none of the base object’s ancestors have previously included the other modules:

def M2.included (base)
  puts "#{self} included in #{base}"
  M1.included base if M1.respond_to?(:included) &&
   (!base.respond_to?(:superclass) || !base.superclass.include?(M1))
end

which, after the change, produces:

M1 included in M2
M2 included in M3
M1 included in M3
M2 class methods: [:included, :cm1]
M1 constant
M3 class methods: [:cm1]
M3 instance methods: [:im1]

Download It

A Ruby gem (called extended_include) based on this posting is available at rubygems.org.