Re: Dispatching callback handed over to C

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

On 03.04.12 22:20, Randy Brukardt wrote:

> The whole idea of the void pointer is so anti-Ada that it is best to get rid
> of it as early as you can. (There is no such thing as "untyped data" in Ada,
> and, as Dmitry likes to point out, not in real life, either. There is only
> data for which the type is not known at compile-time, and that sort of data
> is a problem - best to avoid it.)

Just to mention that there is more to (void*) than "untyped data"!
The C standard guarantees certain properties of objects when
the programmer applies C's type conversion rules in many
directions. (void*) is not a demolition operation.

There is also a ptrdiff_t both in C and in Ada, the ptrdiff_t rules
again implying anything but untyped data. And the definitions of
either language's type system seem to join here and there.

Unchecked_Union works for discriminated records, but it does not
extend to the union of tag-discriminated records of some R'Class.

I see that Maciej's solution has two hooks in it that I would hope are
hooks for addressing the concerns. They both have body visibility only.
One declares subtype Void_Ptr is System.Address, so
System.Address can be replaced with something that better matches O-O
arguments of type (void*). And the solution has a pointer translation
mechanism involving an instance of System.Address_To_Access_Conversions.

What else can we do to enable O-O mechanics in existing C libraries?

(If there isn't anything Ada can do, then I-can-emulate-all-your-language
in my not-that-many-CVE-generating C programs, so why don't you just make
your great Ada language work with today's libraries after only 30 years...
;-)