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Date: Tue, 8 Sep 2015 08:10:38 -0700 (PDT)
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Subject: working with diferent instances of the same generic interface
From: Aitor Alcrudo Sangros <aitor.alcrudo@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:27743
Date: 2015-09-08T08:10:38-07:00
List-Id: <comp.lang.ada>

Hi, my google-fu can't find how to do this. I think it should be doable but I'm not 100% sure. I'm thankful if anyone helps.

I have a generic interface some of my tagged records instance, similar to:

package Intro is
   type Data_type is interface;

   generic
      type Readable_Data is abstract new Data_type with private;
   package Readable is
      type Readable_Element is interface;
      type Readable_Element_Access is access all Readable_Element'Class ;
      function Get_Values (this : Readable_Element ) return Readable_Data'Class
                         is abstract;
   end Readable;

   type state is (OK,KO);

     generic
      with package Readable_Reporting is new Readable(<>);
      use Readable_Reporting ;
   package Reporting is
      type Reporting_Element is interface and Readable_Element ;
      type Reporting_Element_Access is access all Reporting_Element'Class;
      function Report (This :Reporting_Element  ) return state is Abstract;
      overriding function Get_Values (this : Reporting_Element ) return
          Readable_Data'Class is abstract;
   end Reporting;
end Intro;

This has worked with no problem. 
I instance it like this
   SomeData is new Data_type with record
     a :float;
   end record;

   package Readable is new Intro.Readable(
      readable_data =>SomeData);
   package Reporting is new Intro.Reporting(Readable);

   type Element_type_A is new Readable.Readable_Element
     				and Reporting.Reporting_Element 
   						with record
      data :float;
   end record;
   type Element_type_A_Access is access all Element_type_A 'Class;

Now, I have something like this

package Reporter is
   package Readable_basic is new Intro.Readable(Data_type );
   package Reporting_basic is new Intro.Reporting(Readable);
   use Reporting;
   type manager is tagged private;
   procedure Add_Element( This : in out manager ;
                         Element : in Reporting_Element_Access );
private
   package Element_Vector is new
     Ada.Containers.Vectors (Natural,Reporting_Element_Access );
   use Element_Vector ;

   type manager  is tagged record
      Element_List : Element_Vector.Vector;
   end record;
end Reporter ;

this compiles.

Now I try to use it elsewhere

procedure try is
   man :manager ;
   elm1 : Element_type_A_Access ;
   elm2 : Element_type_B_Access ;
begin
   man.add(Reporter.Reporting_basic.Reporting_Element_Access(elm1)  );
   man.add(Reporter.Reporting_basic.Reporting_Element_Access(elm2)  );
end;

This compiles, but produces a runtime exception with invalid interface conversion.

Is there any way to get this to work?.