why are metaclasses created in ruby?

Question

I m trying to understand the Ruby Object Model. I understood that the instance methods are saved in the class rather than in the objects of the class because it removes redundan

Answer 1

This isn't quite an answer to your question, but it might be useful. Two things to think about that might help:

• metaclass is not really a good name for what's going on here when you think of how the meta prefix is used in other scenarios. eigenclass which you will see used in other documentation is probably a better name, meaning "an object's own class"
• It's not just classes that have an eigenclass, every object does

The eigenclass is used to store methods that are specific to a particular object. e.g. we can add a method to a single String object:

my_string = 'Example'
def my_string.example_method
  puts "Just an example"
end

This method can only be called on my_string and not on any other String object. We can see that it is stored in my_string's eigenclass:

eigenclass = class << my_string; self; end # get hold of my_string's eigenclass
eigenclass.instance_methods(false) # => [:example_method]

Remembering that classes are objects, in this context, it makes sense that the methods specific to a particular class should be stored in that class's eigenclass.

---

Update: actually, there is an eigenclass for the eigenclass. We can see this more easily if we add eigenclass as a method to Object:

class Object 
  def eigenclass 
    class << self
      self
    end 
  end 
end

and then we can do:

irb(main):049:0> my_string.eigenclass
=> #>
irb(main):050:0> my_string.eigenclass.eigenclass
=> #>>
irb(main):051:0> my_string.eigenclass.eigenclass.eigenclass # etc!
=> #>>>

whilst this seemingly creates an infinite regress, this is avoided because Ruby only creates the eigenclasses on as they are needed. I think the name "metaclass" really is a source of part your confusion because you are expecting a "metaclass" to hold some kind of information that it actually doesn't.