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From: nr@labrador.cs.virginia.edu (Norman Ramsey)
Subject: How to write TYPECASE in Ada 95?
Date: 1999/02/05
Message-ID: <79fct8$9k3$1@murdoch.acc.Virginia.EDU>#1/1
X-Deja-AN: 441078935
Organization: University of Virginia Computer Science
Newsgroups: comp.lang.ada
Date: 1999-02-05T00:00:00+00:00
List-Id: <comp.lang.ada>


I'm trying to figure out from the reference manual how to identify
which type extension of a type I have.  For example, if I have

   type Point is tagged
         record
           X, Y : Real := 0.0;
         end record;

   type Painted_Point is new Point with
         record
           Paint : Color := White;
         end record;


   type Tall_Point is new Point with
         record
           Height : Real := 0.0;
         end record;

I'd like to be able to discriminate at run time:

   p : Point;

     ...

   if p in Painted_Point then
      ... do something with Painted_Point'(p)
   else if p in Tall_Point then
      ... do something with Tall_Point'(p)

Even better would be something like Modula-3 TYPECASE:

  (* modula-3 *)
  TYPECASE p OF
  Painted_Point (painted) => ... do something with painted
  Tall_Point    (tail)    => ... do something with tall
    ...
  END

What's the idiomatic way to express this in Ada?


Norman

-- 
Norman Ramsey
http://www.cs.virginia.edu/~nr