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From: Simon Wright <simon.j.wright@amsjv.com>
Newsgroups: comp.lang.ada
Subject: Re: problem with abstract types
Date: 01 Nov 2002 12:05:08 +0000
Organization: Alenia Marconi Systems, ISD, Farlington
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Date: 2002-11-01T12:05:08+00:00
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Ulrich Eckhardt <uli.e@gmx.de> writes:

> Robert I. Eachus wrote:

> >   with registry; use registry;
> >   procedure main is
> >     declare
> >       reg := openRegistry(CLASS_USER,"registrytest");
> 
> It's not so easy. My function get_Registry sould estimat what
> type of registry should be used (mybe i want to add later on
> also an LDAP, Database or some sort of pluggable support). So it
> seems that i have to use access parameters for this.

Reg was of type (access to) Registry'Class, which means that the
actual value can be of whichever type you decide at run time.

But to make this work you have to do things the right way: the
"proper" idiom is to initialize using a function,

   declare
      Reg : Registry'Class := Open_Registry (...);
   begin

where you might need to include some indication of what sort of
registry to choose in the parameters to Open_Registry. Often people
use strings. This is a classic example of the Factory pattern ..