Re: c++/8297: problem with preprocessor

[Zack Weinberg <zack@codesourcery.com>]

The following reply was made to PR c++/8297; it has been noted by GNATS.

From: Zack Weinberg <zack@codesourcery.com>
To: riteshg@pune.tcs.co.in
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: c++/8297: problem with preprocessor
Date: Sun, 20 Oct 2002 23:42:52 -0700

 On Mon, Oct 21, 2002 at 05:46:30AM -0000, riteshg@pune.tcs.co.in wrote:
 > when i run 
 > 
 > #define comment /##/
 > comment
 > 
 > the output is coming as space between twp front slash "/ /".
 > and it is supposed to come "//" . I am using gcc 3.0 . Till
 > older versions than gcc3.0 it is comming correctly as "//" .
 > And also in older version preprocessor is not processing the user
 > defined comments as
 >
 > #define comment /##/
 > comment 
 >
 > when ran with -E option gives output in preprocessed file
 > as
 > //
 
 I'm sorry, but you have it backward.  The 2.x preprocessor was buggy;
 the behavior of the 3.0 and later preprocessor is correct.
 
 // is not a preprocessing token.  If it appears in source text, it
 introduces a line comment, which is stripped from the program _before_
 the text is broken into tokens.  Otherwise, it is simply two "/"
 tokens adjacent to each other.  You attempted to force the creation of
 a single token "//" with the ## operator.  This induces undefined
 behavior: GCC can do whatever it likes.  We choose to have it issue a
 warning and pretend there was no ## operator; thus, two "/" tokens are
 passed to the output routines.  They then have to insert a space, so
 that these two "/" tokens are not later misinterpreted as introducing
 a line comment.
 
 If you describe what you are actually trying to do, we may be able to
 offer advice on how to do it properly.
 
 zw