From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,
	T_FILL_THIS_FORM_SHORT autolearn=ham autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,f7a9613bbc2bd8c9
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 2002-05-13 08:20:14 PST
Path: 
 archiver1.google.com!news1.google.com!newsfeed.stanford.edu!logbridge.uoregon.edu!fu-berlin.de!uni-berlin.de!pec-19-25.tnt6.hh2.uunet.DE!not-for-mail
From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Generic default parameters
Date: Mon, 13 May 2002 17:21:23 +0200
Message-ID: <60lvdu4ifauunm4qob6ir7rlas4soe0cp4@4ax.com>
References: <MPG.1745f522613b2fb598968d@news.ip-plus.net>
 <e1uudust53uc95gecju9hujrjfrn3svemr@4ax.com> <ulmaop2px.fsf@gsfc.nasa.gov>
NNTP-Posting-Host: pec-19-25.tnt6.hh2.uunet.de (149.225.19.25)
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
X-Trace: fu-berlin.de 1021303209 20935476 149.225.19.25 (16 [77047])
X-Newsreader: Forte Agent 1.8/32.548
Xref: archiver1.google.com comp.lang.ada:23962
Date: 2002-05-13T17:21:23+02:00
List-Id: <comp.lang.ada>

On 13 May 2002 10:00:26 -0400, Stephen Leake
<stephen.a.leake.1@gsfc.nasa.gov> wrote:

>Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:
>
>> 7. Incomplete instantiations:
>> 
>>    package X is new Y (<>);
>>    --
>>    -- No actual parameters given.
>>    -- All declarations of the visible part of X
>>    -- are incomplete here in usual sense.
>>    -- I.e. if X declares a a type XX then
>>    --    type XX_Ptr is access X.XX;
>>    -- would be OK.
>> . . .
>> private
>>    package X is new Y (<parameter-list>);
>> 
>> The formal parameters which actuals are not visible in the public
>> part, shall be only used in the private part or the body of the
>> generic. The idea is to instantiate a generic with some private things
>> and yet to have the instance visible.
>
>This I'm less happy with. I think you can get the same effect with
>library instantiations and/or child packages.

I do not know how. Moreover I suppose that there should be no way,
otherwise, "private" would be leaky, which we definitely do not want
to have.

A concrete example. Let we want to implement smart pointers / handles.
It could go as follows:

package Object is
   type Entity is new
      Ada.Finalization.Limited_Controlled with
   record
      Use_Count : Natural := 0; -- Reference count
   end record;
...
end Object;

generic
   type Object_Type (<>) is abstract new Entity with private;
   type Object_Type_Ptr is access all Object_Type'Class;
package Object.Handle is
   type Handle is tagged private;
...
private
   type Handle is new Ada.Finalization.Controlled with record
      Ptr : Object_Type_Ptr := null;
   end record;
end Object.Handle;

Now the problem, how to instantiate Object.Handle having Object_Type
private? Let we have:

package IO is
...
private
   type File_Descriptor is new Entity with ...;
   type File_Descriptor_Ptr is access all File_Descriptor'Class;
end IO;

package IO.Handles is
   package Handles is
       new Object.Handle (File_Descriptor, File_Descriptor_Ptr);
           -- Illegal, File_Descriptor is not visible here
   subtype IO_Handle is Handles.Handle;
   ...
end IO.Handles;

If we move the instantiation into the private part it would work, but
then it would be private. Once private, always private. With 7 it
could be:

package IO.Handles is
   package Handles is new Object.Handle (<>);
   subtype IO_Handle is Handles.Handle;
   ... -- Declaration of proxy operations on handles
private
   package Handles is
       new Object.Handle (File_Descriptor, File_Descriptor_Ptr);
end IO.Handles;

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de