Ruby metaclass madness

I'm stuck. I'm trying to dynamically define a class method and I can't wrap my head around the ruby metaclass model. Consider the following class:

class Example

  def self.meta; (class << self; self; end); end

  def self.class_instance; self; end

end

Example.class_instance.class # => Class
Example.meta.class           # => Class

Example.class_instance  == Example      # => true
Example.class_instance  == Example.meta # => false

Obviously both methods return an instance of Class. But these two instances are not the same. They also have different ancestors:

Example.meta.ancestors            # => [Class, Module, Object, Kernel]
Example.class_instance.ancestors  # => [Example, Object, Kernel]

What's the point in making a difference between the metaclass and the class instance?

I figured out, that I can send :define_method to the metaclass to dynamically define a method, but if I try to send it to the class instance it won't work. At least I could solve my problem, but I still want to understand why it is working this way.

Update Mar 15, 2010 13:40

Are the following assumptions correct.

• If I have an instance method which calls self.instance_eval and defines a method, it will only affect the particular instance of that class.
• If I have an instance method which calls self.class.instance_eval (which would be the same as calling class_eval) and defines a method it will affect all instances of that particular class resulting in a new instance method.
• If I have a class method which calls instance_eval and defines a method it will result in a new instance method for all instances.
• If I have a class method which calls instance_eval on the meta/eigen class and defines a method it will result in a class method.

I think it starts to make sense to me. It would certainly limit your po