Re: preprocessor/4923: Concatenation appears to handle whitespace incorrectly

['Zack Weinberg' <zack at codesourcery dot com>]

The following reply was made to PR preprocessor/4923; it has been noted by GNATS.

From: 'Zack Weinberg' <zack@codesourcery.com>
To: "Baum, Nathan I" <s0009525@chelt.ac.uk>
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: preprocessor/4923: Concatenation appears to handle whitespace incorrectly
Date: Fri, 23 Nov 2001 10:51:01 -0800

 On Thu, Nov 22, 2001 at 01:47:51PM -0000, Baum, Nathan I wrote:
 > 
 > > At that point the tokens on either side of ## are ")" and "def".  
 > > Pasting them together produces an invalid token,
 > > ")def", which triggers undefined behavior - we choose to pretend the
 > > ## never happened.  Then "FOO ( abc )" gets expanded to produce "abc".
 > > Since "abc" and "def" were not concatenated, cpp has to put a space
 > > between them so they are interpreted sa separate tokens.
 > 
 > Would it break existing programs if ## were to concatenate the two tokens
 > regardless (in some later version of gcc)? I'd imagine that no sensible
 > program would rely upon undefined behaviour, so it shouldn't. Presumeably,
 > it'd have to throw the two tokens away and rescan the newly created string,
 > but that doesn't _seem_ like a major problem (I don't know how CPP handles
 > these things internally, so it might be).
 
 It does, in fact, concatenate "the two tokens" in the textual output -
 but the two tokens which get concatenated are ")" and "def".
 Contrast
 
 #define a(x) FOO(abc) ## x
 #define b(x) FOO(abc) x
 
 a(def)	-> FOO(abc)def
 b(def)	-> FOO(abc) def
 
 To do what you apparently want, when the ")" was the last character of
 a macro invocation, it would have to remember that it had been pasted
 after and apply the paste to the last token of the macro expansion.
 This would be a lot of work and frankly I don't see the point.  Show
 me real code that desperately needs this functionality and I'll show
 you how to fix it so it's standard conforming C.
 
 > > gcc 3.x will warn you when this happens:
 > > test.c:5:1: warning: pasting ")" and "def" does not give a valid
 > preprocessing token
 > 
 > Hmm. Shouldn't the message say 'concatenating' rather than 'pasting'?
 
 The manual uses the two terms interchangeably, and I think the error
 message is already long enough.
 
 zw