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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Mon, 09 Aug 2010 15:10:56 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2010-08-09T15:10:56-04:00
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Natacha Kerensikova <lithiumcat@gmail.com> writes:

> That's something I'm quite uncomfortable with. However I see the point
> of the other related posts here. So my question is, if I assume
> Stream_Element is an octet, is there a way to make compilation fail
> with an explicit error message when the assumption doesn't hold? C's
> preprocessor allows this through a #if on CHAR_BITS and a #error to
> stop compilation with a custom message. Any Ada equivalent?

Put:

    pragma Assert(Stream_Element'Modulus = 2**8);
    pragma Assert(Stream_Element'Size = 8);

in any library package that depends on this fact.

This will fail at run time if it's wrong.  But it will fail on
EVERY execution of the program, so it's pretty-much just as
good as a compile-time error.  Also, if you're lucky,
the compiler will warn you at compile time (and you can
use warnings-as-errors mode if you like).

You might also want to look up the GNAT-specific Compile_Time_Error
and Compile_Time_Warning pragmas.

- Bob