Generating function declaration using a macro iteration

Question

I'm trying to generate a function declaration using a macro

/* goal: generate int f(int a, float b) */
template<typename P>
struct ptype;

template<typename P>
struct ptype<void(P)> { typedef P type; };

#define NAMEe
#define COMMAe
#define COMMA ,

#define NAME(N) N PARAMS
#define PARAMS(P, ...) COMMA ## __VA_ARGS__ P NAME ## __VA_ARGS__
#define PARAM_ITER(P) P NAME

#define PROTO(R, N, P)  \
  ptype<void R>::type N (PARAM_ITER P (,e))

PROTO((int), f, (int)(a)(float)(b));

It will iteratively process the next (name) or (type) by NAME or PARAMS respectively, with the ... having an empty macro argument. But GCC complains with

prototype.hpp:20:35: warning: ISO C99 requires rest arguments to be used

And clang complains with

ptype<void (int)>::type f (int aprototype.hpp:20:1: warning: varargs argument missing, but tolerated as an extension [-pedantic]

I think this happens because of the following

#define FOO(X, ...)
FOO(A);

Because I'm not passing an argument for the ... or each of those (name) or (type). Is there any simple work around I can apply?

---

I've now used a technique similar to the technique used by @James to find the length of a parameter list. If as second argument, instead of O, ONT is passed, I will print the comma and NAME. The following is the final solution:

/* goal: generate void f(int a, float b) */
template<typename P>
struct ptype;

template<typename P>
struct ptype<void(P)> { typedef P type; };

#define TYPE_DO(X) X
#define TYPE_DONT(X)
#define TYPE_MAYBE(X, A, ...) TYPE_D ## A (X)

#define COMMA_DO ,
#define COMMA_DONT
#define COMMA_MAYBE(A, B, ...) COMMA_D ## B

#define NAME_DO NAME
#define NAME_DONT
#define NAME_MAYBE(A, B, ...) NAME_D ## B

#define NAME(N) N PARAMS
#define PARAMS(...) COMMA_MAYBE(__VA_ARGS__,O,O) TYPE_MAYBE(__VA_ARGS__,O,O) \
                    NAME_MAYBE(__VA_ARGS__,O,O)
#define PARAM_ITER(P) P NAME

#define PROTO(R, N, P)  \
  ptype<void R>::type N (PARAM_ITER P (D,ONT))

Test:

#define STR1(X) #X
#define STR(X) STR1(X)

int main() {
  // prints correctly
  std::cout << STR(PROTO((int), f, (int)(a)(float)(b)));
}

Answer 1

To solve the "FOO" problem, you can select different macros depending on the arity of the variable arguments pack. Here's a first shot at that:

// These need to be updated to handle more than three arguments:
#define PP_HAS_ARGS_IMPL2(_1, _2, _3, N, ...) N
#define PP_HAS_ARGS_SOURCE() MULTI, MULTI, ONE, ERROR

#define PP_HAS_ARGS_IMPL(...) PP_HAS_ARGS_IMPL2(__VA_ARGS__)
#define PP_HAS_ARGS(...)      PP_HAS_ARGS_IMPL(__VA_ARGS__, PP_HAS_ARGS_SOURCE())

#define FOO_ONE(x)     ONE_ARG:    x
#define FOO_MULTI(...) MULTI_ARG:  __VA_ARGS__

#define FOO_DISAMBIGUATE2(has_args, ...) FOO_ ## has_args (__VA_ARGS__)
#define FOO_DISAMBIGUATE(has_args, ...) FOO_DISAMBIGUATE2(has_args, __VA_ARGS__)
#define FOO(...) FOO_DISAMBIGUATE(PP_HAS_ARGS(__VA_ARGS__), __VA_ARGS__)

Usage example:

FOO(1)     // replaced by ONE_ARG:   1
FOO(1, 2)  // replaced by MULTI_ARG: 1, 2

(I'll try to revisit this to clean it up; I think there are definitely some unneccessary macros in there. I haven't had a chance to look into the broader problem that you describe, so I'm not sure if this solves that too. There may be a simpler way to solve that problem, too... I'm not particularly familiar with variadic macros. This preprocesses cleanly on mcpp.)