Message Propagation with Singleton Class

Instances never contain a method they only carry data. However we can define a singleton class for any object including an instance of a class.

When a message is passed to an object (method is called) Ruby first checks if a singleton class is defined for that object and if it can reply to that message otherwise Ruby checks instance’s class’ ancestors chain and walks up on that.

class Example
  def foo
    :example
  end
end

Example.new.foo #=> :example

module PrependedModule
  def foo
    :prepend
  end
end

class Example
  prepend PrependedModule
end

Example.ancestors #=> [Prepended, Example, Object, Kernel, BasicObject]
e = Example.new
e.foo #=> :prepended

def e.foo
  :singleton
end

e.foo #=> :singleton