Date: Thu, 11 Aug 2005 08:12:58 -0300
From: romildo at uber.com.br
To: Remi Vanicat <remi.vanicat at gmail.com>
Cc: lablgtk at math.nagoya-u.ac.jp
Subject: Re: [newbie] Creating widget classes
Message-ID: <20050811111257.GA7991 at malaquias>
References: <20050811042345.GA29721 at malaquias> <6b8a914205081102267481b29b at mail.gmail.com>
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On Thu, Aug 11, 2005 at 11:26:27AM +0200, Remi Vanicat wrote:
> 2005/8/11, romildo@uber.com.br <romildo@uber.com.br>:

> > If I am not wrong, the button variable introduced
> > by the let binding is common to all instances
> > of the class (class variable).
> 
> Well, the fact is that you are wrong. Those let binding let you define
> variable that are bound before the intialization of the object, when
> the instance variable are bound just before intialization method.
> Otherwise, they (instance variable and let binding before the object
> keyword) are private to each object.

I am only partial wrong regarding the sharing of a variable binded
in a let expression prior to the object keyword. As is shown by the
attached test program. Its output is:

  $ ./t1
  x1 has counter = 1
  y1 has counter = 2
  x1 has counter = 3
  y1 has counter = 4
  x2 has counter = 1
  y2 has counter = 1
  x2 has counter = 2
  y2 has counter = 2

If the class has no parameter,

  class myclass =
    let myvar = ... in
  object
    ...
  end

then the let is evaluated only once, when the class declaration is
evaluated. In this case the class constructor is constant,
and myvar is shared with all instances of myclass.

That was the only test I have made before sending my prior
message to the listing, and it led me to a wrong conclusion.

But if there are any class parameters,

  class myclass mypar =
    let myvar = ... in
  object
    ...
  end

then the class constructor is not constant. It is a function and
should be applied to an argument to create an object. In this
case the let is evaluated only when the class constructor is
aplied to some arguments to build an instance of the class.
This way the variable myvar cannot be shared by the instances
of the class.

I hope I got things right now.

Regards.

Romildo

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class c1 =
  let counter = ref 0 in
object
  val mutable name = ""
  method set_name new_name =
    name <- new_name;
  method bump =
    counter := !counter + 1;
    print_string (name ^ " has counter = ");
    print_int (!counter);
    print_newline ()
end

class c2 () =
  let counter = ref 0 in
object
  val mutable name = ""
  method set_name new_name =
    name <- new_name;
  method bump =
    counter := !counter + 1;
    print_string (name ^ " has counter = ");
    print_int (!counter);
    print_newline ()
end

let main () =
  let x1 = new c1 in
  let y1 = new c1 in
  let x2 = new c2 () in
  let y2 = new c2 () in
    x1#set_name "x1";
    y1#set_name "y1";
    x2#set_name "x2";
    y2#set_name "y2";
    x1#bump;
    y1#bump;
    x1#bump;
    y1#bump;
    x2#bump;
    y2#bump;
    x2#bump;
    y2#bump

let _ = main ()

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