Ruby singleton class

In ruby world singleton class is term used frequently to describe something that is in java world called anonymous class. The term is sometime very confusing. I have seen this technique being used to extend class's behavior (as instance of Class). Suppose that we need to define a class instance variable the variable associated with the class (different from class variable, which is global variable)

class Foo
end
class Foo
  class << self
    attr_accessor :instances
  end
end

the equivalent variant of above code is

class Foo
end
class_foo = Foo
class << class_foo
    attr_accessor :instances
end

now do some test

Foo.instances =[] #=>[]
Foo.instance_variables #=>[@instances]
Foo.class_variables #=>[]
Foo.instances << Foo.new #=> [#<Foo:0x8241cbc>]
class Foo
  @@total = 0
end
Foo.class_variables #=>["@@total"]

The great tutorial about ruby singleton class can be found here