Re: Dispatching callback handed over to C

[Natasha Kerensikova <lithiumcat@gmail.com>]

On 2012-04-03, Maciej Sobczak <see.my.homepage@gmail.com> wrote:
> On 3 Kwi, 09:19, Natasha Kerensikova <lithium...@gmail.com> wrote:
>
>> The problem I'm facing currently is binding to a very common idiom for
>> callback in C.
>
> [...]
>
> I had to solve this poblem in the context of the YAMI4 poject and I
> managed to get the essential parts of the solution here:
>
> http://inspirel.com/articles/Polymorphic_Callbacks_Ada_Cpp.html

Thanks a lot for the solution.

It turns out this is almost what my tentative solution looked like, the
only step I missed was to use System.Address to carry the data part,
instead of some access type.

But still, I can't help but think there is one needless subprogram call
in that solution. With a way to obtain the address of the actual
subprogram the dispatching call would dispatch to, the address could be
handed over to C code so that it can call it directly.

> The callback, as seen by Ada, is polymorphic (the user sees it as a
> dipatching call). The solution does not involve any compiler-specific
> magic any more than what could be expected, so should be quite
> portable.

Actually it uses the assumption that System.Address and void* have the
same representation.

I wonder how reliable the assumption is.



Thanks for your insights,
Natasha