mixed functional / object style

mixed functional / object style

[Guillaume Hennequin]

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Dear list,

this is a somewhat naive question
let's define

class a = object
 val mutable v = ...
 method v = v
 method m = something that uses v
end ;;

now assume that I want to create a lot of those a objects, so many that I
may encounter memory problems.

I thought the following would be better, memory wise, but when I test it
doesn't seem to be the case

class a = object
 val mutable v = ...
 method v = v
end ;;

and instead of each object having its own method m, I define it separately
let m x = something that calls x#v

This question is somewhat equivalent to: what is the memory consumption of a
simple method
method m = let _ = self#v in () ??

Thanks a lot
Guillaume.

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Re: [Caml-list] mixed functional / object style

[Jacques Garrigue]

From: Guillaume Hennequin <gje.hennequin@gmail.com>
> Dear list,
> 
> this is a somewhat naive question
> let's define
> 
> class a = object
>  val mutable v = ...
>  method v = v
>  method m = something that uses v
> end ;;
> 
> now assume that I want to create a lot of those a objects, so many that I
> may encounter memory problems.
> 
> I thought the following would be better, memory wise, but when I test it
> doesn't seem to be the case
> 
> class a = object
>  val mutable v = ...
>  method v = v
> end ;;
> 
> and instead of each object having its own method m, I define it separately
> let m x = something that calls x#v
> 
> This question is somewhat equivalent to: what is the memory consumption of a
> simple method
> method m = let _ = self#v in () ??

A better answer is that objects have a two-level structure.
They are represented by a record containing their fields, an object id
(use for comparison) and a pointer to a shared method table (share by
all objects generate from the same class).
So adding a method to a class does not change the memory consumption
for generated objects.

However, beware that the number of fields in an object is not only the
number of val in its class definition. Constructor arguments or
intermediate values generated by let statements are automatically
converted into implicit fields if they are used by methods or
initializers.

class c x y z =
  let t = y + z in
  object
    method x = x
    method t = t
  end
The above class has just two implicit fields, x and t.

Jacques Garrigue

Re: [Caml-list] mixed functional / object style

[Goswin von Brederlow]

Guillaume Hennequin <gje.hennequin@gmail.com> writes:

> Dear list,
> this is a somewhat naive question
> let's define
> class a = object
>  val mutable v = ...
>  method v = v
>  method m = something that uses v
> end ;;
> now assume that I want to create a lot of those a objects, so many that I may
> encounter memory problems.
> I thought the following would be better, memory wise, but when I test it
> doesn't seem to be the case
> class a = object
>  val mutable v = ...
>  method v = v
> end ;;
> and instead of each object having its own method m, I define it separately
> let m x = something that calls x#v
> This question is somewhat equivalent to: what is the memory consumption of a
> simple method
> method m = let _ = self#v in () ??
> Thanks a lot
> Guillaume.

A class is a reference to the virtual table of a class and its
instance variables. There is exactly one virtual table for every class
you declrare and all instances of that class share the same virtual
table. All the methods are listed in that one virtual table.

So a method will cost you 4 or 8 byte (or maybe more, don't know how
big the entry for a method is) but it will be something small and
const no matter how many instances you have.

MfG
        Goswin