Counting Variadic Macro Arguments in C99 and C++

countargs.h

/*
   Credits: Jens Gustedt
   https://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments
*/

#include <stdio.h>

#define EXPAND(x) x

#define CONCAT_(a, b) a##b
#define CONCAT(a, b) CONCAT_(a, b)

#define IF_0(t, f) f
#define IF_1(t, f) t
#define IF(x, t, f) CONCAT(IF_, x)(t, f)

#define ARG(                                                                   \
     _0,  _1,  _2,  _3,  _4,  _5,  _6,  _7,                                    \
     _8,  _9, _10, _11, _12, _13, _14, _15,                                    \
    _16, _17, _18, _19, _20, _21, _22, _23,                                    \
    _24, _25, _26, _27, _28, _29, _30, _31,                                    \
    _32, _33, _34, _35, _36, _37, _38, _39,                                    \
    _40, _41, _42, _43, _44, _45, _46, _47,                                    \
    _48, _49, _50, _51, _52, _53, _54, _55,                                    \
    _56, _57, _58, _59, _60, _61, _62, _63,                                    \
    _64, ...) _64

#define IS_EMPTY_TRIGGER(...) ,

#define IS_EMPTY_HAS_COMMA(...)                                                \
    EXPAND(ARG(                                                                \
        __VA_ARGS__,                                                           \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 1,                                                \
        1, 1, 1, 1, 1, 1, 1, 0))

#define IS_EMPTY_PASTE(_0, _1, _2, _3, _4) _0##_1##_2##_3##_4

#define IS_EMPTY_0000 0
#define IS_EMPTY_0001 1
#define IS_EMPTY_0010 0
#define IS_EMPTY_0011 0
#define IS_EMPTY_0100 0
#define IS_EMPTY_0101 0
#define IS_EMPTY_0110 0
#define IS_EMPTY_0111 0
#define IS_EMPTY_1000 0
#define IS_EMPTY_1001 0
#define IS_EMPTY_1010 0
#define IS_EMPTY_1011 0
#define IS_EMPTY_1100 0
#define IS_EMPTY_1101 0
#define IS_EMPTY_1110 0
#define IS_EMPTY_1111 0

#define IS_EMPTY_(_0, _1, _2, _3)                                              \
    IS_EMPTY_PASTE(IS_EMPTY_, _0, _1, _2, _3)
#define IS_EMPTY(...)                                                          \
    IS_EMPTY_(                                                                 \
        IS_EMPTY_HAS_COMMA(__VA_ARGS__),                                       \
        IS_EMPTY_HAS_COMMA(IS_EMPTY_TRIGGER __VA_ARGS__),                      \
        IS_EMPTY_HAS_COMMA(__VA_ARGS__ ()),                                    \
        IS_EMPTY_HAS_COMMA(IS_EMPTY_TRIGGER __VA_ARGS__ ()))

#define COUNT_ARGS_(...)                                                       \
    EXPAND(ARG(                                                                \
        __VA_ARGS__,                                                           \
        64, 63, 62, 61, 60, 59, 58, 57,                                        \
        56, 55, 54, 53, 52, 51, 50, 49,                                        \
        48, 47, 46, 45, 44, 43, 42, 41,                                        \
        40, 39, 38, 37, 36, 35, 34, 33,                                        \
        32, 31, 30, 29, 28, 27, 26, 25,                                        \
        24, 23, 22, 21, 20, 19, 18, 17,                                        \
        16, 15, 14, 13, 12, 11, 10,  9,                                        \
         8,  7,  6,  5,  4,  3,  2,  1,                                        \
         0))

#define COUNT_ARGS(...)                                                        \
    IF(IS_EMPTY(__VA_ARGS__), 0, COUNT_ARGS_(__VA_ARGS__))

static const int count_0 = COUNT_ARGS();
static const int count_1 = COUNT_ARGS(a);
static const int count_8 = COUNT_ARGS(a, b, c, d, e, f, g, h);

int
main(void)
{
    printf("count_0: %d\n", count_0);
    printf("count_1: %d\n", count_1);
    printf("count_8: %d\n", count_8);
    return 0;
}