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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?= =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: Parameterless generic instance in place of opaque type instance
Date: Sun, 28 Jul 2013 09:26:44 +0200
Organization: Ada @ Home
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Date: 2013-07-28T09:26:44+02:00
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Le Sun, 28 Jul 2013 09:02:22 +0200, Yannick Duch=C3=AAne (Hibou57)  =

<yannick_duchene@yahoo.fr> a =C3=A9crit:
> Say a GADO still defines an ADT, with an unknown discriminant, so that=
  =

> it cannot be instantiated without initialization. What the private  =

> declares, is just an access type to a deferred type (defined in the  =

> body). The body defines an instance of the type it fully defines. The =
 =

> package offers a single function returning an (opaque) access to this =
 =

> unique and per=E2=80=91paquage instance, instance. The function could =
be named  =

> `Instance_Reference`, to clearly show it's always the same behind,  =

> however a new distinct one per generic instance (not strictly a  =

> singleton, a per=E2=80=91package singleton).

I should have posted an Ada sample, to be more meaningful. Here is:


     generic
     package Samples is
        type Reference_Type (<>)
           is private;
        function Reference
           return Reference_Type;
        procedure Operation
          (Reference : Reference_Type);
     private
        type Data_Type;
        type Reference_Type
           is not null access all Data_Type;
     end;

     package body Samples is
        type Data_Type
           is record
              null;
           end record;
        Data : aliased Data_Type;
        function Reference
           return Reference_Type
           is (Data'Access);
        procedure Operation
          (Reference : Reference_Type)
        is begin
           null;
        end;
     end;

     procedure Use_A_Sample
     is
        package Sample
           is new Samples;
        Reference :
           Sample.Reference_Type :=3D
             (Sample.Reference);
        procedure Receive_A_Parameter
          (Reference : Sample.Reference_Type)
        is begin
           Sample.Operation (Reference);
        end;
     begin
        Receive_A_Parameter (Reference);
     end;


-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity