Advancep OOP

ruby_oop_1.rb

A = Class.new do
  attr_reader :a
  def initialize(a)
    @a = a
  end
end

a = A.new 1
a.class.class #class

ruby_oop_2.rb

# http://www.integralist.co.uk/posts/eigenclass.html

class B
  def self.eigenclass 
    class << self
      self
    end 
  end
  
  def self.b
    p 'B'
  end
end

B.instance_methods.include? :b # false
B.methods.include? :b # true
B.eigenclass.instance_methods.include? :b # true

B.class  # Class
B.superclass # Object

B.eigenclass.class # Class
B.eigenclass.superclass # <Class:Object>
B.eigenclass.new # TypeError: can't create instance of singleton class

ruby_oop_3.rb

class B
  def eigenclass
    class << self
      self
    end
  end
end

b = B.new
b.eigenclass # <Class:#<B:0x007f99130465d8>>
b.eigenclass.superclass # B
b.eigenclass.equal? B.new.eigenclass # false

def b.b
  p 'b'
end
b.eigenclass.instance_methods.include? :b # true
B.new.eigenclass.instance_methods.include? :b # false

ruby_oop_4.rb

class A
  def self.b
    p @@b
  end

  def self.b=(b)
    @@b = b
  end

  def self.a
    p @a
  end

  def self.a=(a)
    @a = a
  end
end

class B < A
end

A.b = 1
A.b # 1
B.b = 2
B.b # 2
A.b # 2

A.a = 1
B.a = 2
A.a # 1
B.a # 2