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Newsgroups: comp.lang.ada
From: Anonymous Coward <bogus_addy@bogus_domain.net>
Subject: Re-exporting primitive operations of a private type (like "-",
 and 'min)
Message-Id: <slrndm8143.52o.bogus_addy@tango.mindfuq.org>
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Date: Sat, 29 Oct 2005 21:19:27 GMT
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Xref: g2news1.google.com comp.lang.ada:6047
Date: 2005-10-29T21:19:27+00:00
List-Id: <comp.lang.ada>

I'm bothered by the fact that the primitive operations of a parent are
not inherited in a private type.  In particular, the basic arithmetic
operations are lost, like "+", "-", 'min, 'max, etc.  There seems to
be no clean way to re-export these operations, or am I missing
something?  Here is some sample code:

[distance.ads]

   package Distance is

      type Public_Distance_Type is new Float;

      procedure Some_Procedure 
        (Sample_Variable : in Public_Distance_Type);


      type Private_Distance_Type is private;

      Zero_Distance  : constant Private_Distance_Type;
      Large_Distance : constant Private_Distance_Type;

      function "-" (Left, Right : in Private_Distance_Type)
        return Private_Distance_Type;

   private

      type Private_Distance_Type is new Public_Distance_Type;

      Zero_Distance  : constant Private_Distance_Type := 0.0;
      Large_Distance : constant Private_Distance_Type := 999_999_999.0;

   end Distance;

[distance.adb]

   package body Distance is

      function "-" (Left, Right : in Private_Distance_Type)
        return Private_Distance_Type is

      begin

         --This is ugly.  Isn't there a way to declare this
         --operation without redefining it?  The type casting
         --is also a nuissance, but the compiler warns of
         --infinite loops if casting is not used.

         return Private_Distance_Type(Public_Distance_Type(Left) -
                                      Public_Distance_Type(Right));

      end "-";

      procedure Some_Procedure (Sample_Variable : in Public_Distance_Type)is
      begin

         --This was another simple test to see whether we can
         --inherit explicitly declared public operations.

         null;

      end Some_Procedure;

   end Distance;

[use_distance.adb]

   with Distance;

   procedure Use_Distance is

      Public_Distance  : Distance.Public_Distance_Type;
      Private_Distance : Distance.Private_Distance_Type;

      use type Distance.Private_Distance_Type;
      use type Distance.Public_Distance_Type;

   begin

      Public_Distance := Distance.Public_Distance_Type'Min(-5.0,7.0);

      --The following line fails to compile because
      --'min is not inherited.
      --
      --Private_Distance := Distance.Private_Distance_Type'Min
      --  (Distance.Zero_Distance,Distance.Large_Distance);

      Distance.Some_Procedure (Sample_Variable => Public_Distance);

      --The following line fails to compile because
      --Some_Procedure is not inherited.
      --
      --Distance.Some_Procedure (Sample_Variable => Private_Distance);

      --This line compiles because it was redefined explicitly
      --
      Private_Distance := Distance.Large_Distance -
                          Distance.Zero_Distance;

   end Use_Distance;

---

Redefining "-" is an annoyance.  I'm not even sure how to redefine
attribute operations like 'min.  Suggestions?