Detect presence or absence of arguments in a C macro

Question

How can one define a C macro IFARGS(YES, NO, ...) such that invoking IFARGS with no additional arguments produces NO, and invoking IFARGS with one or more arguments produces YES?

I have an answer using GCC (see below), but I'd prefer one for C99 if possible (or a proof of its impossibility).

Answer 1

In C99 it is possible to detect if a macro argument is empty, but making that robust against all odds that may appear in that argument (arguments that are themselves expanding, contain () and stuff like that) is difficult. My macro package P99 implements such a thing, so you wouldn't have to worry too much. With that your macro can be implemented as

#define IFARGS(YES, NO, ...) P99_IF_EMPTY(__VA_ARGS__)(YES(__VA__ARGS__))(NO())

As its name indicates, P99 is only building on C99 features for that.

Answer 2

#define GET(_0, _1) _0  // Return the first of two arguments
#define GET_(_0, _1) _1  // Return the second of two arguments

#define JOIN(_0, _1) _0 ## _1  // Concatenate two arguments
#define EJOIN(_0, _1) JOIN(_0, _1)  // Expand macros and concatenate

#define FIRST(_, ...) _  // Truncate everything after first comma
#define EFIRST(_) FIRST(_)  // Expand argument and pass to FIRST

#define REST(_0, ...) __VA_ARGS__  // Remove everything before first comma

#define GET_GET(...) \
    EJOIN(GET, EFIRST(REST(,,##__VA_ARGS__ _)))  // Branch between GET and GET_

#define IFARGS(YES, NO, ...) GET_GET(__VA_ARGS__)(YES, NO)

Note that if this were possible in C99, then it would be possible to simulate ##__VA_ARGS__, like so:

#define PREPEND_COMMA(...) , __VA_ARGS__
#define NO_COMMA()
#define PREPEND_COMMA_IF_NONEMPTY(...) IFARGS(PREPEND_COMMA, NO_COMMA, __VA_ARGS__)(__VA_ARGS__)

Then any instance of , ##__VA_ARGS__ could be replaced by PREPEND_COMMA_IF_NONEMPTY(__VA_ARGS__).