Re: preprocessor token pasting, 2.95 vs 3.0

[James Dennett <jdennett at acm dot org>]

Justin Guyett wrote:
> 
> On Wed, 20 Jun 2001, Neil Booth wrote:
> 
> > > ../test.h:32:9: warning: pasting "bar2" and "." does not give a valid
> > > preprocessing token
> >
> > CPP is correct, and the message is pretty clear, at least if you
> > understand what the semantics of ## really are.
> >
> > I suspect you have a macro somewhere that is doing
> >
> > #define foo(A, B) foo.A ## B ## .baz
> 
> except B is always a number.. see below.
> 
> > In this particular case, the 2nd ## is not helping and you should just
> > delete it.
> 
> gcc 3.0 errors if you do that.
> 
> sorry if this was unclear.  The code has some monstrous multi-line macros.
> the portion in question was originally this:
> 
> #define A(x)  do { \
>    func.x ## 2.clear(); \
>    } while(0)
> 
> gcc 2.95 has no problems pasting func.x with 2.clear(), but gcc 3 does
> (error "invalid suffix on floating constant", it thinks 2 is a number,
> since functions can't start with numeric digits and it hasn't done the
> pasting yet).  gcc 2.95's interpretation makes sense, gcc 3.0 doesn't.
> Nobody cares if it's a number, if it's right after a ##, it's getting
> pasted onto something else so any ideas about what it is or what it's part
> of should be suspended until after the pasting.
> 
> with gcc 3.0, changing to "func.x ## 2 ## .clear();" works, but gives
> the error: (presuming foo is passed to the macro)
> 
> ../test.h:32:9: warning: pasting "foo" and "." does not give a valid
> preprocessing token
> 
> so e.g. if foo is passed as x, is there any way to do the equivalent of
> func.foo2.clear();
> without getting that warning, and if it's changed to an error, how would
> you do something like that?
> 
> simple test code:  try compiling this with gcc 2.95.x and gcc 3... this
> won't compile at all with gcc 3, compiles perfectly with 2.95, but
> illustrates the point i think.
> 
> #include <list>
> #define T(x) (func.x ## 2.clear())
> 
> struct a {
>   std::list<int> foo2;
> };
> 
> int main(int argc, char **argv) {
>   struct a func;
>   T(foo);
>   return(0);
> }
> 
> with gcc 3.0 you'll find it doesn't compile, and requires you to change
> the macro to
> ... ## 2 ## .clear())
> 
> and you still get the warning, which is what I was trying to find a way
> around...

Write it correctly!  You're trying to paste x onto the start of a
token which is "2."

#define T(x) (func.x ## 2 .clear())

(with a space after the 2) is what you want.

This is not a bug in gcc 3; your macro definition was broken.

-- James Dennett <jdennett@gcc.gnu.org>