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From: jimr@col.hp.com (Jim Rogers)
Newsgroups: comp.lang.ada
Subject: Please explain why this is wrong...
Date: 7 Feb 1995 23:01:48 GMT
Organization: HP Colorado Springs Division
Message-ID: <3h8u4s$ea3@nonews.col.hp.com>
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Date: 1995-02-07T23:01:48+00:00
List-Id: <comp.lang.ada>

I have been playing with simple inheritance models.  When I try to declare
a type as:

  type Cube is new Square with null record;

I get the following message from the gnatf tool:

shapes.ads:87:09: type must be declared abstract or else "new_square" overriden

Why do I get this message.  I think I am using the same syntax I read in the
Instr.ads file found in the GNAT examples directory.

The source for my code follows:

-- file shapes.ads
-----------------------------------------------------------------------------
-- Experimenting with the simple designs of Ada shapes classes
-----------------------------------------------------------------------------

with Ada.Numerics.Aux;
use  Ada.Numerics.Aux;

package shapes is

-----------------------------------------------------------------------------
-- Abstract definition for shape
-----------------------------------------------------------------------------
   
   type shape is abstract tagged private;

   function area( Object : shape) return double is abstract;

   function get_name( Object : shape ) return string is abstract;

-----------------------------------------------------------------------------
-- Definition of rectangle as an inheritor of shape
-----------------------------------------------------------------------------

   type rectangle is new shape with private;

   function new_rectangle( height : double; width : double ) return rectangle;

   function area ( Object : rectangle ) return double;

   function get_name (Object : rectangle ) return string;


-----------------------------------------------------------------------------
-- Definition of circle as an inheritor of shape
-----------------------------------------------------------------------------

   type circle is new shape with private;

   function new_circle( Radius : double ) return circle;

   function area ( Object : circle ) return double;

   function get_name ( Object : circle ) return string;


-----------------------------------------------------------------------------
-- Definition of square as an inheritor of rectangle
-----------------------------------------------------------------------------

   type square is new shape with private;

   function new_square ( Side : double ) return square;

   function area ( Object : square ) return double;

   function get_name ( Object : square ) return string;

-----------------------------------------------------------------------------
-- Definition of square as an inheritor of rectangle
-----------------------------------------------------------------------------

   type cube is new square with private;

   function new_cube (Side : double) return cube;

   function volume ( Object : cube ) return double;

   function get_name ( Object : cube ) return string;

private

   type shape is tagged null record;

   type rectangle is new shape with record
      height : double := 0.0;
      width  : double := 0.0;
   end record;

   type circle is new shape with record
      radius : double := 0.0;
   end record;

   type square is new shape with record
      side   : double := 0.0;
   end record;

   type cube is new square with null record;

end shapes;


-- file shapes.adb
with Ada.Numerics.Aux;
use  Ada.Numerics.Aux;

package body shapes is

   function new_rectangle( height : double; width : double ) return rectangle is
      tmp : rectangle;
   begin
      tmp.height := height;
      tmp.width  := width;
      return tmp;
   end new_rectangle;

   function area ( Object : rectangle ) return double is
   begin
      return Object.height * Object.width;
   end area;

   function get_name (Object : rectangle ) return string is
   begin
      return "rectangle";
   end get_name;


   function new_circle( Radius : double ) return circle is
      tmp : circle;
   begin
      tmp.radius := radius;
      return tmp;
   end new_circle;

   function area ( Object : circle ) return double is
   begin
      return double(3.14159) * Object.radius * Object.radius;
   end area;

   function get_name ( Object : circle ) return string is
   begin
      return "circle";
   end get_name;


   function new_square ( Side : double ) return square is
      tmp : square;
   begin
      tmp.side := side;
      return tmp;
   end new_square;

   function area ( Object : square ) return double is
   begin
      return Object.side * Object.side;
   end area;

   function get_name ( Object : square ) return string is
   begin
      return "square";
   end get_name;


   function new_cube ( Side : double ) return cube is
      tmp : cube;
   begin
      tmp.side := side;
      return tmp;
   end new_cube;

   function volume ( Object : cube ) return double is
      S : double := Object.side;
   begin
      return S * S * S;
   end volume;

   function get_name ( Object : cube ) return string is
   begin
      return "cube";
   end get_name;

end shapes;


--
------------------------------------------------------------------------------
| Jim Rogers                    | Dead Reckoning:                            |
| Hewlett-Packard Company       | Traditional form of rough-estimate         |
| Colorado Springs Division     | navigation used for hundreds of years by   |
|                               | sailors, almost all of whom are dead.      |
| jimr@col.hp.com               |                                            |
------------------------------------------------------------------------------