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From: =?ISO-8859-15?Q?Markus_Sch=F6pflin?= <no.spam@spam.spam>
Newsgroups: comp.lang.ada
Subject: Access to procedure and generics
Date: Tue, 28 Aug 2012 11:46:41 +0200
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Date: 2012-08-28T11:46:41+02:00
List-Id: <comp.lang.ada>

Hello,

please consider the following program. What it is trying to achieve (in Ada 
95) is that a callback is registered for call as soon as the generic is 
instantiated.

---%<---
package CALLBACK is
    type CALLBACK_T is access procedure;
    procedure REGISTER_CALLBACK(CALLBACK : CALLBACK_T);
end CALLBACK;
--
package body CALLBACK is
    procedure REGISTER_CALLBACK(CALLBACK : CALLBACK_T)
    is
    begin
       null;
    end;
end CALLBACK;
--
generic
package USE_CALLBACK is
    procedure HANDLER;
end USE_CALLBACK;
--
with CALLBACK;
package body USE_CALLBACK
is
    procedure HANDLER is
    begin
       null;
    end;
begin
    CALLBACK.REGISTER_CALLBACK(HANDLER'Access);
end USE_CALLBACK;
--
with USE_CALLBACK;
procedure TEST is
    package FOO is new USE_CALLBACK;
begin
    null;
end TEST;
--->%---

When trying to compile this with GNAT 4.4.5 in Ada95 mode, I get the following 
error:

 > gnatmake -gnat95 test
gcc-4.4 -c -gnat95 test.adb
use_callback.adb:10:38: 'Access attribute not allowed in generic body
use_callback.adb:10:38: because access type "CALLBACK_T" is declared outside 
generic unit (RM 3.10.2(32))
use_callback.adb:10:38: move 'Access to private part, or (Ada 2005) use 
anonymous access type instead of "CALLBACK_T"

Now, after reading the reference cited by GNAT I think I understand why it 
doesn't compile, but I have a hard time figuring out what GNAT means by "move 
'Access to private part".

And I'm still at a loss on how to achieve what I actually need, namely 
automatically registering a callback as soon as the generic is instantiated.

Any help would be much appreciated.

Regards,
Markus