Re: Efficency in OCaml

[Nicolas Ollinger <nollinge@cs.utu.fi>]

On Thu, 2 Sep 1999, Jerome Vouillon wrote:

> On Wed, Sep 01, 1999 at 02:40:21PM -0400, chet@watson.ibm.com wrote:
> > Is there a description of the Ocaml object and
> > "virtual-function-table" format?

(snip description of buckets use)

I played a little with objects representation in OCaml 2.xx. As far as
I understand, at least in bytecode, class instances are represented as
boxed values tagged object_tag with n+2 fields : then first field is the
method array array described by Jerome in last mail, the second field
seems to be a unique id associate to the object, other fields are used
for instance variables in the order of declaration, inherited variables
first. As the method array array is unique for each class, it can be used
to identify the class (notice that classes are represented as global
variables). I'm intrigued by this second field, what is the use of this
id ? Where is the necessity to identify uniquely every object ?

Concerning marshaling of objects, a simple solution is to use a function
like:

let crunch o =
  let r = Obj.dup (Obj.repr o) in
  let idclass = compute_id (Obj.field r 0) in
  Obj.set_field r 1 idclass;
  Obj.set_tag r Obj.marshaled_object_tag
  r;;

with compute_id a function that deduce a unique class id of the object.

Then unmarshaling is just doing the inverse operation. Of course, if
you want to share objects between different programs then you must add
some informations about the module in which the class is declared, and
so one.

Any comment ?

N.
--