From: "Eustáquio Rangel de Oliveira Jr." <eustaquiorangel@...>
Date: 2005-07-04T00:16:26+09:00
Subject: Re: class variable leading a double life

Hey Robert!

> Let me add to that that usually you want to associate a variable with 
> the class instance or with instances.  Class variables (the ones with 
> @@) have some strange properties that sometimes lead to surprising 
> behavior.  I generally recommend to not use them.  Instance variables in 
> classes are much simpler to handle and much clearer and cleaner IMHO.

Just to make things more clear,

irb(main):001:0> class Test
irb(main):002:1>    @@v1 = 0    # class variable?
irb(main):003:1>    @v2  = 1    # ???
irb(main):004:1>    def initialize
irb(main):005:2>       @v3 = 2  # instance variable?
irb(main):006:2>    end
irb(main):007:1>    def one
irb(main):008:2>       @@v1
irb(main):009:2>    end
irb(main):010:1>    def two
irb(main):011:2>       @v2
irb(main):012:2>    end
irb(main):013:1>    def three
irb(main):014:2>       @v3
irb(main):015:2>    end
irb(main):016:1> end
=> nil
irb(main):017:0> t = Test.new
=> #<Test:0xb7dc6574 @v3=2>
irb(main):018:0> t.one
=> 0
irb(main):019:0> t.two
=> nil
irb(main):020:0> t.three
=> 2

What is the correct names of v1, v2 and v3? v1 is a class variable, but 
what about v2 and v3? We can say that v3 belongs to the class instance 
(we saw that after creating the class v3 is on the output 
<Test:0xb7dc6574 @v3=2>)on t, but v2 belongs to the class Test?

The answer is the difference to create the variables just after the 
class statement and before the initialize method OR create it in the 
initialize method, right?

And v2 is not reached by the two method, but if I change the class and 
insert

irb(main):026:0> class Test
irb(main):027:1>    def self.four
irb(main):028:2>       @v2
irb(main):029:2>    end
irb(main):030:1> end

and use

irb(main):032:0> t.class.four
=> 1

I can reach it. So where v1 and v2 exactly are named and belongs to? I 
can see that each t haves it's own v3, but still didn't get the way the 
or ones are. How v1 and v2 are located on different places.
Kind of

+-------+     +-----+
| Test  +-----+  t  |
+-------+     +-----+
| @@v1? |     | @v3 |
| @v2?  |     +-----+
+-------+

Maybe v1 and v2 are on the same place (the Test instance - v1 needs to 
be shared by all Test's, right?) but just can be reached on different 
ways (v2 using four)?

On the pickaxe we have an example

class SongList
   MaxTime = 5*60           #  5 minutes
   def SongList.isTooLong(aSong)
     return aSong.duration > MaxTime
   end
end

On this case, MaxTime resides on the same place that v2 or not? Because 
if I change SongList and add

class SongList
   def isTooLong(aSong)
     return aSong.duration > MaxTime
   end
end

and use

s = SongList.new
s.isTooLong(<song here>)

I have a valid result, but something like

irb(main):057:0> class SongList2
irb(main):058:1>    @maxtime = 5*60
irb(main):059:1>    def SongList2.isTooLong(t)
irb(main):060:2>       return t > @maxtime
irb(main):061:2>    end
irb(main):062:1>    def isTooLong(t)
irb(main):063:2>       return t > @maxtime
irb(main):064:2>    end
irb(main):065:1> end
=> nil
irb(main):066:0> s2 = SongList2.new
=> #<SongList2:0xb7d733d8>
irb(main):067:0> s2.isTooLong(301)
ArgumentError: comparison of Fixnum with nil failed
         from (irb):63:in `>'
         from (irb):63:in `isTooLong'
         from (irb):67
         from (null):0
irb(main):068:0> SongList2.isTooLong(301)
=> true
irb(main):069:0> SongList2.isTooLong(30)
=> false

gives me that error, so there is also a scope difference on a constant 
like MaxTime and a variable like @maxtime, defined on the class body?

Thanks! :-)

----------------------------
Eust�quio "TaQ" Rangel
eustaquiorangel@yahoo.com
http://beam.to/taq
Usu�rio GNU/Linux no. 224050