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Date: Thu, 31 Oct 2002 13:06:53 +0100
From: Ulrich Eckhardt <uli.e@gmx.de>
Organization: Private at home
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Subject: problem with abstract types
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Date: 2002-10-31T13:06:53+01:00
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Hi,

i am starting my first project in ada (A complete rewrite
of WHFC, see my Homepage). For this i want to
write an OS independend way to store configuration
informations (on windows in the registry and on unix in
human readable config files).

Therefore my idea was to created a package registry wich defines
an abstract type registry and several abstract functions
and (at this time) one real function .

package registry is
   type registry is abstract tagged limited private;
   type registry_Access is access all registry'Class;

   procedure openRegistry(reg : in out registry; ..) is abstract;
   function getRegistry( .. ) return registry_Access
   [..]

Also i have created a child package registry.unix,
which implements the abstract functions for using on
unix:

  package registry.unix is
     type file_registry is new registry with private;
     procedure openRegistry(reg : in out file_registry; ..);  	
     [..]

Also i will create a package registry.win which does the
same for windows.
package registry.win is
     type win_registry is new registry with private;
     procedure openRegistry(reg : in out win_registry; ..);

The function getRegistry of the main package should return
and initialize now depending on the os a package registry.win
or registry.unix, since for the use of this package it
should not be visible on which platform it was compiled.

function getRegistry (a : in sys_access; p_name : in Unbounded_String)
   return registry_Access is
   r : registry_Access;
   begin
     if compiled = SYS_UNIX then
        r := new file_registry(s_access => a);
     else
        r := new win_registry();
     end if;
     return r;
end getRegistry;

So in a testprogramm i use :

with registry; use registry;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure main is
    reg : registry_Access;
    err : r_error;
begin
    reg := getRegistry(CLASS_USER,To_Unbounded_String("registrytest"));
    openRegistry(reg'access,err);
end main;

But when compiling this code i get a
main.adb:10:20: prefix of "access" attribute must be aliased

Any hints what i am missing, or comments how such things are better
done in ada?

Uli
-- 
Ulrich Eckhardt
http://www.uli-eckhardt.de