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From: "Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>
Newsgroups: comp.lang.ada
References: <67ac8b2c.0305020925.6c27576d@posting.google.com>
Subject: Re: variant record and pointer
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Date: Fri, 02 May 2003 19:00:02 GMT
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Xref: archiver1.google.com comp.lang.ada:36883
Date: 2003-05-02T19:00:02+00:00
List-Id: <comp.lang.ada>

This is your question, but have you considered using tagged types instead of
variant records? You could declare your shape types as follows:

   type Shape;
   type Pointer is access all Shape'Class;

   type Shape is abstract tagged
      record
         Ptr     : Pointer;
         Special : Unbounded_String;
      end record;

   type Circle is new Shape with
      record
         Diameter : Natural;
      end record;

   type Square is new Shape with
      record
         Side : Natural;
      end record;

   type Triangle is new Shape with
      record
         Edge_1,
         Edge_2,
         Edge_3 : Natural;
      end record;

This would allow you to derive additional shapes (for example, Rectangle)
later, without impacting existing code. Moreover, you could write overloaded
subprograms that can dispatch at runtime, e.g.

    function Perimeter( X : in Shape ) return Float is abstract;

    function Perimeter( X : in Circle ) return Float;
    function Perimeter( X : in Square ) return Float;
    function Perimeter( X : in Triangle ) return Float;

    -- ...
    Pointer s;
    --

    Len := Perimeter( s.all );
    -- This calls the right Perimeter function,
    -- even if we cannot determine the type of s.all until runtime.