General speaking, itâ??s about the relationship between virtual
class(metaclass ) and Class.
Itâ??s all in chapter 24.
In Figure 24.2, Dave gives the relationship between object
instance(a_guitar), its class(Guitar) and its metaclass(Guitar). . Then in section â??Class and Module Definitionsâ? , as we see in the code example, any class (for example, class Test) is a object instance of class Class. So I think Class is a metaclass of any class. Since Guitar is a
metaclass of Guitar, can I think Guitar` is a subclass of Class. If my
understanding is right, I think itâ??s better Dave can add this
relationship in Figure 24.2 and make it clearer.
Thanks in advance
uncutstone
The following is quoted from ruby-1.8.4-core-rdocs
Classes in Ruby are first-class objectsâ??each is an instance of class Class.
All metaclasses are instances of the class `Classâ??.
This makes me confused. In my understanding, all metaclasses are
subclass of the class ‘Class’ as I posted in previous post.
Somebody can explain it?
Thanks in advance.
uncutstone
uncutstone wu wrote:
The following is quoted from ruby-1.8.4-core-rdocs
Classes in Ruby are first-class objectsâ??each is an instance of class Class.
All metaclasses are instances of the class `Classâ??.
This makes me confused. In my understanding, all metaclasses are
subclass of the class ‘Class’ as I posted in previous post.
My new understanding is: any metaclass(singleton class of a class
object) is a subclass of the class ‘Class’ and also a instance of the
class ‘Class’. Am I right?
I write a snippet of code to test this understanding:
class Object
def metaclass
class <<self
self
end
end
end
class AClass
end
puts AClass.metaclass.class
puts AClass.metaclass.superclass
puts AClass.metaclass.ancestors
the result:
Class
#Class:Object
Class
Module
Object
Kernel
It almost proves my understanding but there is still something I don’t
fully understand. It is line 2 of the result, say, #Class:Ojbect.
Somebody can explain?
Thanks in advance.
uncutstone
Maybe this helps:
/*
* Ruby's Class Hierarchy Chart
*
* +------------------+
* | |
* Object---->(Object) |
* ^ ^ ^ ^ |
* | | | | |
* | | +-----+ +---------+ |
* | | | | |
* | +-----------+ | |
* | | | | |
* +------+ | Module--->(Module) |
* | | ^ ^ |
* OtherClass-->(OtherClass) | | |
* | | |
* Class---->(Class) |
* ^ |
* | |
* +----------------+
*
* + All metaclasses are instances of the class `Class'.
*/
T.
P.S. Recently noticed Smalltalk uses the term metaclass. I wonder why
singleton took it’s place?
unknown wrote:
P.S. Recently noticed Smalltalk uses the term metaclass. I wonder why
singleton took it’s place?
Only classes have metaclasses, but any object can have a singleton
class. And although singleton classes of classes are similar to
metaclasses in some ways, they are not quite the same thing as the
Smalltalk metaclasses.
– Jim W.
uncutstone wu wrote:
puts AClass.metaclass.class
puts AClass.metaclass.superclass
puts AClass.metaclass.ancestors
the result:
Class
#Class:Object
Class
Module
Object
Kernel
Can somebody explain:
Why AClass.metaclass.superclass gets “#Class:Object”?
Why AClass.metaclass.ancestors gets “[Class,Module,Object,Kernel]”?
Acctually, I understand this one.
Why ancestor’s doesn’t include superclass?
Thanks in advance.
uncutstone
“u” == uncutstone wu [email protected] writes:
u> Why AClass.metaclass.superclass gets “#Class:Object”?
What do you expect ?
u> Why ancestor’s doesn’t include superclass?
#ancestors don’t include singleton classes
Guy Decoux
“u” == uncutstone wu [email protected] writes:
u> Thanks very much. I think I already understand it now.
Well, if you understand it, try it with ruby 1.8.4 
You use ruby 1.8.2, no ?
Guy Decoux
ts wrote:
“u” == uncutstone wu [email protected] writes:
u> Why AClass.metaclass.superclass gets “#Class:Object”?
What do you expect ?
u> Why ancestor’s doesn’t include superclass?
#ancestors don’t include singleton classes
Thanks very much. I think I already understand it now.
uncutstone
“u” == uncutstone wu [email protected] writes:
u> I think it is because 1.8.2 and 1.8.4 have different object models.
no, the object model is not changed.
the problem is here
moulon% ./ruby -ve ‘a = Object.new; p class << a; self end.superclass’
ruby 1.8.2 (2004-12-25) [i686-linux]
Object
moulon%
moulon% /usr/bin/ruby -ve ‘a = Object.new; p class << a; self
end.superclass’
ruby 1.8.4 (2005-12-24) [i486-linux]
#Class:Object
moulon%
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.2 (2004-12-25) [i686-linux]
#Class:Object
moulon%
moulon% /usr/bin/ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.4 (2005-12-24) [i486-linux]
#Class:Class
moulon%
trying to correct a bug has perhaps introduced a problem
Guy Decoux
ts wrote:
“u” == uncutstone wu [email protected] writes:
u> Thanks very much. I think I already understand it now.
Well, if you understand it, try it with ruby 1.8.4
You use ruby 1.8.2, no ?
Guy Decoux
Yes, I use 1.8.2.
And I try it in 1.8.4 and get “#Class:Class” from
AClass.metaclass.superclass instead of “#Class:Object”.
I think it is because 1.8.2 and 1.8.4 have different object models.
What is the defference between object models of ruby 1.8.2 and 1.8.4?
Where can I find this kind of information?
Thanks in advance.
uncutstone
ts wrote:
“u” == uncutstone wu [email protected] writes:
u> I think it is because 1.8.2 and 1.8.4 have different object models.
no, the object model is not changed.
the problem is here
moulon% ./ruby -ve ‘a = Object.new; p class << a; self end.superclass’
ruby 1.8.2 (2004-12-25) [i686-linux]
Object
moulon%
moulon% /usr/bin/ruby -ve ‘a = Object.new; p class << a; self
end.superclass’
ruby 1.8.4 (2005-12-24) [i486-linux]
#Class:Object
moulon%
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.2 (2004-12-25) [i686-linux]
#Class:Object
moulon%
moulon% /usr/bin/ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.4 (2005-12-24) [i486-linux]
#Class:Class
moulon%
trying to correct a bug has perhaps introduced a problem
So, you mean this is a bug of version 1.8.4.
Best Regards,
uncutstone
ts wrote:
“u” == uncutstone wu [email protected] writes:
u> So, you mean this is a bug of version 1.8.4.
Well, for me this is strange
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.4 (2006-06-19) [i686-linux]
#Class:Class
moulon%
I prefer when ruby say this
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.9.0 (2006-06-20) [i686-linux]
#Class:Object
moulon%
Guy Decoux
It’s reasonable. I agree with you.
Best regards,
uncutstone
“u” == uncutstone wu [email protected] writes:
u> So, you mean this is a bug of version 1.8.4.
Well, for me this is strange
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.8.4 (2006-06-19) [i686-linux]
#Class:Class
moulon%
I prefer when ruby say this
moulon% ./ruby -ve ‘class A < Object; end; p class << A; self
end.superclass’
ruby 1.9.0 (2006-06-20) [i686-linux]
#Class:Object
moulon%
Guy Decoux