Re: [boost] [wave] pp-number bug?

From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2006-01-21 16:01:29

Alberto Ganesh Barbati wrote:

> this post originates from thread "concatenation of power of 
> negative number" in comp.std.c++ (http://tinyurl.com/asn95). 
> Consider this file:
> 
> ----
> #include <boost/preprocessor/cat.hpp>
> BOOST_PP_CAT(1e, -1)
> ----
> 
> According to Paul Mensonides' reply in the thread, it should 
> be eventually interpreted as
> 
>   <1e> ## <-> <1>
> 
> thus the pp-number "1e" is concatenated with "-" obtaining 
> the pp-number "1e-" and then "1" is appended as a separate 
> token. The final result would thus be "1e- 1" (notice the 
> blank between "-" and "1").
> 
> However, if run into Wave, the program above produces the 
> error message:
> 
> error: pasting the following two tokens does not give a valid 
> preprocessing token: "e" and "-"
> 
> Who's right? I'd agree with Paul Mensonides, because "1e" is 
> a valid pp token which is to be formed before applying the ## 
> operator. But my understanding is quite limited in this area 
> so I commit myself to the Gurus.

Paul is right and Wave is wrong. That's a known issue with Wave, it stems
from the fact that Wave currently works on C++ tokens, not pp-tokens as
mandated by the Standard. This will be fixed in the future, when I change
Wave to operate on pp-tokens, rescanning these into C++ tokens after
preprocessing. If you need to preprocess this kind of constructs with Wave,
you may want to use the --variadics command line option, allowing
concatenation of unrelated tokens.

Regards Hartmut