Ruby craziness: Class vs Object?

Question

I just started playing with JRuby. This is my first ruby post. I had a hard time understanding classes vs objects in Ruby. It doesnt mean like what classes & objects in other Object oriented laguages. for an example

Class.is_a? Object 

returns true and

Object.is_a? Object 

too.

so class & Object are both objects

here comes another one

Class.is_a? Class 

returns true and

Object.is_a? Class 

too.

wait, i am not done yet

 Object.instance_of? Class  Class.instance_of? Class 

Both are true

 Object.instance_of? Object  Class.instance_of? Object 

Both are false. right, nothing can be instance of object.

And

 Class.kind_of? Class  Object.kind_of? Class 

both are true

 Class.kind_of? Object  Object.kind_of? Object 

both are true

So both are exactly same, then why do we have both these.?

After some more digging, i wrote this simple method to return method list supported by both

irb(main):054:0> def print_methods(obj) irb(main):055:1>    obj.methods.each do |mm| irb(main):056:2*        puts mm irb(main):057:2>    end irb(main):058:1> end 

Only method difference between print_methods(Object) and print_methods(Class) is

     Nesting 

if Nesting means inheritance, Is Object similar to the sealed class??

Can someone clarify me what is all this?

Update: To Edds comment

Interestingly i see lot of differences in the method list in

c=Class.new print_methods(c) 

&

o=Object.new print_methods(o) 

Now I understand Instance of a class is really an class instance (And this class instance is actually a Object) not an object instance. And even this instance allow me to span another instances

  xx = c.new //works - c is an Object / and xx is a instance of an Object c   yy = o.new //nope  - o is already a instance of an Object, so it cannot be instantiated again 

So Finally, Object is really an instance of a Class. Because

  xx.is_a? Class  

is false, but

  xx.is_a? Object  

returns true

Am i right, ??

Answer 1

Basically the key thing to understand is that every class is an instance of the Class class and every class is a subclass of Object (in 1.8 - in 1.9 every class is a subclass of BasicObject). So every class is an object in the sense that it is an instance of a subclass of Object, i.e. Class.

Of course this means that Class is an instance of itself. If that makes your brain hurt, just don't think about it too deeply.

Object and Class are is_a? Object

x.is_a? y returns true if x.class == y or x.class < y, i.e. if x's class is y or x's class inherits from y. Since every class inherits from object x.is_a? Object returns true no matter what x is. (In 1.8 anyway, in 1.9 there's also BasicObject which is now the most basic class in the inheritance hierarchy).

They are also is_a? Class

Both Object and Class are indeed classes, so that should not be surprising.

They are also instance_of? Class, but not instance_of? Object.

Unlike is_a?, x.instance_of? y only returns true if x.class == y, not if x.class is a subclass of y. So since both x and y are instance_of? Class, they're not instance_of? Object.

right, nothing can be instance of object.

That's not true. Object.new.instance_of? Object is true.

kind_of?

kind_of? is an alias for is_a?, so see above.

So both are exactly same, then why do we have both these.?

It should be pointed out that everything up to now is true for all classes. E.g. String.is_a? Object, String.is_a? Class and String.instance_of? Class are true and String.instance_of? Object is false for the same reasons as above. (Also String.is_a? String and String.instance_of? String are both false for the same reasons - String is a class, not a string).

You can not conclude from this that all the classes are the same. They're just all instances of the same class.

Comparing methods

Since both Object and Class are classes, they both have all the instance methods defined by Class. Class additionally has the singleton method nesting. nesting tells you which module you're currently nested in, it has nothing to do with inheritance.

For any given class TheClass.methods will return the instance methods defined by Class (e.g. superclass, which returns the class which TheClass inherits from, and new which creates a new instance of TheClass) plus the singleton methods defined by that class.

Anyway methods only tells you which methods can be called directly on a given object. It does not tell you which methods can be called on an instance of a class. For that you can use instance_methods, which returns significantly different results for Object and Class.

Answer 2

In Ruby, everything is an Object including classes and modules. Object is the most low-level class (well, in Ruby 1.9.2 there is also BasicObject but this is an other story).

See the following output.

> Object.ancestors # => [Object, Kernel, BasicObject]  > Class.ancestors # => [Class, Module, Object, Kernel, BasicObject]  > Module.ancestors # => [Module, Object, Kernel, BasicObject]  > String.ancestors # => [String, Comparable, Object, Kernel, BasicObject] 

As you can see, both Class and Module inherits from Object.

Back to your original assertions, you have to understand the difference bewteen

• is_a?
• kind_of'
• instance_of?

They are not interchangeable. is_a? and kind_of? returns true if other is the same class or an ancestor. Conversely, instance_of? returns true only if other is the same class.

> Class.is_a? Object # => true  > Class.kind_of? Object # => true  > Class.instance_of? Object # => false