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From: "Steve Doiel" <nospam_steved@pacifier.com>
Subject: Alternate to Unchecked_Conversion - Portable?
Date: 1999/02/21
Message-ID: <36d05e39.0@news.pacifier.com>#1/1
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X-Trace: 21 Feb 1999 11:27:53 PST, 216.65.138.96
Newsgroups: comp.lang.ada
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Date: 1999-02-21T00:00:00+00:00
List-Id: <comp.lang.ada>

I discovered a technique for performing a similar function to
Unchecked_Conversion.  Locating two data structures at the same address by
defining the first normally and then using an address clause to locate the
second at the same address as the first (as shown in the following sample):


WITH Ada.Text_Io;
WITH Interfaces;
 USE Interfaces;
PROCEDURE TestAlias IS
  PACKAGE Text_Io RENAMES Ada.Text_Io;

  TYPE aRec IS
    RECORD
      f1 : Integer_32;
      f2 : Integer_32;
    END RECORD;

  TYPE Integer_16_Array IS ARRAY( Positive RANGE <> ) OF Integer_16;

  v1 : aRec;
  v2 : Integer_16_Array( 1..4 );
  FOR v2'ADDRESS USE v1'ADDRESS;    -- This causes v1 and v2 to occupy the
same memory

BEGIN
  v1.f1 := 1;
  v1.f2 := 2;
  FOR ii IN v2'RANGE LOOP
    Text_Io.Put( Integer_16'IMAGE( v2( ii ) ) );
  END LOOP;
  Text_Io.New_Line;
END TestAlias;


My question is: is this portable?

I expect that record layouts will be dependent on different endian machines,
and I know that this technique is inherently unsafe, but it makes things
very simple.

SteveD