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From: colbert@hermix.UUCP (colbert)
Newsgroups: comp.lang.ada
Subject: Re: Characters with codes >= 128
Message-ID: <8709100440.AA04224@rand-unix.rand.org>
Date: Wed, 9-Sep-87 23:47:24 EDT
Article-I.D.: rand-uni.8709100440.AA04224
Posted: Wed Sep  9 23:47:24 1987
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In response to my answer to his question about character types
sommar@seismo.css.gov  (Erland Sommarskog) writes:

>I think Martin Moore's solution was much more simple and elegant. It will
>work on any Ada system that doesn't check character assignments for 
>Constraint_error.
>  This solution requires one hell lot of work and it isn't portable from
>OS to another. Yes, I can write my own Text_IO, but guess how fun I find
>that. And, I will have to write one Text_IO for each OS I want to work
>with. Guess why there is a standard Text_IO. It gives you a standard 
>interface.

Unfortunately, Martin Moore's solution is NOT portable either.  It only works
because:

	1) Unchecked_Conversion is implemented in DEC Ada.

	2) The size of type Character objects in DEC is 8 bits.

	3) DEC did not give a Constraint_Error on the assignemt (which may
	   be a bug in DEC's implementation).

	4) DEC does not "place restrictions on unchecked conversions"
	   (13.10.2 P2);

	5) DEC truncates high order bits if the source value if its size is
	   greater than the size of the target type (this is really only a
	   problem with the specific example given by Moore, in that he used
	   the type Integer as the source type as opposed to an 8 bit type).

The principle benefit of my proposed solution is the creation of a portable
abstraction that represents the problem. Re-implementing a Text I/O for this
type is a small price to pay for this benefit (especially when Moore's
technique can be used in the implementation of this Text I/O - Sufficiently
issolated to prevent major impact on the system that I'm implementing and later
porting [as pointed out by another reader of this group]).


Take Care,
Ed Colbert
hermix!colbert@rand-unix.arpa

P.S.

As an additional comment, at the recent SIGAda Conference, Dr. Dewer indicated
that Unchecked_Conversion could be legally implemented to always return 0 no
matter what the "value" of the source object was.  I did not get a chance to
full nail him down on what he ment by this comment, so may be he will respond
to this message.