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From: mfeldman@seas.gwu.edu (Michael Feldman)
Newsgroups: comp.lang.ada
Subject: Tagged type feature, or GNAT bug?
Date: 27 Sep 1994 12:36:30 -0400
Organization: George Washington University
Message-ID: <369hme$e9j@felix.seas.gwu.edu>
NNTP-Posting-Host: 128.164.9.3
Date: 1994-09-27T12:36:30-04:00
List-Id: <comp.lang.ada>

-- I'm exploring tagged types as an alternative to variant records.
-- Seemingly, one should be permitted to use a classwide type as
-- one would an unconstrained variant record. But see below:

PROCEDURE TestTags IS

-- First a classical Ada 83 variant record
-- Note that the record is unconstrained

  TYPE Kinds IS (A, B, C);

  TYPE VariantRec (Discrim: Kinds := A) IS RECORD
    FixedPart: String (1..4);
    CASE Discrim IS
      WHEN A =>
        F1: Integer;
      WHEN B =>
        F2: Float;
      WHEN C =>
        F3: Boolean;
    END CASE;
  END RECORD;

-- Now a tagged type equivalent

  TYPE TaggedRec IS TAGGED RECORD
    FixedPart: String(1..4);
  END RECORD;

  TYPE A_TaggedRec IS NEW TaggedRec WITH RECORD
    F1: Integer;
  END RECORD;

  TYPE B_TaggedRec IS NEW TaggedRec WITH RECORD
    F2: Float;  
  END RECORD;

  TYPE C_TaggedRec IS NEW TaggedRec WITH RECORD
    F3: Boolean;
  END RECORD;

-- OK, some arrays now.

-- CASE 1: array of unconstrained variant records. OK in Ada 83.

  TYPE List IS ARRAY (1..4) OF VariantRec; -- unconstrained variant rec
  Stuff: List;

-- CASE 2: Now an array intended to hold any of the four types
-- (by direct analogy with Case 1)

-- GNAT rejects this - says the array elements must be constrained

  TYPE TaggedList IS ARRAY (1..4) OF TaggedRec'Class;
  TaggedStuff: TaggedList;

-- CASE 3: Array of_pointers_ to tagged records
-- This will compile OK, but seems a gratuitously required use
-- of pointers - very un-Ada-like.

  TYPE TaggedRecPtr IS ACCESS TaggedRec'Class; 
  TYPE TaggedListPtr IS ARRAY (1..4) OF TaggedRecPtr;
  TaggedStuffPtr: TaggedList;

-- So what is the solution? Perhaps I missed something in 9XRM 5.0,
-- but I found nothing to prohibit what I tried to do in Case 2.

-- The Ada 9X textbook examples make much of the fact that one
-- _can_ use pointers more flexibly in 9X, because e.g. the
-- designated objects need not be dynamic. They also show pointers
-- as a way that one _can_ do dispatching of operations on tagged-type
-- derivatives. It is not clear where (or why) this is the _only_
-- allowed way.

-- So did I bump into an Ada 9X feature, or just a GNAT bug?

BEGIN

  NULL;

END TestTags;

-- Mike Feldman