Re: [Boost-Users] newbie Question about the macro BOOST_JOIN

From: John Maddock (john_maddock_at_[hidden])
Date: 2002-09-28 05:30:34

> The idea behind BOOST_JOIN is to force the preprocessor to scan the macro
> parameters for replacement before merging the results with ##. So, it
> changes the meaning if one of the parameters is a macro:
>   #define Z_BOOST_JOIN(X, Y) X##Y
>   Z_BOOST_JOIN(id_, __LINE__) // Expands to "id___LINE__"
>   BOOST_JOIN(id_, __LINE__) // Expands to "id_13" if __LINE__ is 13
>
> However, I'm not sure why BOOST_DO_JOIN2 is used; it seems to me that only
> one layer of indirection should work. I assume it's to work around some
> compiler's non-conforming preprocessor.

I think it depends on whether BOOST_JOIN is called from another macro or not
(if not you need the extra indirection), or at least I think that's the
answer ;-)

John Maddock
http://ourworld.compuserve.com/homepages/john_maddock/index.htm