Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: Preprocessor macro to quote its argument
Message-ID: <1990Aug26.002710.9343@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <1112@mti.mti.com> <1114@mti.mti.com> <1990Aug20.012839.3545@zoo.toronto.edu> <11517@ogicse.ogi.edu> <1990Aug21.175138.24633@zoo.toronto.edu> <21594@grebyn.com>
Date: Sun, 26 Aug 90 00:27:10 GMT

In article <21594@grebyn.com> ckp@grebyn.UUCP (Checkpoint Technologies) writes:
>Why can't this work?
>#define CTRL(ch) ((* #ch) - 64)
>Now, I know this *could* be a constant expression...

3.4 says that the value of an object may not be accessed by a `*' operator
in a constant expression, so this isn't a constant expression in a portable
sense.  However, it is not a constraint violation, so the compiler is not
*required* to complain about it, and 3.4 does explicitly authorize compilers
to accept "other forms" of constant expression.  So your comment is correct:
it *could be* a constant expression, but isn't dependably one.  Alas, one
use of the `CTRL(D)' hack is in case labels, where constant expressions are
required.
-- 
Committees do harm merely by existing. | Henry Spencer at U of Toronto Zoology
                       -Freeman Dyson  |  henry@zoo.toronto.edu   utzoo!henry