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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Experimenting with the OOP features in Ada
Date: Wed, 4 Jan 2017 13:44:28 +0100
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Date: 2017-01-04T13:44:28+01:00
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On 2017-01-04 13:18, Laurent wrote:

> Other thing I don't understand:
>
> For the persistent storage solution I need those handles.

Actually the base type for persistent objects is 
Object.Archived.Deposit. Object.Entity is merely a reference-counted 
object. Object.Archived.Deposit is that plus operations to 
serialize/deserialize and handle inter-object dependencies/references.

> So I tried to adapt
> the Base_Types package by copying the Safe_Strings example.
>
> with My_Strings;
> with My_Strings.Handle;
> with Object;
>
> package Base_Types is
>
>    type Object is abstract new Object.Entity with private;
>
>    subtype Name_Type is My_Strings.Handle.My_Safe_String;
>    subtype Code_SIL_Type is My_Strings.Handle.My_Safe_String;
>
>    procedure Set_Name (Item : in out Object; Name : String) is abstract;
>    procedure Set_Code_SIL (Item : in out Object; Code_SIL : String) is abstract;
>
>    function Name_Value (Item : Object) return String is abstract;
>    function Code_SIL_Value (Item : Object) return String is abstract;
>
> private
>
>    type Object
>    is abstract new Object.Entity with
>       record
>          Name     : Name_Type;
>          Code_SIL : Code_SIL_Type;
>       end record;

If you are not going to change string components you could simply do

   type Object (Name_Length : Positive; Code_SIL_Length : Positive) is
      abstract new Object.Entity with
   record
      Name     : String (1..Name_Length);
      Code_SIL : String (1..Code_SIL_Length);
   end record;

>    type Base_Type_Ptr is access Object'Class;
>
> end Base_Types;
>
> There seems to be no place where I can insert an tagged keyword?

Object.Entity and Object.Archived.Deposit are already tagged.

> When I compile this I get a few errors:
>
> and some more of the following but they are just a symptom not the cause:
>
> test.adb:16:23: invalid prefix in selected component "Test_Antibiotic"
> test.adb:16:38: prefixed call is only allowed for objects of a tagged type
>
> base_types.ads:7:32: object "Object" cannot be used before end of its
> declaration
> base_types.ads:22:20: type "Object" cannot be used before end of its declaration
> base_types.ads:28:33: tagged type required, found type "Object" defined at line
> 21

The package name Object gets hidden by the declaration the type Object. 
Change it to:

   type Object is abstract new Standard.Object.Entity ...

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de