[Caml-list] prevent more than one object

[Caml-list] prevent more than one object

[Mirko Aigner]

Hi !

I'm looking for a way to prevent that from a class more than one object 
is builded. Therefor I wrote this little files

(*singleton.ml*)
class singleton x =
   object
     val mutable data : int = x;
     method get_data = data;
     method set_data newdata = data <- newdata;
   end;;

let instance = new singleton 0

let get_instance = instance

(*singleton.mli*)
class singleton :
   int ->
   object
     method get_data : int
     method set_data : int -> unit
   end;;

val get_instance : singleton

The problem is the class is still public and instances can be build, any 
way to prevent this ?!

thx for helping
Mirko Aigner

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Re: [Caml-list] prevent more than one object

[Nick Name]

On Wed, 02 Jul 2003 18:14:23 +0200
Mirko Aigner <Mirko.Aigner@alcatel.de> wrote:

> 
>  The problem is the class is still public and instances can be build,
>  any way to prevent this ?!

I would make the class private in the module and supply only access
functions; if you need it to be a class you should expose only the class
type, but there is Oo.copy so your value will never be granted unique.

Try this

module M : sig
  class type singleton = object
    method f : int
  end

  val x : singleton
end = 
struct
  class singleton = object
    method f = 0
  end

  let x = new singleton
end

You can't create a new singleton with "new" but still can do that with
Oo.copy M.x. It is better to completely hide the type if you can, as in

module M : sig
  type singleton

  val x : singleton
end = struct ...

Vincenzo

-- 
Ho dato al mio dolore la forma di parole abusate
che mi prometto di non pronunciare mai più
[CSI]

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Re: [Caml-list] prevent more than one object

[John Max Skaller]

Mirko Aigner wrote:

> Hi !
> 
> I'm looking for a way to prevent that from a class more than one object 
> is builded. 


Why? Its a design error. Fix your design.

There's never any need for a singleton object,

because there is no resource for which there
cannot be more than one. I have seen (C++) designs
using singletons and they create an utter mess
because someone wrongly assumed something was
unique.

If you want one object, define the class in your
mainline, create it, and then coerce it to
a shared class type which does not provide a constructor.

-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850


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Re: [Caml-list] prevent more than one object

[Mirko Aigner]

John Max Skaller wrote:

> Why? Its a design error. Fix your design.
> 
> There's never any need for a singleton object,
> 
> because there is no resource for which there
> cannot be more than one. I have seen (C++) designs
> using singletons and they create an utter mess
> because someone wrongly assumed something was
> unique.
> 
> If you want one object, define the class in your
> mainline, create it, and then coerce it to
> a shared class type which does not provide a constructor.
> 

Maybe you're right, but this is not the right place to discuss the 
usefullnesss of the design patterns described by the GoF.
Vincente and Wagner describe in their paper, that singleton pattern 
could be implemented with modules. 
sdg.lcs.mit.edu/~dnj/6898/projects/vicente-wagner.pdf

I was testing it and got some problems, mentioned it here and got some 
hints how to solve the problem.

THANKS to all helping me with it!!

Mirko Aigner


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