Re: [Caml-list] How safe is caml_get_public_method() to check if an object has a method?

[Goswin von Brederlow <goswin-v-b@web.de>]

On Mon, Mar 07, 2016 at 02:05:48PM +0900, Jacques Garrigue wrote:
> On 2016/03/07 11:43, Goswin von Brederlow wrote:
> > 
> > Hi,
> > 
> > /usr/include/caml/mlvalues.h says:
> > 
> > CAMLextern value caml_get_public_method (value obj, value tag);
> > /* Called as:
> >   caml_callback(caml_get_public_method(obj, caml_hash_variant(name)), obj) */
> > /* caml_get_public_method returns 0 if tag not in the table.
> >   Note however that tags being hashed, same tag does not necessarily mean
> >   same method name. */
> > 
> > In QT5 the classes have virtual methods that can be overloaded. I'm
> > considering allowing the ocaml classes to have methods by the same
> > name. The C glue would then check if the ocaml class has such a method
> > and call it. Otherwise the original method from QT5 would be called.
> > 
> > The question now is wether that is save to do? Is it save as long as
> > the names produce unique hash_variant values? Or are the hash tables
> > in objects only parts of the bits of the hash_variant and therefore
> > collide far more often (given a non-existant method name)?
> 
> 
> The function used is exactly hash_variant, so if it gives different results on all
> your method names, this is safe.
> For debugging purposes, I'm also considering adding the method names
> as string to the vritual method table. It could help, but you can already extract
> all the ocaml method names from your program, and check that there is
> no discrepancy. (The probability of having a conflict is very low.)
> 
> Jacques

That is good to know. Thx.

MfG
	Goswin