a.rb

# fork it
#
# make it print all true with only ONE LINE OF CODE

class A
  def A.foo
    @foo ||= (
      if self == A
        '42.0'
      else
        inherited = :THIS_NEEDS_TO_INHERIT_THE_DEFAULT_VALUE_FROM_THE_PARENT_CLASS
      end
    )
  end

  def A.foo=(foo)
    @foo = foo
  end
end

class B < A
end

class C < B
end

p( A.foo == B.foo )
p( A.foo.object_id != B.foo.object_id )

p( B.foo == C.foo )
p( B.foo.object_id != C.foo.object_id )

p( A.foo == C.foo )
p( A.foo.object_id != C.foo.object_id )

A.foo = 1
B.foo = 2
C.foo = 3

p( A.foo != B.foo )
p( B.foo != C.foo )
p( A.foo != C.foo )

module M; end

A.send(:extend, M)
B.send(:extend, M)
C.send(:extend, M)

p( A.foo == 1 )
p( B.foo == 2 )
p( C.foo == 3 )