From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-0.8 required=5.0 tests=BAYES_00,INVALID_DATE
	autolearn=no autolearn_force=no version=3.4.4
Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!cbatt!ihnp4!qantel!lll-lcc!lll-crg!nike!ucbcad!ucbvax!telesoft.UUCP!gary
From: gary@telesoft.UUCP (Gary Dismukes @lizard)
Newsgroups: net.lang.ada
Subject: Re: "=" for limited private access types
Message-ID: <8609250236.AA02292@sdcsvax.UCSD.EDU.ucsd.edu>
Date: Wed, 24-Sep-86 15:10:55 EDT
Article-I.D.: sdcsvax.8609250236.AA02292
Posted: Wed Sep 24 15:10:55 1986
Date-Received: Tue, 30-Sep-86 06:51:40 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
List-Id: <comp.lang.ada>


Implementing equality for limited private access types
------------------------------------------------------

The same technique that is used to implement equality for a limited private
type whose full type is derived from type INTEGER can be used when the full
type is an access type (or, in fact, any type for which "=" is defined):


    package P is

      type LP is limited private;

      function "=" ( LEFT, RIGHT: LP ) return BOOLEAN;

    private

      type ACCESS_TYPE is access SOME_TYPE;
      
      type LP is new ACCESS_TYPE;

    end P;

 
    package P is

      function "=" ( LEFT, RIGHT: LP ) return BOOLEAN is
      begin
        return ACCESS_TYPE( LEFT ) = ACCESS_TYPE( RIGHT );
      end "=";

    end P;


It is just a matter of introducing a type via a separate type declaration,
and then deriving from that type in the private type's full type declaration.

>From your example it appears that you actually want to define equality for
your limited type to be equality of the designated objects.  This can be
done by implementing the "=" function as follows:

    function "=" ( LEFT, RIGHT: LP ) return BOOLEAN is
    begin
      If ACCESS_TYPE( LEFT ) = null or ACCESS_TYPE( RIGHT ) = null then
        return FALSE;
       -- or perhaps: return ACCESS_TYPE( LEFT ) = ACCESS_TYPE( RIGHT );
      else
        return LEFT.all = RIGHT.all;
      end if;
    end "=";

Also, for efficiency, one might want to compare LEFT and RIGHT directly in
the else part before comparing the designated objects in the case of objects
that are large:
 
    return ACCESS_TYPE( LEFT ) = ACCESS_TYPE( RIGHT )
             or else LEFT.all = RIGHT.all;



Gary Dismukes
TeleSoft

gary@telesoft.uucp                          ucbvax--|   hp-sdd--|
telesoft!gary@ucsd.arpa                     decvax--+--sdcsvax--+--telesoft
                                            ihnp4--|  noscvax--|