NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/lang/c/20301

Internet Message Format | 1993-01-28 | 1.8 KB

Path: sparky!uunet!think.com!rpi!crdgw1!rdsunx.crd.ge.com!bart!volpe
From: [e-mail] (Christopher R Volpe)
Newsgroups: comp.lang.c
Subject: Re: Preprocessor question
Message-ID: <[e-mail]>
Date: 27 Jan 93 19:50:17 GMT
References: <[e-mail]> <[e-mail]>
Sender: volpe@bart (Christopher R Volpe)
Reply-To: [e-mail]
Organization: GE Corporate Research & Development
Lines: 35
Nntp-Posting-Host: bart.crd.ge.com

In article <[e-mail]>, [e-mail] (Steve Langstaff) writes:
|> In article [e-mail], [e-mail] (Walter Misar) writes:
|> : In article <mick.728072797@picus>, [e-mail] (Mick Farmer) writes:
|> : > Hi,
|> : >
|> : > Karim asks how to get a preprocessor macro recognised inside
|> : > a string.  The answer, at least in ANSI C, is to use the
|> : > stringising operator and string concatenation:
|> : >
|> : > #define JUNK 50
|> : > static char *junk = #JUNK " ways to leave your lover"
|> :                       ^^^^^
|> : Nope, #token works only with #defines like in:
|> : #define stringify(x) #x
|> : #define str_exp(x) stringify(x)        /* this extra level IS necessary */
|> : static char *junk = str_exp(JUNK) "ways ....";
|> :
|> [snip]
|>
|> Why is the extra level necessary?

Without the extra level, it quotes the macro argument as written. For
example, the result of `stringify(JUNK)' using the above definitions
would be `"JUNK"'. I.e., the argument JUNK gets quoted before being
expanded. The extra indirection gives the argument JUNK a chance to
be macro-expanded before being quoted. Hence, the result of `str_exp(JUNK)'
is `"50"'.

-Chris

--
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Chris Volpe
G.E. Corporate R&D
[e-mail]