Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!ames!decwrl!sdd.hp.com!ucsd!nosc!cod!sampson
From: sampson@cod.NOSC.MIL (Charles H. Sampson)
Newsgroups: comp.lang.ada
Subject: Re: limited private types, "=" operator
Message-ID: <2036@cod.NOSC.MIL>
Date: 4 Aug 90 00:01:13 GMT
References: <1152@cs.nps.navy.mil>
Reply-To: sampson@cod.nosc.mil.UUCP (Charles H. Sampson)
Organization: Computer Sciences Corporation
Lines: 35

In article <1152@cs.nps.navy.mil> erickson@cs.nps.navy.mil (David Erickson) writes:
>Is there any way to define "=" for a limited private type which is an
>access type (which may be null)?  The problem arises trying to test for
>null without producing an infinitely recursive definition.

     This is a special case of the problem of breaking the recursion when
defining an overloaded operator on a private type.  The LRM contains an
example, 7.4.2(13).  In that example, the private type was realized as a
named type (Integer) and the name was used to convert from the private
type to a type different from that of the function's arguments.  In your
case, the realization of your private type has no name, in the straight-
forward implementation, so you have to circle around the problem in order
to give it a name:

   ...
   TYPE Ltd_pvt IS LIMITED PRIVATE;
   ...
PRIVATE
   ...
   TYPE Type_name IS ACCESS List_link;
   TYPE Ltd_pvt IS NEW Type_name;
   ...
END PACKAGE ...;

PACKAGE BODY ...
   ...
   FUNCTION "="(L, R : Ltd_pvt) RETURN Boolean IS

   BEGIN
      ...
      IF Typ_name(L) = NULL THEN ...
      ...
   END "=";

                               Charlie Sampson