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From: minow@decvax.UUCP (Martin Minow)
Newsgroups: comp.lang.c
Subject: #if inside #define -- easy to do, but is it a good idea?
Message-ID: <157@decvax.UUCP>
Date: Fri, 25-Sep-87 21:11:49 EDT
Article-I.D.: decvax.157
Posted: Fri Sep 25 21:11:49 1987
Date-Received: Sun, 27-Sep-87 07:06:55 EDT
Reply-To: minow@decvax.UUCP (Martin Minow)
Organization: Digital Eq. Corp. - Merrimack NH.
Lines: 28

From time immemorial, C has forbidden # control lines inside a
macro definition.  When a collegue begged for this capability,
I discovered it was trivial to add it to Decus CPP (which was posted
to Usenet mod.sources about 2-3 years ago).  It might be used
as follows:
	#define set_bit(bit)				\
	#if debug					\
	    printf("setting %04x\n", bit),		\
	#endif						\
		(device_register = (bit))

Decus CPP compiles macros by replacing formal parameters with the
appropriate actual parameter and "pushing" the entire string back on
the input file reader.  The only change I had to make was to recognize
the string "\ <newline>" during macro compilation and "expand" it as
"<newline>".  (Of course, the program is now non-transportable.)

While the process may be more difficult for a tokenizing pre-processor,
it shouldn't be too hard to implement.

I'm sure it's too late in the review cycle to put this into the Standard
(and it does violate the formal model), but it might be worth thinking
of for the future.

Or, did I forget something obvious?

Martin Minow
decvax!minow