How can objects have class methods in ruby?

Question

Object , Class, Module , NilClass are all instances of Class.

1) First Doubt how can something be an instance of itself ? (i.e 'Class' is an instance of 'Class') or is my assumption wrong ?

2) If Object , Class ,Module etc ... are all objects then how can they have class methods ? Because class methods can only be called from classes and are not present in objects. (or is my assertion incorrect that Object, Class , Module are all objects ?)

3)If Object , Class , Module are not objects then what are they ?

3) Does a class method account for the missing method in instance a of Class and ultimately a decrease in method count ?

>> Class.methods.count

=> 82

>> a = Class.new

=> #<Class:0x1005519b8>

>> a.methods.count

=> 81

FYI You can easily find out which methods differ between Class.methods and Class.new.methods: Class.methods - Class.new.methods => ["nesting"] — user229044
– user229044 ♦, Commented Aug 31, 2011 at 23:15

C. K. Young · Accepted Answer · 2011-08-31 23:20:36Z

1

Class objects are indeed objects.

Class methods are actually methods defined in the class's eigenclass (singleton class). That is why those methods are not available to actual instances of said classes.

Here's a way to help you see this: first, add a singleton_class method if you don't already have it:

module Kernel
  def singleton_class
    class << self
      self
    end
  end
end

Now, try the following:

String.instance_methods
String.singleton_class.instance_methods
Class.instance_methods
Class.singleton_class.instance_methods
c = Class.new
c.instance_methods
c.singleton_class.instance_methods

This will help you gain an appreciation for what methods are available to instances of a class, versus what methods are methods on the class (i.e., instances of the class's singleton class).

(You can pass a false argument to each of those instance_methods calls to see which methods are defined for that class, and not any superclasses.)

edited Aug 31, 2011 at 23:20

answered Aug 31, 2011 at 23:13

C. K. Young

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3 Comments

pankajdoharey Over a year ago

OK now here's as answer i was looking for ... i have been onto this for days now .. been asking a similar question on stack overflow for days, and people have been ridiculing and twisting and seem to be not getting it. I get it now. Thank-you very much Chris. :)

C. K. Young Over a year ago

I'm glad to be of help. I vastly prefer the name "eigenclass" also, but I've been overruled: see stackoverflow.com/questions/2505067/class-self-idiom-in-ruby/… and its attached comments. :-)

pankajdoharey Over a year ago

This is very close and now i know what i have to look for further (i.e eigenclass)

Sony Santos · Accepted Answer · 2011-08-31 23:10:48Z

0

The Ruby core is composed by Class, Object, Module and Kernel. They're predefined, so the Class class can be an instance of itself.
They have class methods because they're classes, too (and classes are objects).
I can't answer it yet. I have to discover which method is missing to think in an answer.

answered Aug 31, 2011 at 23:10

Sony Santos

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3 Comments

Sony Santos Over a year ago

And Kernel#method_missing won't help me on answer 3. :)

pankajdoharey Over a year ago

Following method is missing : Class.methods - a.methods => ["nesting"]

pankajdoharey Over a year ago

Kernel#method_missing why do i get this joke every-time and still i dont get it.

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