Overloading Macro on Number of Arguments

Question

I have two macros FOO2 and FOO3:

#define FOO2(x,y) ...
#define FOO3(x,y,z) ...

I want to define a new macro

Answer 1

I was just researching this myself, and I came across this here. The author added default argument support for C functions via macros.

I'll try to briefly summarize the article. Basically, you need to define a macro that can count arguments. This macro will return 2, 1, 0, or whatever range of arguments it can support. Eg:

#define _ARG2(_0, _1, _2, ...) _2
#define NARG2(...) _ARG2(__VA_ARGS__, 2, 1, 0)

With this, you need to create another macro that takes a variable number of arguments, counts the arguments, and calls the appropriate macro. I've taken your example macro and combined it with the article's example. I have FOO1 call function a() and FOO2 call function a with argument b (obviously, I'm assuming C++ here, but you can change the macro to whatever).

#define FOO1(a) a();
#define FOO2(a,b) a(b);

#define _ARG2(_0, _1, _2, ...) _2
#define NARG2(...) _ARG2(__VA_ARGS__, 2, 1, 0)

#define _ONE_OR_TWO_ARGS_1(a) FOO1(a)
#define _ONE_OR_TWO_ARGS_2(a, b) FOO2(a,b)

#define __ONE_OR_TWO_ARGS(N, ...) _ONE_OR_TWO_ARGS_ ## N (__VA_ARGS__)
#define _ONE_OR_TWO_ARGS(N, ...) __ONE_OR_TWO_ARGS(N, __VA_ARGS__)

#define FOO(...) _ONE_OR_TWO_ARGS(NARG2(__VA_ARGS__), __VA_ARGS__)

So if you have

FOO(a)
FOO(a,b)

The preprocessor expands that to

a();
a(b);

I would definitely read the article that I linked. It's very informative and he mentions that NARG2 won't work on empty arguments. He follows this up here.