Can I define variadic C preprocessor macros with __VA_ARGS in the middle instead of the end?

问题

GCC complains if I do this:

#define M(obj,met, ..., contents) obj##_##met(const void * self, __VA_ARGS__) { \
   contents \
   }

Giving me these 2 reasons:

error: missing ')' in macro parameter list
warning: __VA_ARGS__ can only appear in the expansion of a C99 variadic macro

Apparently, C99 - style variadic macros expect the closing parenthesis immediately after the ellipsis, effectively demanding that the variadic list be the last arguments of the macro. I need it to be in the middle to produce my shorthand notation described in the above macro. Does GCC support this feature, using another (non-C99) variadic macro style? Can I emulate it doing it someway else? I don't want the variadic list at the end, it will make my notation confusing. And I can only use GCC.

回答1:

No you can't. The ... must appear at the end.

But you could define M as

#define M(obj,met, ...) obj##_##met(const void * self, __VA_ARGS__)

and use it as

void M(foo, bar, int x, char y, double z) {
   content;
}

回答2:

You have to put the ... at the end, but using LAST and POP_LAST macros, you can keep the same order of arguments for your macro and define it like this:

#define M(obj,met, ...) obj##_##met(const void * self, POP_LAST(__VA_ARGS__)) { \
   LAST(__VA_ARGS__) \
   }

Heres how you can define these macros:

/* This counts the number of args */
#define NARGS_SEQ(_1,_2,_3,_4,_5,_6,_7,_8,N,...) N
#define NARGS(...) NARGS_SEQ(__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1)

/* This will let macros expand before concating them */
#define PRIMITIVE_CAT(x, y) x ## y
#define CAT(x, y) PRIMITIVE_CAT(x, y)

/* This will pop the last argument off */
#define POP_LAST(...) CAT(POP_LAST_, NARGS(__VA_ARGS__))(__VA_ARGS__)
#define POP_LAST_1(x1)
#define POP_LAST_2(x1, x2) x1
#define POP_LAST_3(x1, x2, x3) x1, x2
#define POP_LAST_4(x1, x2, x3, x4) x1, x2, x3
#define POP_LAST_5(x1, x2, x3, x4, x5) x1, x2, x3, x4
#define POP_LAST_6(x1, x2, x3, x4, x5, x6) x1, x2, x3, x4, x5
#define POP_LAST_7(x1, x2, x3, x4, x5, x6, x7) x1, x2, x3, x4, x5, x6
#define POP_LAST_8(x1, x2, x3, x4, x5, x6, x7, x8) x1, x2, x3, x4, x5, x6, x7

/* This will return the last argument */
#define LAST(...) CAT(LAST_, NARGS(__VA_ARGS__))(__VA_ARGS__)
#define LAST_1(x1) x1
#define LAST_2(x1, x2) x2
#define LAST_3(x1, x2, x3) x3
#define LAST_4(x1, x2, x3, x4) x4
#define LAST_5(x1, x2, x3, x4, x5) x5
#define LAST_6(x1, x2, x3, x4, x5, x6) x6
#define LAST_7(x1, x2, x3, x4, x5, x6, x7) x7
#define LAST_8(x1, x2, x3, x4, x5, x6, x7, x8) x8

These macros will work for up to 8 arguments. You can easily extend them to handle more if you like.

来源：https://stackoverflow.com/questions/3850421/can-i-define-variadic-c-preprocessor-macros-with-va-args-in-the-middle-instead

标签

gcc

c-preprocessor

c99

variadic-macros