[boost] Re: missing ## ... juxtaposition?

From: Dan W. (danw_at_[hidden])
Date: 2003-12-02 11:43:34

I reported earlier that there are some boost files where it appears that 
##'s are missing; which with the Digital Mars compiler results in a space 
being inserted; eg.:

in file "...boost\mpl\list.hpp":
#   define MPL_AUX_LIST_HEADER 
BOOST_PP_CAT(list,BOOST_MPL_LIMIT_LIST_SIZE).hpp /**/
which I was able to work-around by adding the ##, such as:
.....,BOOST_MPL_LIMIT_LIST_SIZE)##.hpp /**/

Paul Mensonides replied:
 >The compilers are wrong to insert a space, and a concatenation here is 
undefined
 >behavior.  A better way to fix it would be to try this:
 >
 >#define MPL_AUX_LIST_HEADER \
 >    MPL_AUX_LIST_HEADER_I(BOOST_PP_CAT(list,BOOST_MPL_LIMIT_LIST_SIZE)) \
 >    /**/
 >#define MPL_AUX_LIST_HEADER_I(name) name.hpp

Subsequently, I exchanged messages at the Digital Mars forum. I am no 
expert in C++ myself, so I'm just trying to be helpful in solving this 
problem by acting as dumb messenger. I'll just copy and paste a summary of 
the DM forum dialog below.

--------------------------------------------------------------------------------------
 > In article <bq8u65$obq$1_at_[hidden]>, Walter says...
 >I don't see how that derives from the spec. The only token concatention
 >allowed by the spec is with ##, extra whitespace should be quite irrelevant.
 >## is defined as token concatenation, so I don't see how that is undefined.
--------------------------------------------------------------------------------------
Even if 'concatenation' per-se is not called for, and against the Standard,
could it be that the "." (dot) relieves the preprocessor from 
responsibility for
adding a space at the end of the preceding string (since the dot already 
acts as
a kind of 'separator'..)?
I just find it hilarious how the boost libraries work with so many compilers,
but only need dozens of ## in many files to work with DM.  I wouldn't be
surprised at all that they'd be all wrong; --won't be the first time that
everybody is wrong, but this bug may be just about ready for acceptance by
ANSI/ISO/whatever...  ;-)
--------------------------------------------------------------------------------------
 >The separator inserted by dmc is to make the preprocessor work right, it
 >isn't easilly removed. I don't really understand why boost seems to want to
 >rely on the 'juxtaposition-equals-concatenation' kludge, the ## operator was
 >added to Standard C specifically to move away from that practice.
--------------------------------------------------------------------------------------
Maybe if someone could paste the section of the Standard dealing with this, 
I'd much appreciate it.
Yours.
dan