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

Subject: Re: [Boost-users] Preprocessor: Expaning empty VA_ARGS
From: Edward Diener (eldiener_at_[hidden])
Date: 2016-09-08 12:06:56

On 9/8/2016 10:35 AM, Florian Lindner wrote:
> Hello,
>
> 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 do have variadic data, only the single variadic data element is 
empty. So the idea is to check for emptiness. You can do that with my 
VMD library and the BOOST_VMD_IS_EMPTY macro.

Your preprocessing logic path should be:

1) Retrieve the first variadic element using 
BOOST_PP_VARIADIC_ELEM(0,__VA_ARGS__).

2) Check for emptiness of what you retrieved using BOOST_VMD_IS_EMPTY.

3) If it is not empty proceed with what you are doing above, else output 
nothing.

I will presume you know enough about BOOST_PP control constructs to 
figure out the rest.

I could add a macro in VMD to check if variadic data is "empty".