thanks for responses On Jan 21, 4:24 pm, Peter Vandenabeele <pe...@vandenabeele.com> wrote: > 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 Vandenabeelehttp://twitter.com/peter_vhttp://rails.vandenabeele.com
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