From: Mark Hubbart <discord@...>
Date: 2004-08-10T01:43:45+09:00
Subject: Re: stupid question: Object#name


On Aug 9, 2004, at 7:21 AM, Benny wrote:

> Robert Klemme wrote:
>
>>
>> "Benny" <linux@marcrenearns.de> schrieb im Newsbeitrag
>> news:2npa39F2ppenU1@uni-berlin.de...
>>> hi all,
>>>
>>> its time for stupid questions again :)
>>>
>>> is there a way to get the name of an object, e.g.
>>>
>>> test = MyClass.new
>>> test.name #=> "test"
>>
>> That's not the name of the object but the name of a variable that
>> references the object.  What would you expect to be the name in this 
>> case:
>>
>> test = MyClass.new
>> t2   = test
>> # name?
>>
> ok, see my other posting:
>
> "substitute Object#name with Object#names
> in my first posting"
> so it would be
>
> test = MyClass.new
> t2   = test
> t2.names #=> ["t2", "test"]
> test.names #=> ["t2", "test"]

This isn't exactly what you want, but it's as close as you can come, 
currently, in ruby. Without a whole bunch of ugliness, anyway.

   def names(obj, bind)
     vars = eval("local_variables", bind)
     vars.select{|var| eval(var, bind).id == obj.id}
   end
     ==>nil
   a = 2.3
     ==>2.3
   b = a
     ==>2.3
   c = 2.3
     ==>2.3
   names(a, binding)
     ==>["a", "b"]
   names(c, binding)
     ==>["c"]
   names(2.3, binding)
     ==>[]

Now I suppose it would be possible to do something like this (untested):

   def add_binding(bind)
     $stored_bindings ||= []
     $stored_bindings << bind
   end

   def do_something
     add_binding(bind)
     #... do other stuff
   end

then...

   def find_names(obj)
     variable_names = []
     $stored_bindings.each do |bind|
        vars = eval("local_variables", bind)
        variable_names << vars.select{|v| eval(v, bind).id == obj.id}
     end
   end

Now, you can call find_names on any object that is in a binding that 
you have indexed.

Still, though, for tis to be a complete solution, you woiuld have to 
come up with a way to index the @vars and @@vars, $globals and 
CONSTANT_VARS, as well. I have a feeling that all this infrastructure 
would cause a considerable slowdown, though.


>>> I know we have it for classes (and yes: I could use a subclass 
>>> instead)
>>
>> That's a special case.
> yes I know: class names are constants
>
>> Consider also:
>>
>>>> class Foo;end
>> => nil
>>>> Foo
>> => Foo
>>>> f = Foo
>> => Foo
>>>> f.name
>> => "Foo"
>>>> f == Foo
>> => true
>>
> I dont get you here :(
> I think as names of classes a constants and names of variables are 
> symbols
> they are a entirely different thing (in the way ruby handles it)
> I only made the statement with the subclass to get no such answer from 
> the
> list ("use a subclass if you need names"). and as I didnt want to use
> constants I also didnt want to use a subclass  :)
>
>>> but is there a way to do it with objects?
>>
>> No.
> from my other posting:
>
> "I think in the kernel we have symbols attached to object-ids (do we?)
> so why not have a method to show us the symbols to a corresponding
> object_id?"

If I understand correctly, they are stored that way in the internal 
symbol table. That symbol table is inaccessible, though. Once you are 
in the runtime, the name -> obj correlations are spread out in various 
constructs. Some are easier to get at than others; it's easy to lookup 
a constant, but local variables are truely a pain.


>>> btw. why didn't matz make Class and Object be the same thing?
>>
>> Err...  Because they are not the same concept.
> if they behave mostly the same way in ruby
> it might be reflected behind the scenes.
> are the differences in the abilities of Object and
> Class so big that they legitimate hard distinction in the
> interpreter? or are we loosing something if Object and Class
> is based on the same fundamentals apart from syntactical restrictions
> and the separated name spaces (constants vs. symbols)?

Class is a template for creating objects. Object is a subclass of 
class. This is a fairly common OO topic, and there is a pretty good 
description of classes and objects here:

http://www.visibleworkings.com/little-ruby/Chapter3.pdf



>> Object is an instance of
>> Class (as Class is also, which sometimes confuses people).  Object 
>> defines
>> everthing an instance is capable of (ok, together with Kernel, but 
>> that
>> contains mostly methods that one uses as functions).  Could be that 
>> the
>> self referenciality confused you - sometimes it's hard to grasp. :-)
>
> I think its not confusing me on the contrary: I even vote for 
> narrowing the
> differences.
>
>
>> Maybe you should get yourself a book about programming languages 
>> concepts
>> and / or about OO in special.  Just an idea.
> I read the whole pragmatic programmer (1st ver.) and "Programmieren mit
> Ruby" /R�hrl/Schmiedl/Wyss several times and I thought it would be
> sufficient. but perhaps you have a good recommendation for a pure OO 
> book.
>
> kind regards,
>
> benny
>