Designated Initializer Macro for C99 and C++

main.c

#include <stdio.h>

/* -------------------------------------------------------------------------- */

#define EXPAND(x) x

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

#define ARG(                                                                   \
     _0,  _1,  _2,  _3,  _4,  _5,  _6,  _7,                                    \
     _8,  _9, _10, _11, _12, _13, _14, _15,                                    \
    _16, ...) _16

#define COUNT_ARGS(...)                                                        \
    EXPAND(ARG(                                                                \
        __VA_ARGS__,                                                           \
        16, 15, 14, 13, 12, 11, 10,  9,                                        \
         8,  7,  6,  5,  4,  3,  2,  1,                                        \
         0,))

#define APPLY_1(x, first) x(first)
#define APPLY_2(x, first, ...) x(first) APPLY_1(x, __VA_ARGS__)
#define APPLY_3(x, first, ...) x(first) APPLY_2(x, __VA_ARGS__)
#define APPLY_4(x, first, ...) x(first) APPLY_3(x, __VA_ARGS__)
#define APPLY_5(x, first, ...) x(first) APPLY_4(x, __VA_ARGS__)
#define APPLY_6(x, first, ...) x(first) APPLY_5(x, __VA_ARGS__)
#define APPLY_7(x, first, ...) x(first) APPLY_6(x, __VA_ARGS__)
#define APPLY_8(x, first, ...) x(first) APPLY_7(x, __VA_ARGS__)
#define APPLY_9(x, first, ...) x(first) APPLY_8(x, __VA_ARGS__)
#define APPLY_10(x, first, ...) x(first) APPLY_9(x, __VA_ARGS__)
#define APPLY_11(x, first, ...) x(first) APPLY_10(x, __VA_ARGS__)
#define APPLY_12(x, first, ...) x(first) APPLY_11(x, __VA_ARGS__)
#define APPLY_13(x, first, ...) x(first) APPLY_12(x, __VA_ARGS__)
#define APPLY_14(x, first, ...) x(first) APPLY_13(x, __VA_ARGS__)
#define APPLY_15(x, first, ...) x(first) APPLY_14(x, __VA_ARGS__)
#define APPLY_16(x, first, ...) x(first) APPLY_15(x, __VA_ARGS__)

#define APPLY(x, ...)                                                          \
    EXPAND(                                                                    \
        CONCAT(APPLY_, COUNT_ARGS(__VA_ARGS__))(x, __VA_ARGS__))

#ifdef __cplusplus
    #define STRUCT_MEMBER_ASSIGN(x) out x;
    #define STRUCT_INITIALIZE_STATIC(id, type, ...)                            \
        static const type id##_initialize()                                    \
        {                                                                      \
            type out = {};                                                     \
            APPLY(STRUCT_MEMBER_ASSIGN, __VA_ARGS__);                          \
            return out;                                                        \
        }                                                                      \
        static const type id = id##_initialize()
#else
    #define STRUCT_MEMBER_ASSIGN(x) x,
    #define STRUCT_INITIALIZE_STATIC(id, type, ...)                            \
        static const type id = { APPLY(STRUCT_MEMBER_ASSIGN, __VA_ARGS__) }
#endif

/* -------------------------------------------------------------------------- */

struct my_struct {
    int foo;
    int bar;
};

STRUCT_INITIALIZE_STATIC(my_data, struct my_struct, .foo=123, .bar=456);

/* -------------------------------------------------------------------------- */

int
main()
{
    printf("foo: %d, bar: %d\n", my_data.foo, my_data.bar);
    return 0;
}