Re: [Boost-users] Boost.Preprocessor with __LINE__ in MSVC71

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2006-03-08 16:38:32

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] 
> [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Wu 
> Yinghui, Freddie

>  > This is just one of the *many* ways in which VC's 
> preprocessor is  > wacky.  It suffices to say it isn't even 
> in the ballpark of  > implementing the phases of translation 
> correctly.
> 
> That's what I feel. The error message indicate that the 
> compiler somehow considers "id" and __LINE__ two identifier 
> when the error occurs. I tried using the /P parameter to dump 
> the preprocessed source, and that looked fine, though. So I 
> guess it is the problem with MSVC compiler mixing 
> preprocessing and parsing phases together...

Yes.  There is nothing that the pp-lib can do about it.

>  > What do you mean by "certain scope"?  Or, more precisely, 
> given a  > finite scope such as that defined by a function, 
> why do you need  > unique identifiers?  Most of the time when 
> people want unique  > identifiers, they want an identifier 
> that is unique across an entire  > program or, at minimum, 
> inside a translation unit.
> 
> Well... The scope can be the whole program, or in a 
> namespace, or in a translation unit. The unique ID is used in 
> a macro in the library for some tidious code that can be 
> automatically generated using preprocessor.

For the record, doing this sort of thing is almost guaranteed to produce ODR
violations.  With that said...

> Anyway, thanks for Stewart Tootill's suggestion. I had to 
> resort to using __COUNTER__ at the meantime, which seemed 
> working so far. (But one thing that worries me is that 
> __COUNTER__ is only unique within a translation unit. So what 
> if I need a unique identifier throughout the
> program?)

The pp-lib contains a similar construct, but it must be manually updated.  E.g.

#include BOOST_PP_UPDATE_COUNTER()

BOOST_PP_COUNTER() // 1

#include BOOST_PP_UPDATE_COUNTER()

BOOST_PP_COUNTER() // 2
BOOST_PP_COUNTER() // 2

#include BOOST_PP_UPDATE_COUNTER()

BOOST_PP_COUNTER() // 3

I could provide the means to seed this value for a translation unit, but I'm not
sure if this is really a good idea.  Thoughts?

Regards,
Paul Mensonides