Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!apple!julius.cs.uiuc.edu!wuarchive!udel!princeton!phoenix.Princeton.EDU!pfalstad
From: pfalstad@phoenix.Princeton.EDU (Paul John Falstad)
Newsgroups: comp.lang.c
Subject: Re: Embedding macro values in strings
Message-ID: <5196@idunno.Princeton.EDU>
Date: 11 Jan 91 08:44:52 GMT
References: <1991Jan10.200806.12745@grep.co.uk>
Sender: news@idunno.Princeton.EDU
Organization: Princeton University, Princeton, New Jersey
Lines: 37

In article <1991Jan10.200806.12745@grep.co.uk> frank@grep.co.uk (Frank Wales) writes:
>For example, suppose I have  foo(__LINE__); in my source, and I want that
>to expand to  bar("Line n"); where n is substituted with the actual value
>of __LINE__ at that point.  Trying
>
>  #define foo(n)	bar("Line "#n)
>
>and invoking as  foo(10); yields  bar("Line 10"); , whereas  foo(__LINE__);
>yields  bar("Line __LINE__"); .

You realize, of course, that foo(10) would not actually produce:

   bar("line 10"),

but will produce:

	bar("line " "10")

wish is concatenated into a single string in a later phase of
compilation.  Just clarifying that.

Anyway, adding another level of macro expansion does the trick:

#define foo(n) bar("Line "fee(n))
#define fee(X) #X
foo(10)
foo(__LINE__)

This produces the intended result (using gcc):

bar("Line ""10" ) 
bar("Line ""4" ) 

--
Paul Falstad, pfalstad@phoenix.princeton.edu PLink:HYPNOS GEnie:P.FALSTAD
"We could nuke Baghdad into glass, wipe it with Windex, tie fatback on
our feet and go skating." - Air Force Times columnist Fred Reed