Re: generic package with procedure paramter gives "not subtype conformant with declaration"

["cl1" <charles.w.lambert@gmail.com>]

Jeffrey R. Carter wrote:
> cl1 wrote:
> > <snip>
>
> The problem is that System.Address is not guaranteed to be the same as a
> C pointer, so using it to interface with C makes your code compiler
> dependent. In all versions of GNAT that I've used and bothered to check,
> System.Address has been the same as a C pointer, but that could change
> at any time.

I did not know this. I am forever learning, it seems.

>
> For types, any convention-C access type will work for a void pointer
> when the values come from C and are never dereferenced by the Ada. I
> generally use "access all Integer" for that case, though some people
> prefer to have a void type to designate:
>
> type Void is null record;
> type Void_Ptr is access all Void;
> pragma Convention (C, Void_Ptr);

This is a very good concept. Is there a mechanism in ada to allow:

My_Void_Ptr := My_C_Float'Access;

where My_C_Float could be My_New_Type or My_Integer, etc.
I ask, because my code does this and i was using the System.Address as
a catch all. Once i store the access in the pointer i no longer need to
know
what type it is. I just need the reference to pass to the C code. For
instance:

package avcall is
type Argument is
  record
    Value_Address : System.Address;
  end record;
end avcall;

generic
  type Any_Type is private;
package avcall.register_type
  package Any_Type_Conversion is new
System.Address_To_Access_Conversions(Any_Type);
  type Argument_Instance is
    record
      Instance_Value : Any_Type_Conversion.Object_Pointer;
    end record;
    function Concat(AList : Var_Args; Arg : Any_Type) return Var_Args
is
      Info : Argument_Instance;
    begin
      Info.Instance_Value := Arg;
      Info.Value_Address :=
Any_Type_Conversion.To_Address(Info.Instance_Value);
      return AList;
    end;
end avcall.register_type;

This is how i'm currently doing things. How would i change my
Argument.Value_Address from System.Address type to something like the
Void_Ptr type you proposed? So that i can change:

Info.Value_Address :=
Any_Type_Conversion.To_Address(Info.Instance_Value);

to work with the Void_Ptr type.


>
> Since values of Void_Ptr are never dereferenced, the designated type
> doesn't matter.
>
> For subprograms, it's more complicated because you have to match the
> subprogram parameter and return type profile. In your case, though
> (IIRC), the subprograms all have the same profile (that of your generic
> formal subprogram), so the use of a convention-C access type seems to be
> possible.
>
> > i'm using GPS which has gcc version 3.4.6 on mac os x 10.4.8
>
> GPS is an IDE, not a compiler, but gcc 3.4.6 is; it's an Ada-95
> compiler. The compiler version is of interest because Ada 95's
> accessibility rules differ from Ada 0X's.
>
> > I agree with you.
> > However, I am confused about this. The procedure could be declared
> > anywhere. How
> > does the compiler know what scope the procedure that is supplied to the
> > generic package is in? I mean can it ever know? If not, that means
> > there
> > is no foreseeable fix to this issue from my point of view.
>
> The compiler can't know the scope of the actual procedure in general;
> that's why you can never store the 'access of a generic formal
> procedure. The language assumes the worst case for safety.

to quote another post in this thread:

-And you wanted to do Av_Param_Instance'Access inside the generic?
-That won't work, because the compiler can't check the rules about
-X'Access, because it doesn't know enough about the actual parameter
-passed to Av_Param_Instance.  The accessibility rules come to mind.
-Also, the fact that you can't do X'Access if X is intrinsic

Now that i know that is the rule. I not only understand what was
causing
the problem, but why.

>
> --
> Jeff Carter
> "Monsieur Arthur King, who has the brain of a duck, you know."
> Monty Python & the Holy Grail
> 09