There's a few things wrong with this. Firstly you write BOOST_PP_IIF but include - you probably meant BOOST_PP_IF. Well, includes which includes , and I thought I stay at the top level of the boost/preprocessor directory as the content of subdirectories may become subject of change.

But more seriously your "condition" is not a number known at preprocessor-time - i.e. it won't evaluate to a number at pre-processor time, only at compile-time when the boost::is_convertible template magic kicks in. Thus BOOST_PP_IF wouldn't expand as you expect. Hmm, but then, what is the advantage compared to an ordinary #if statement? Actually, I thought that BOOST_PPP_IF is a wrapper for some template "if", e.g. like

template struct IF {typedef Then RET;}; template struct IF {typedef Else RET; }; Was this assumption wrong?

However, you aren't even seeing an error related to that problem.

BOOST_PP_COMMA() is indeed being evaluated before the BOOST_PP_IIF() and thus the compiler is complaining about 4 arguments to the latter macro - compilers do vary in how they expand macros - search the developer's list for Paul Mensonides' essay "How macro expansion works" for more - but you will be safe if your _force_ the COMMA macro to be expanded after the IF one. The following trick occurs over and over in writing preprocessor code:

BOOST_PP_IF( X , BOOST_PP_COMMA , BOOST_PP_EMPTY )()

Because BOOST_PP_COMMA and BOOST_PP_EMPTY are function-like macros they cannot possibly expand until they are "next to" the (). This can only happen once the BOOST_PP_IF has been evaluated. I.e. the above example will evaluate to a comma if X is non-zero and nothing otherwise....

Thanks for this explaination, that's really an interesting trick. It's a pity that it doesn't help in this case, but it may be useful otherwise.

I'd like to know if there is common name for this trick as I find '"macro name inside the IF, macro arguments outside"-trick' a bit verbose when discussing with colleagues.

Pete

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