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From: nr@hart.Princeton.EDU (Norman Ramsey)
Newsgroups: comp.lang.modula3
Subject: Specializing objects
Message-ID: <6929@rossignol.Princeton.EDU>
Date: 14 Feb 91 21:02:31 GMT
Sender: news@cs.Princeton.EDU
Organization: Dept. of Computer Science, Princeton University
Lines: 27


Suppose I have two object types

TYPE
  T = OBJECT
        mumble, fritz: INTEGER;
      END;

  TA = T OBJECT
         a: BOOLEAN;
       END;

and suppose further that I am handed a T and want to produce a TA with
the property that every field of the TA has the same value it had in
the T.  I can't think of anything any better than:

PROCEDURE Narrow(o:T):TA =
BEGIN
  RETURN NEW(TA, mumble := o.mumble, fritz := o.fritz, a := <whatever>);
END Narrow;

Is there some way for me to do this without having to enumerate all
the fields of T?  If not, I am bound to break my programs by extending
T and forgetting to update Narrow.
--
Norman Ramsey
nr@princeton.edu