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

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

cl1 wrote:
> 
> 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:

If the access values come from Ada, it gets a bit more complicated. 
However, C pointers are convertible from one type to another, IIRC, so 
that works for you. You need to declare a convention-C access type for 
the type you use, store the 'access in one of those, then unchecked 
convert that value to the void pointer type. A generic can make that easier:

with Ada.Unchecked_Conversion;
...
type Void_Ptr is ...
...
generic -- Void_Conversion
    type T (<>) is limited private;
package Void_Conversion is
    type T_Ptr is access all T;
    pragma Convention (C, T_Ptr);

    function To_Void_Ptr is new Ada.Unchecked_Conversion
    (Source => T_Ptr, Target => Void_Ptr);
end Void_Conversion;

Then you can do

package C_Float_Convert is new Void_Conversions (T => C_Float);

T_Ptr : constant C_Float_Convert.T_Ptr := My_C_Float'access;

My_Void_Ptr : Void_Ptr := C_Float_Convert.To_Void_Ptr (T_Ptr);

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

That's usually a good thing.

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