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

Simple as:

#define GET_MACRO(_1,_2,_3,NAME,...) NAME
#define FOO(...) GET_MACRO(__VA_ARGS__, FOO3, FOO2)(__VA_ARGS__)

So if you have these macros:

FOO(World, !)         # expands to FOO2(World, !)
FOO(foo,bar,baz)      # expands to FOO3(foo,bar,baz)

If you want a fourth one:

#define GET_MACRO(_1,_2,_3,_4,NAME,...) NAME
#define FOO(...) GET_MACRO(__VA_ARGS__, FOO4, FOO3, FOO2)(__VA_ARGS__)

FOO(a,b,c,d)          # expeands to FOO4(a,b,c,d)

Naturally, if you define FOO2, FOO3 and FOO4, the output will be replaced by those of the defined macros.