Re: [Boost-users] passing comma to macro

From: Warlich, Christof \(Christof\) (cwarlich_at_[hidden])
Date: 2008-03-13 12:48:57

> There's a few things wrong with this.
> Firstly you write BOOST_PP_IIF but include <boost/preprocessor/if.hpp>
> - you probably meant BOOST_PP_IF.
Well, <if.hpp> includes <control/if.hpp> which includes
<control/iif.hpp>,
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<bool condition, class Then, class Else> struct IF {typedef Then
RET;};
template<class Then, class Else> struct IF<false, Then, Else> {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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