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=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,e1e2bc096a996632,start
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: g2news1.google.com!postnews.google.com!not-for-mail
From: gate02@wrandyke.demon.co.uk (Michael Mounteney)
Newsgroups: comp.lang.ada
Subject: why can't we declare unconstrained objects ?
Date: 12 Dec 2004 07:43:21 -0800
Organization: http://groups.google.com
Message-ID: <fa59671.0412120743.5cc44da6@posting.google.com>
NNTP-Posting-Host: 80.176.143.194
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-Trace: posting.google.com 1102866202 26117 127.0.0.1 (12 Dec 2004 15:43:22
 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Sun, 12 Dec 2004 15:43:22 +0000 (UTC)
Xref: g2news1.google.com comp.lang.ada:6901
Date: 2004-12-12T07:43:21-08:00
List-Id: <comp.lang.ada>

Is there a simple way in Ada of simulating C/C++ unions ?  It seems to
me that this is gratuitously prevented, that is, it can be done with
safety, by extending an existing run-time check, but it is in fact
prevented by the compiler.

Hopefully the following commented source will illustrate my point.

with Ada.text_IO;

procedure unconstrained is

	-- Very simple discriminated type
	type thing (what : Boolean) is
		record
			case what is
				when false =>
					letter : character;
				when true =>
					number : natural;
			end case;
		end record;

	-- No problems here:  we provide a discriminant.
	sample : thing := (false, 'X');

	-- This is alright as well of course.
	type thing_pointer is access all thing;

	-- This is also fine:  a pointer to any `thing'.
	indirect : thing_pointer;

	-- This causes a problem:  I want an unconstrained `thing', one
	-- that can be switched between holding a character and a number
	-- but the initialisation makes it constrained.
	sample2 : thing := (true, 12);

	-- Omitting the initialisation doesn't work:
	-- this just fails at compile-time.
	sample3 : thing;

begin
	-- Just reference a field in the `thing'.
	Ada.text_IO.put (sample.letter);

	-- Create a new access object and access its field
	-- This requires a RUN-TIME check that indirect.what is true.
	indirect := new thing (true);
	indirect.number := 12;

	-- This will generate a RUN-TIME failure of course.
	indirect.letter := 'A';

	-- Warning at compile-time, failure at run-time.  I want to change
the
	-- discriminant.  Since the compiler will insert a run-time check
	-- for field selection via an access, why not for a direct variable ?
	sample2 := thing'(false, 'Z');
end;