CODEWITHSUNDEEP
ruby
ovhaag
ovhaag

Reputation: 1278

why does Object.superclass.respond_to? :object_method ?

I am confused. When I define a method in Object I can call it in Objects superclass BasicObject!

Like so:

class Object
  def object_method
    "object_method called"
  end
end

Object.superclass.respond_to? :object_method
# => true

Object.superclass.object_method
# => "object_method called"

I have expected that only derived classes inherit the new method!

P.S.: I come to this question from an exercise on rubymonk

.. implement a method superclasses inside Object ..

where the recursion stop criterion is "affected".

Upvotes: 2

Views: 319

Answers (2)

Sergey Bolgov
Sergey Bolgov

Reputation: 806

When you call Object.superclass, you get an object that describes BasicObject class.

This object is an instance of Class class that inherits from Object. Therefore, it has all the methods that Object has, including the one you added.

However, instances of the BasicObject class do not have this method:

irb(main):122:0> Object.object_method
=> "object_method called"

irb(main):123:0> Object.new.object_method
=> "object_method called"

irb(main):124:0> Object.superclass.object_method  # Same as BasicObject.object_method
=> "object_method called"

irb(main):125:0> Object.superclass.new.object_method  # Same as BasicObject.new.object_method
NoMethodError: undefined method `object_method' for #<BasicObject:0x441743be>
        from (irb):125:in `evaluate'
        from org/jruby/RubyKernel.java:1065:in `eval'
        from org/jruby/RubyKernel.java:1390:in `loop'
        from org/jruby/RubyKernel.java:1173:in `catch'
        from org/jruby/RubyKernel.java:1173:in `catch'
        from ~/.rvm/rubies/jruby-1.7.0/bin/irb:13:in `(root)'

Here is more stuff to meditate on:

irb(main):129:0> Object
=> Object
irb(main):130:0> Object.class
=> Class
irb(main):131:0> Object.new.class
=> Object
irb(main):132:0> Object.superclass
=> BasicObject
irb(main):133:0> Object.superclass.class
=> Class
irb(main):134:0> Object.superclass.new.class
NoMethodError: undefined method `class' for #<BasicObject:0x1c944d4a>
        from (irb):134:in `evaluate'
        from org/jruby/RubyKernel.java:1065:in `eval'
        from org/jruby/RubyKernel.java:1390:in `loop'
        from org/jruby/RubyKernel.java:1173:in `catch'
        from org/jruby/RubyKernel.java:1173:in `catch'
        from ~/.rvm/rubies/jruby-1.7.0/bin/irb:13:in `(root)'

Have fun!

Upvotes: 3

oldergod
oldergod

Reputation: 15010

As you can see, it is a derived class.

#ruby 1.8.7
Object.superclass
# => nil
nil.kind_of? Object
# => true

#ruby 2.0.0
Object.superclass
# => BasicObject 
BasicObject.kind_of? Object
# => true

Upvotes: 3

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