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From: Simon Wright <simon.j.wright@amsjv.com>
Newsgroups: comp.lang.ada
Subject: Re: problem with abstract types
Date: 31 Oct 2002 15:31:23 +0000
Organization: Alenia Marconi Systems, ISD, Farlington
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Date: 2002-10-31T15:31:23+00:00
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Ulrich Eckhardt <uli.e@gmx.de> writes:

> 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
>    [..]

[...]

> 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

reg'access is an access-to-registryAccess, while openRegistry requires
a registry, not an access-to-registryAccess or even a registryAccess,
so your call should have read

   openRegistry (reg.all, err);

Some people would declare your openRegistry as (pardon my Ada-isation
of your naming conventions):

   procedure Open (Reg : access Registry; ...);

in which case you could have said

   Registry.Open (Reg);

Passing a parameter called err to openRegistry implies you aren't
thinking of using exceptions, this is one place where you definitely
should.

It seems needlessly painful for users to make them use unbounded
strings in the getRegistry call, a plain string would be much easier.