Hi all,

What I'm trying to achieve: add a variable to a class. Fill it with
data. Every nstance of that class should have access to the data in that
variable. But I want every instance be able to modify that variable,
without modifying the original value of the variable.

It's like adding methods to the singleton class of an instance (if I
explain it properly).

I think it becomes a lot clearer if you see the code I've written:
http://pastie.caboo.se/194982

I thought this guy described what I needed:
http://briancarper.net/2007/03/22/ruby-singleton-classes/
But even then I can't get it to work.

Thanks in advance!
Leon
-- 
Posted via http://www.ruby-forum.com/.

Hi --

On Sun, 11 May 2008, Leon Bogaert wrote:

> Hi all,
>
> What I'm trying to achieve: add a variable to a class. Fill it with
> data. Every nstance of that class should have access to the data in that
> variable. But I want every instance be able to modify that variable,
> without modifying the original value of the variable.

I'm not sure what you mean exactly. Do you mean each instance should
have a copy of the object? If so, I would just do:

class Tester

   def self.vars
     @vars ||= []
   end

   def self.add_var(var)
     self.vars << var
   end

   def vars
     @vars || self.class.vars
   end

   def add_var(var)
     @vars ||= self.class.vars.dup
     vars << var
   end
end

That gives the results you expect in your test.


David

-- 
Rails training from David A. Black and Ruby Power and Light:
   INTRO TO RAILS         June 9-12            Berlin
   ADVANCING WITH RAILS   June 16-19           Berlin
   INTRO TO RAILS         June 24-27           London (Skills Matter)
See http://www.rubypal.com for details and updates!

David A. Black wrote:
> Hi --
> 
> On Sun, 11 May 2008, Leon Bogaert wrote:
> 
>> Hi all,
>>
>> What I'm trying to achieve: add a variable to a class. Fill it with
>> data. Every nstance of that class should have access to the data in that
>> variable. But I want every instance be able to modify that variable,
>> without modifying the original value of the variable.
> 
> I'm not sure what you mean exactly. Do you mean each instance should
> have a copy of the object? If so, I would just do:
> 
> class Tester
> 
>    def self.vars
>      @vars ||= []
>    end
> 
>    def self.add_var(var)
>      self.vars << var
>    end
> 
>    def vars
>      @vars || self.class.vars
>    end
> 
>    def add_var(var)
>      @vars ||= self.class.vars.dup
>      vars << var
>    end
> end
> 
> That gives the results you expect in your test.
> 
> 
> David

Thanks David. That's a whole lot simpler. I think I'll stick with your 
example.
Except I think I'll merge @vars & self.class.vars each time vars() gets 
called. So you can update the original Tester class with new vars and 
that the instances have access to these new vars too.

Thanks!
-- 
Posted via http://www.ruby-forum.com/.

btw, you can simply do @@var to have a var that is shared among objects 
of the same class.
-- 
Posted via http://www.ruby-forum.com/.

On Sun, May 11, 2008 at 8:33 AM, Leon Bogaert <leon@tim-online.nl> wrote:
> David A. Black wrote:
>> On Sun, 11 May 2008, Leon Bogaert wrote:

>>> What I'm trying to achieve: add a variable to a class. Fill it with
>>> data. Every nstance of that class should have access to the data in that
>>> variable. But I want every instance be able to modify that variable,
>>> without modifying the original value of the variable.

>> I'm not sure what you mean exactly. Do you mean each instance should
>> have a copy of the object? If so, I would just do:
>>
>> class Tester
>>
>>    def self.vars
>>      @vars ||= []
>>    end
>>
>>    def self.add_var(var)
>>      self.vars << var
>>    end
>>
>>    def vars
>>      @vars || self.class.vars
>>    end
>>
>>    def add_var(var)
>>      @vars ||= self.class.vars.dup
>>      vars << var
>>    end
>> end
>>
>> That gives the results you expect in your test.


> Thanks David. That's a whole lot simpler. I think I'll stick with your
> example.
> Except I think I'll merge @vars & self.class.vars each time vars() gets
> called. So you can update the original Tester class with new vars and
> that the instances have access to these new vars too.

You might also want to have a look at the class_inheritable_accessor
and related methods from the activesupport gem (part of Rails), which
never seems to come up in similar threads on Ruby talk.  It sort of
does what you want, although it sets things up when a class is
subclassed and doesn't keep up with changes.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

On Sun, May 11, 2008 at 10:09 AM, Albert Schlef <albertschlef@gmail.com> wrote:
> btw, you can simply do @@var to have a var that is shared among objects
> of the same class.

There be dragons!  Class variables (@@var) in Ruby are also shared by
subclasses (with some quirks in some cases such as when a superclass
acquires a class variable AFTER one or more of its subclasses) and
violate the expectations of almost everyone who tries to use them.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

On May 10, 2008, at 6:40 PM, David A. Black wrote:
>>
>> Hi all,
>>
>> What I'm trying to achieve: add a variable to a class. Fill it with
>> data. Every nstance of that class should have access to the data in  
>> that
>> variable. But I want every instance be able to modify that variable,
>> without modifying the original value of the variable.
>
> I'm not sure what you mean exactly. Do you mean each instance should
> have a copy of the object? If so, I would just do:
>
> class Tester
>
>  def self.vars
>    @vars ||= []
>  end
>
>  def self.add_var(var)
>    self.vars << var
>  end
>
>  def vars
>    @vars || self.class.vars
>  end
>
>  def add_var(var)
>    @vars ||= self.class.vars.dup
>    vars << var
>  end
> end
>
> That gives the results you expect in your test.
>



careful, any ruby code that uses 'dup' or clone has serious bugs 9 out  
of 10 times, in the case it doesn't meet the OP's *requirements*  
(although the simple test case passes) at all:

cfp:~ > cat a.rb
#
# doesn't let instance modify the var without modifying the class var
#

   Tester.add_var 'oops'
   t = Tester.new
   t.vars.first << ', this modifies the original'
   p Tester.vars  #=> "oops, this modifies the original"


#
# and doesn't take a copy either
#
   Tester.vars.clear
   Tester.add_var ['oops']
   t = Tester.new
   t.vars.first << ', this modifies the original'
   p Tester.vars  #=> [["oops", ", this modifies the original"]]


BEGIN {
   class Tester
    def self.vars
      @vars ||= []
    end

    def self.add_var(var)
      self.vars << var
    end

    def vars
      @vars || self.class.vars
    end

    def add_var(var)
      @vars ||= self.class.vars.dup
      vars << var
    end
   end
}


cfp:~ > ruby a.rb
["oops, this modifies the original"]
[["oops", ", this modifies the original"]]




you really need one of two things: a factory or a smarter copy  
algorithm:

using a factory:

cfp:~ > cat a.rb

class Shares
   Factory = lambda { |var|
     case var.to_s
       when 'a'
         'not shared'
       when 'b'
         'but created fresh for class and instance cases'
       else
     end
   }

   def Shares.a
     @a ||= Factory['a']
   end

   def a
     @a ||= Factory['a']
   end
end

p Shares.a

shares = Shares.new
shares.a << ", but it's own copy"

p Shares.a
p shares.a


cfp:~ > ruby a.rb
"not shared"
"not shared"
"not shared, but it's own copy"



or, using a smart copy method:

cfp:~ > cat a.rb
class Shares
   def Shares.a
     @a ||= 'not shared'
   end

   def a
     @a ||= Marshal.load(Marshal.dump(Shares.a))
   end
end

p Shares.a

shares = Shares.new
shares.a << ", but it's own copy"

p Shares.a
p shares.a


cfp:~ > ruby a.rb
"not shared"
"not shared"
"not shared, but it's own copy"


this is nicer, but only works for objects you can marshal - obvisouly.


a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being  
better. simply reflect on that.
h.h. the 14th dalai lama