macros overridden by arg count

macros_overridden_by_arg_count

// Define two overrides that can be used by the expansion of 
// our main macro.
#define _MY_CONCAT3(a, b, c) a b c
#define _MY_CONCAT2(a, b) a b

// Define a macro that uses the "paired, sliding arg list"
// technique to select the appropriate override. You should
// recognize this as similar to the GET_NTH_ARG() macro in
// previous examples.
#define _GET_OVERRIDE(_1, _2, _3, NAME, ...) NAME

// Define a macro that concats either 3 or 2 strings together.
#define MY_CONCAT(...) _GET_OVERRIDE(__VA_ARGS__, \
    _MY_CONCAT3, _MY_CONCAT2)(__VA_ARGS__)

int main() {
    printf("3 args: %s\n", MY_CONCAT("a", "b", "c"));
    printf("2 args: %s", MY_CONCAT("a", "b"));
}
 
// ------ output --------
3 args: abc
2 args: ab