From: Robert Klemme <bob.news@...>
Date: 2004-08-10T15:46:16+09:00
Subject: Re: stupid question: Object#name


"Benny" <linux@marcrenearns.de> schrieb im Newsbeitrag
news:2npjbbF3ak29U1@uni-berlin.de...
> <snip: everything other is discussed widely to my
> full satisfaction :) >
> Robert Klemme wrote:
>
> >>> o1 = Class.new
> > => #<Class:0x1018ab58>
> >>> o2 = o1.new
> > => #<#<Class:0x1018ab58>:0x10188d70>
> >>> o2.class
> > => #<Class:0x1018ab58>
> >>> p1 = Object.new
> > => #<Object:0x10185b30>
> >>> p2 = p1.new
> > NoMethodError: undefined method `new' for #<Object:0x10185b30>
> > from (irb):6
> >
> > You can create instances of a class but not of an object.
> class C1
>   include Singleton
> end
> p2 = C1.new #=> NoMethodError: private method `new' called...
>
> You cannot create instances of some classes as well. Now is a singleton
>  class more like a class or more like an object?

A singleton class is a special class.  You can create at least one
instance from it although not directly.  But it is nevertheless a class.
It doesn't loose the capability of creating instances just because you
can't access it directly:

>> require 'singleton'
=> true
>> class C1
>>   include Singleton
>> end
=> C1
>> C1.instance
=> #<C1:0x10170a90>
>> C1.instance
=> #<C1:0x10170a90>
>> C1.instance.id
=> 134972744
>> c1 = C1.instance_eval { new }
=> #<C1:0x100dbef8>
>> c1.id
=> 134668156
>> c1.class
=> C1
>> c2 = C1.instance_eval { new }
=> #<C1:0x100c29c8>
>> c2.id
=> 134616292
>> c2.class
=> C1
?> c1 == C1.instance
=> false
>> c2 == C1.instance
=> false
>> c1 == c2
=> false

> I am not yet convinced that the differences are that big.
> ok, its called in another manner but largely behaving the same way. why
> should Object not just be another special class as C1 is?

The instance "Object" is a class.  But instances of Object are not classes
and hence cannot create instances (see above).  While the object "Class"
is a class as well as instances that are created from it; so they can
create new instances.

Regards

    robert