seeing_the_included_class.rb

require 'objspace'

class A
  # When we include a module, it makes an invisible "included class" (T_ICLASS),
  # and places it in the inheritance hierarchy, as the superclass.
  # Hence A the inclusion, A no longer has a direct reference to Object,
  # it now has a direct reference to the included class
  ObjectSpace.reachable_objects_from(self)  # => [#<InternalObject:0x007f8cd29343b8 T_CLASS>, "A", Object]
  m = Module.new
  include m
  ObjectSpace.reachable_objects_from(self)  # => [#<InternalObject:0x007f8cd29343b8 T_CLASS>, "A", #<InternalObject:0x007f8cd2934020 T_ICLASS>]


  # The included class keeps a reference to the module, thus when we ask for things like .ancestors,
  # it gives us the module instead of the included class. This is seen as an implementation detail.  
  A.ancestors # => [A, #<Module:0x007f8cd2934048>, Object, PP::ObjectMixin, Kernel, BasicObject]


  # It must go through the included class to access Object, or the module
  iclass = ObjectSpace.reachable_objects_from(self).last
  iclass                                      # => #<InternalObject:0x007f8cd2934020 T_ICLASS>
  ObjectSpace.reachable_objects_from(iclass)  # => [#<Module:0x007f8cd2934048>, "#<Module:0x007f8cd2934048>", Object]

  # The module does not have a name until it is assigned to a constant.
  # But even then, that name is not generated until it is asked for.
  Object.const_set(:M, m)
  ObjectSpace.reachable_objects_from(iclass)  # => [M, "#<Module:0x007f8cd2934048>", Object]
  m.name                                      # => "M"
  ObjectSpace.reachable_objects_from(iclass)  # => [M, "M", Object]
end

the_singleton_class.rb

require 'objspace'
 
# When we define our first singleton method, that creates a new class to house the method,
# and places the class it at the object's class pointer.
# This means the object has no direct reference to its original class, it must go through the superclass.
# This also explains how every object can have a singleton class: they are created lazily
o = Object.new
ObjectSpace.reachable_objects_from(o) # => [Object]
def o.zomg; end
ObjectSpace.reachable_objects_from(o) # => [#<InternalObject:0x007ffb94814c90 T_CLASS>]

sclass = ObjectSpace.reachable_objects_from(o).first
ObjectSpace._id2ref(sclass.internal_object_id)  # => #<Class:#<Object:0x007ffb94814f60>>
o.singleton_class                               # => #<Class:#<Object:0x007ffb94814f60>>