Re: [Boost-users] Preprocessor: Expaning empty VA_ARGS

Subject: Re: [Boost-users] Preprocessor: Expaning empty VA_ARGS
From: niXman (i.nixman_at_[hidden])
Date: 2016-09-08 11:01:21

Florian Lindner 2016-09-08 17:35:
> Hello,
Hi,

> I have this preprocessor macros:
> 
> -----------
> 
> #include <boost/preprocessor/variadic/to_seq.hpp>
> #include <boost/preprocessor/seq/for_each_i.hpp>
> #include <boost/preprocessor/stringize.hpp>
> 
> #include <iostream>
> 
> #define TRACE(...) \
>   std::cout << "foo"                                                    
>       \
>   BOOST_PP_SEQ_FOR_EACH_I(LOG_ARGUMENT,, 
> BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__));
> 
> #define LOG_ARGUMENT(r, data, i, elem)                                  
> \
>   << std::endl << "  Argument " << i << ": " <<
> BOOST_PP_STRINGIZE(elem) << " == " << elem
> 
> 
> int main(int argc, char *argv[])
> {
>   TRACE();
> 
>   return 0;
> }
> 
> -----------
> 
> They compile when TRACE has arguments, but not without. Problem is,
> that LOG_ARGUMENT is called even when no args are
> supplied, leading to the expansion:
> 
>  std::cout << "foo" << std::endl << "  Argument " <<
>  0
>  << ": " << "" << " == " << ;;
> 
> elem is empty, therefore there is no expression after the last <<
> 
>  expected primary-expression before »;« token
> 
> How can I deal with that?

You need to check if '__VA_ARGS__' is not empty using 
'BOOST_VMD_TUPLE_SIZE()'[1] or something like this[2][3].

[1] 
http://www.boost.org/doc/libs/1_61_0/libs/vmd/doc/html/BOOST_VMD_TUPLE_SIZE.html
[2] 
https://github.com/niXman/nanolog/blob/master/include/nanolog/nanolog.hpp#L97
[3] 
https://github.com/niXman/nanolog/blob/master/include/nanolog/nanolog.hpp#L591

-- 
Regards, niXman
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