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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
Lines: 66


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

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