Boost preprocessor skip if variadic is empty

Question

I have the following boost preprocessor macro to generate a function

extern "C" EXPORT out name(BOOST_PP_SEQ_FOR_EACH_I(PARAMETER_LIST, 0, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)))

This works great unless if __VA_ARGS__ is empty. After some searching I found a way to count the numer of arguments in __VA_ARGS__ using BOOST_PP_VARIADIC_SIZE. After some thinking I wrote this MACRO:

extern "C" EXPORT out name(BOOST_PP_IF(BOOST_PP_VARIADIC_SIZE(__VA_ARGS__), BOOST_PP_SEQ_FOR_EACH_I(PARAMETER_LIST, 0, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)), void))

I think this should work, however I keep getting the following warning

 warning C4002: too many actual parameters for macro 'BOOST_PP_IIF_1'

Although this is a warning it still seems to break the preprocessor. When passing multiple arguments it will only process the first one. I find this so strange, how will adding this if break everything is such a weird way? I've checked the comma's and brackets hundred times but they seem fine. How can I fix this preprocessor?

http://www.boost.org/doc/libs/1_54_0/libs/preprocessor/doc/ref/if.html

Edit: this regression seems relevant: https://svn.boost.org/trac/boost/ticket/8606

I found out the reason why the IF breaks. The arguments for BOOST_PP_IF are expanded before BOOST_PP_IF is evaluated. This means that BOOST_PP_IF receives a variable number of arguments because one if its arguments I pass it expands to 'x, y, z' unfortunately I have no idea how to approach this problem now. — Roy T., Sep 19 '13 at 07:24
That and of course the pre-processor directive expecting a non-empty __VA_ARGS__ is stil being called always :( — Roy T., Sep 19 '13 at 09:04

niXman · Accepted Answer · 2014-10-28T20:04:26.637

implementation:

#include <boost/preprocessor.hpp>

// based on the: http://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments
#define __ARG16(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, ...) _15
#define __HAS_COMMA(...) __ARG16(__VA_ARGS__, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0)
#define __TRIGGER_PARENTHESIS_(...) ,
#define __PASTE5(_0, _1, _2, _3, _4) _0 ## _1 ## _2 ## _3 ## _4
#define __IS_EMPTY_CASE_0001 ,
#define __IS_EMPTY(_0, _1, _2, _3) __HAS_COMMA(__PASTE5(__IS_EMPTY_CASE_, _0, _1, _2, _3))

#define TUPLE_IS_EMPTY(...) \
    __IS_EMPTY( \
        /* test if there is just one argument, eventually an empty one */ \
        __HAS_COMMA(__VA_ARGS__), \
        /* test if _TRIGGER_PARENTHESIS_ together with the argument adds a comma */ \
        __HAS_COMMA(__TRIGGER_PARENTHESIS_ __VA_ARGS__),                 \
        /* test if the argument together with a parenthesis adds a comma */ \
        __HAS_COMMA(__VA_ARGS__ (/*empty*/)), \
        /* test if placing it between _TRIGGER_PARENTHESIS_ and the parenthesis adds a comma */ \
        __HAS_COMMA(__TRIGGER_PARENTHESIS_ __VA_ARGS__ (/*empty*/)) \
    )

#define __GEN_EMPTY_ARGS(...) \
    void

#define __GEN_NONEMPTY_ARGS_CB(unused, data, idx, elem) \
    BOOST_PP_COMMA_IF(idx) elem arg##idx

#define __GEN_NONEMPTY_ARGS(seq) \
    BOOST_PP_SEQ_FOR_EACH_I( \
         __GEN_NONEMPTY_ARGS_CB \
        ,~ \
        ,seq \
    )

#define GEN(out, name, ...) \
    extern "C" EXPORT out name( \
        BOOST_PP_IF( \
             TUPLE_IS_EMPTY(__VA_ARGS__) \
            ,__GEN_EMPTY_ARGS \
            ,__GEN_NONEMPTY_ARGS \
        )(BOOST_PP_TUPLE_TO_SEQ((__VA_ARGS__))) \
    ) {}

// test
GEN(void, finc0, int, char, long)
GEN(void, func1)

output:

extern "C" EXPORT void finc0( int arg0 , char arg1 , long arg2 ) {}
extern "C" EXPORT void func1( void ) {}

Its been a while since I used this code (over a year ago). But this answer seems a correct solution. Thanks! — Roy T., Oct 29 '14 at 08:41
This causes compiler warnings with newer g++/clang++ ([full error msg here](http://coliru.stacked-crooked.com/a/4d998bff8e42f2c3)) `main.cpp:49:1: warning: must specify at least one argument for '...' parameter of variadic macro [-Wgnu-zero-variadic-macro-arguments]` the C++11 Standard (unsure about C99) forbids zero arguments for variadic macros (https://stackoverflow.com/a/21266771/6490710). But "GNU CPP has a pair of extensions which deal with this problem. First, you are allowed to leave the variable argument out entirely" https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html — Jakob, Dec 16 '16 at 13:06

Boost preprocessor skip if variadic is empty

1 Answers1