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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Convert between C "void*" pointer and an access
Date: Thu, 12 Oct 2017 09:05:00 +0100
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Xref: news.eternal-september.org comp.lang.ada:48439
Date: 2017-10-12T09:05:00+01:00
List-Id: <comp.lang.ada>

Victor Porton <porton@narod.ru> writes:

> I NEED to convert between access to tagged records (and access to
> class-wide types) and C void* pointers.
>
> I need this because it is C's way to pass objects by using void* and I
> need to pass Ada objects to C functions (not just C objects).

What we think you need to do is to pass a class-wide pointer to a C
function so that it can do something with it.

One would hope that you could say e.g.

      type T is tagged null record;
      type T_P is access all T'Class with Convention => C;
      procedure Put (Obj : T_P)                 -- <<< warning here
      with
        Import,
        Convention => C,
        External_Name => "demo_put";
      function Get return T_P                   -- <<< warning here
      with
        Import,
        Convention => C,
        External_Name => "demo_get";
      type U is new T with null record;
      O : aliased U;
   begin
      Put (O'Access);
      declare
         Ret : constant T_P := Get;
      begin
         Put_Line ("ret's class is " & Ada.Tags.Expanded_Name (Ret'Tag));
      end;

and that the compiler would refuse to compile the code if it couldn't
map the type's conventions as specified.

Trying this on macOS results in warnings

   class_pointers.adb:6:19: warning: "Put.Obj" involves a tagged type which
    does not correspond to any C type
   class_pointers.adb:11:13: warning: return type of "Get" does not
    correspond to C type

but the resulting executable works as expected. (You would have expected
a similar warning on the type definition; perhaps this is a bug?)

In the past I've had success by specifying the size of the classwide
type (this doesn't suppress the warnings).