Rails - Jan 2012 - why doesn't an instance of Object get Class's new instance method?

"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
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). 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, and hence that''s why we can say Object.new, since Class class
defines "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 Clas, not visa versa.

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>


When we instantiate B, the instance gets B''s instance methods. But why
doesn''t it get Class''s instance method (new)?

B inherits from Object which inherits from Class. It should have
looked up scope chain until it reached Class, since Object is an
instance of Class and therefore inherits from it.

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Let me phrase question differently, after finding out that new is
actually a class method of Class class, as is all methods of Class
class.

m = Class.new

creates an m object that has access to Class''s class methods. So
"new"
created an object that has access to class methods in this case.

Then why:

n = m.new

doesnt "new" create an object that gives n access to m''s
class
methods, as it did in the other case?

On Jan 21, 3:03 pm, John Merlino <stoici...-YDxpq3io04c@public.gmane.org>
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
> 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). 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, and hence that''s why we can say Object.new, since Class
class
> defines "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 Clas, not visa versa.
>
> 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>
>
> When we instantiate B, the instance gets B''s instance methods. But
why
> doesn''t it get Class''s instance method (new)?
>
> B inherits from Object which inherits from Class. It should have
> looked up scope chain until it reached Class, since Object is an
> instance of Class and therefore inherits from it.
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On 21 January 2012 21:06, John Merlino
<stoicism1-YDxpq3io04c@public.gmane.org> wrote:
> Then why:
>
> n = m.new
>
> doesnt "new" create an object that gives n access to m''s
class
> methods, as it did in the other case?
>
> On Jan 21, 3:03 pm, John Merlino
<stoici...-YDxpq3io04c@public.gmane.org> wrote:
>> This fails:
>> 1.9.2p290 :007 > B.new
>> NoMethodError: undefined method `new'' for
#<Object:0x007ff597d27820>
>>
>> When we instantiate B, the instance gets B''s instance methods.
But why
>> doesn''t it get Class''s instance method (new)?
>>
>> B inherits from Object which inherits from Class. It should have
>> looked up scope chain until it reached Class, since Object is an
>> instance of Class and therefore inherits from it.
You''re getting "Class" and "class", and
"Object" and "object" confused.
".new" is a class method of Class, not an instance method of Class. So
when you instanciate an object, you can''t call ".new" on it.
Although
you could call "B.class.new".

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On Sat, Jan 21, 2012 at 9:03 PM, John Merlino
<stoicism1-YDxpq3io04c@public.gmane.org> 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

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thanks for responses

On Jan 21, 4:24 pm, Peter Vandenabeele
<pe...-jNuWw7i2w7syMbTcgqFhxg@public.gmane.org>
wrote:
> On Sat, Jan 21, 2012 at 9:03 PM, John Merlino
<stoici...-YDxpq3io04c@public.gmane.org> 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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