On Sat, Jan 21, 2012 at 9:03 PM, John Merlino <stoici...@aol.com> wrote:
> "Object is the root of Ruby's class hierarchy. Its methods are > available to all classes unless explicitly overridden." > > Wouldn't Class class be at the root of the class hierarchy? > > > After all, look at this: > > 1.9.2p290 :006 > Object.instance_of? Class > => true > > Object is an instance of class, after all we can use one of Class' > instance methods on Object: > > 1.9.2p290 :017 > Object.new > => #<Object:0x007faad047ecd0> > > > > But Class is not instance of Object: > > 1.9.2p290 :007 > Class.instance_of? Object > => false > You might look at the difference between "instance_of?" and "kind_of?" to understand this. peterv@ASUS:~$ irb 001:0> Class => Class # the Class constant is defined 002:0> Class.class => Class # it's class is Class 003:0> Class.instance_of? Object => false # it is not an instance of the class Object 004:0> Class.kind_of? Object => true # but ... the class of Class (Class that is), is derived from Object 005:0> Class.ancestors => [Class, Module, Object, Kernel, BasicObject] # and here you can see ancestor chain (proving that Class, as the class of Class, is derived from Object). # Since newer rubies (1.9.3 I believe ??) There is even a BasicObject on top of the chain # (Object would not longer be at the top/root of the chain). 006:0> Class.object_id => 74033900 # this object_id is an example of a method that is available on Class, but really inherited from Object. 007:0> class C 008:1> end => nil 009:0> C.respond_to?(:new) => true # Now when we create an instance 'C' of class Class (that is we define a class C) # that instance has the method "new" (that is why we can call 'new' on a class) 010:0> module M 011:1> end => nil 012:0> M.respond_to?(:new) => false # But when we create an instance 'M' of class Module (that is we define a # module M) that does not have the method "new" available. This is another proof that Class is not at the top/root of the hierarchy. If that was the case, then _all_ objects (also a Module instance M) would have to inherit the 'new' method from Class. But really all objects (except instances of BasicObject have the object_id method). 016:0> bo = BasicObject.new (Object doesn't support #inspect) => 017:0> bo.object_id NoMethodError: undefined method `object_id' for #<BasicObject:0x8f90224> from (irb):17 from /home/peterv/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>' 1.9.2p290 :008 > defined? Class > => "constant" > 1.9.2p290 :009 > Class.class > => Class > 1.9.2p290 :010 > Object.class > => Class > > Class is an instance of Class. That is, Class is an instance of > itself. And therefore "Class" just is (kind of like the idea if > something stems from something else, how was the very first thing > created - it just was). It is not because the class of Class is Class, that Class cannot have a different superclass. Maybe you are confusing the relationship "is an instance of" with the relationship "superclass". And yes, all classes (and also Class itself) point to Class for "is an instance of". But in the superclass hierarchy, Class is certainly not at the top. I actually think it is a leaf, since an experiment to make a subclass of "Class" failed ... 021:0> class Test < Class 022:1> puts "a derived class of Class" 023:1> end TypeError: can't make subclass of Class from (irb):21 from /home/peterv/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>' Class is just an internal construct, built > part of the language for templating. > > So Object is a constant that represents an object allocated in memory. > When it's methods are searched for it looks up the scope chain, first > at the singleton class (just in case any methods are extended on > Object) and after that, it looks at its immediate parent which is Class, I think that is incorrect: 027:0> Object.superclass => BasicObject # the superclass of Object is BasicObject > and hence that's why we can say Object.new, since Class class > defines "new". > The reason we can say Object.new is because the class of Object is Class and that class Class has defined "new". 048:0> Object.class => Class 049:0> Object.methods.grep(/new/) => [:new] For a module, e.g. that does not work: 053:0> module M 054:1> end => nil 055:0> M.methods.grep(/new/) => [] > > Now this is the interesting part. There obviously is a difference > between Object and Class. As already stated, Object is an instance of > Class and therefore inherits from Class, not visa versa. > Check the difference between the "class" (is an instance of) and the "superclass" relationship. > The difference is made clear as shown below: > > We have a constant "A". We want that constant to be a class, so we do > it easily: > > A = Class.new > > We know that A is a class because we can run A.new. > > However, when we create an object: > > B = Object.new > > This fails: > 1.9.2p290 :007 > B.new > NoMethodError: undefined method `new' for #<Object:0x007ff597d27820> > That just works here ... 058:0> B = Object.new => #<Object:0x9013cdc> 059:0> B.class => Object 060:0> B.superclass NoMethodError: undefined method `superclass' for #<Object:0x9013cdc> from (irb):60 from /home/peterv/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>' > > > When we instantiate B, the instance gets B's instance methods. But why > doesn't it get Class's instance method (new)? > I don't understand this ... > > B inherits from Object B is an instance of Object, it does not "inherit" from it. On B, there is no "superclass" method (only classes have a superclass method). HTH, Peter -- Peter Vandenabeele http://twitter.com/peter_v http://rails.vandenabeele.com -- You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to rubyonrails-talk@googlegroups.com. To unsubscribe from this group, send email to rubyonrails-talk+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en.
