Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!mit-eddie!genrad!decvax!minow
From: minow@decvax.UUCP (Martin Minow)
Newsgroups: comp.lang.c
Subject: Re: #if inside #define -- easy to do, but is it a good idea?
Message-ID: <163@decvax.UUCP>
Date: Sun, 4-Oct-87 13:30:42 EDT
Article-I.D.: decvax.163
Posted: Sun Oct  4 13:30:42 1987
Date-Received: Wed, 7-Oct-87 06:24:47 EDT
References: <157@decvax.UUCP> <29339@sun.uucp> <10898@orchid.waterloo.edu> <7510@elsie.UUCP>
Reply-To: minow@decvax.UUCP (Martin Minow)
Organization: Digital Eq. Corp. - Merrimack NH.
Lines: 53

Several responses suggested that the capabilities of # rescanning
could be accomplished at execution time -- especially since modern
compilers prune "dead-code" so a sequence like:
	#define DEBUG	0
	#define DLOG(a)	((DEBUG) ? printf a : 0)
	   ... DLOG(("%d", x));
generates no extraneous code.

There are other needs, however.  For example, here is example where
it would be nice to be able to generate # commands inside of macros.
It builds a symbol table of keywords:

	char *table[] = {
	    "foo",
	#		define SY_foo	0
	    "bar",
	#		define SY_bar	1
	    NULL
	};

The macro would look something like

	#define SYM(name, n)			\
(1)	    #name,				\
(2)	    #define SY_##name	n

And would be used as

	SYM(foo, 0)
	SYM(bar, 1)

Notes:
(1)	# is used here to "stringize" its argument.  This means that
	SYM(define) or SYM(if) will not work.  This problem is due
	to the ambiguity of the unary # operator.  I suppose you
	could hack around it by writing (1) as
		"" ## #name,

(2)	Note that C doesn't give the programmer the tools to generate
	flexible counters.  I.e, CPP is not a general purpose macro
	processor.  Of course, K&R said this first.

It should be pointed out that if you use command files, makefiles or
their equivalent, it is very easy to build special purpose table
generators that are automatically invoked when the appropriate module
is compiled.  The realworld (say, constructing menus for a window
system) requires more complex table constructions than my simple
extension would allow.  Yacc and lex users have known this for years.

Martin Minow
decvax!minow