C Macros: How to map another macro to variadic arguments?

Question

I'd like to know how to apply a unary function (or another macro) to variadic arguments of a macro, like

int f(int a);

#define apply(args...) <the magic>
apply(a, b, c)

which unrolls

f(a)
f(b)
f(c)

Note that the number of arguments is unknown.

Answer 1

The code below is working for what you've asked for with up to 1024 arguments and without using additional stuff like boost. It defines an EVAL(...) and also a MAP(m, first, ...) macro to do recursion and to use for each iteration the macro m with the next parameter first.

With the use of that, your apply(...) looks like: #define apply(...) EVAL(MAP(apply_, __VA_ARGS__)).

It is mostly copied from C Pre-Processor Magic. It is also great explained there. You can also download these helper macros like EVAL(...) at this git repository, there are also a lot of explanation in the actual code. It is variadic so it takes the number of arguments you want.

But I changed the FIRST and the SECOND macro as it uses a Gnu extension like it is in the source I've copied it from. This is said in the comments below by @HWalters:

Specifically, 6.10.3p4: "Otherwise [the identifier-list ends in a ...] there shall be more arguments in the invocation than there are parameters in the macro definition (excluding the ...)".

Main function part:

int main()
{
   int a, b, c;
   apply(a, b, c) /* Expands to: f(a); f(b); f(c); */

   return 0;
}

Macro definitions:

#define FIRST_(a, ...) a
#define SECOND_(a, b, ...) b

#define FIRST(...) FIRST_(__VA_ARGS__,)
#define SECOND(...) SECOND_(__VA_ARGS__,)

#define EMPTY()

#define EVAL(...) EVAL1024(__VA_ARGS__)
#define EVAL1024(...) EVAL512(EVAL512(__VA_ARGS__))
#define EVAL512(...) EVAL256(EVAL256(__VA_ARGS__))
#define EVAL256(...) EVAL128(EVAL128(__VA_ARGS__))
#define EVAL128(...) EVAL64(EVAL64(__VA_ARGS__))
#define EVAL64(...) EVAL32(EVAL32(__VA_ARGS__))
#define EVAL32(...) EVAL16(EVAL16(__VA_ARGS__))
#define EVAL16(...) EVAL8(EVAL8(__VA_ARGS__))
#define EVAL8(...) EVAL4(EVAL4(__VA_ARGS__))
#define EVAL4(...) EVAL2(EVAL2(__VA_ARGS__))
#define EVAL2(...) EVAL1(EVAL1(__VA_ARGS__))
#define EVAL1(...) __VA_ARGS__

#define DEFER1(m) m EMPTY()
#define DEFER2(m) m EMPTY EMPTY()()

#define IS_PROBE(...) SECOND(__VA_ARGS__, 0)
#define PROBE() ~, 1

#define CAT(a,b) a ## b

#define NOT(x) IS_PROBE(CAT(_NOT_, x))
#define _NOT_0 PROBE()

#define BOOL(x) NOT(NOT(x))

#define IF_ELSE(condition) _IF_ELSE(BOOL(condition))
#define _IF_ELSE(condition) CAT(_IF_, condition)

#define _IF_1(...) __VA_ARGS__ _IF_1_ELSE
#define _IF_0(...)             _IF_0_ELSE

#define _IF_1_ELSE(...)
#define _IF_0_ELSE(...) __VA_ARGS__

#define HAS_ARGS(...) BOOL(FIRST(_END_OF_ARGUMENTS_ __VA_ARGS__)())
#define _END_OF_ARGUMENTS_() 0

#define MAP(m, first, ...)           \
  m(first)                           \
  IF_ELSE(HAS_ARGS(__VA_ARGS__))(    \
    DEFER2(_MAP)()(m, __VA_ARGS__)   \
  )(                                 \
    /* Do nothing, just terminate */ \
  )
#define _MAP() MAP

#define apply_(x) f(x);
#define apply(...) EVAL(MAP(apply_, __VA_ARGS__))

---

To test macro expansion it is useful to use gcc with the command line argument -E:

$ gcc -E srcFile.c

because your're getting concrete error messages and understand what's going on.