Re: [Boost-users] passing comma to macro

Quoting "Warlich, Christof \(Christof\)" :

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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.

Oops sorry I'm misread the include. What you're doing is fine.

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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?

I'm afraid so. BOOST_PP_IF is a preprocessor-time "if". I.e. different code is generated by the preprocessor depending on the value of the first argument. Most compilers have a switch so that you can use do the preprocessing part of compilation if you wish. In your actual case, you'll need to do clas template specicalisation (no macros) to achieve what you want.