Variable arguments inside a macro

Question

I have two functions foo1(a,b) & foo2(a,b,c) and a macro

#define add(a,b) foo(a,b)

I need to re-define macro to accomplish,

1.if a

Answer 1

If you must use variadic macros, then here is a trick.

#define add(...) _Generic ( &(int[]){__VA_ARGS__}, \
                            int(*)[2]: add2,       \
                            int(*)[3]: add3) (__VA_ARGS__)

• Have the macro create a compound literal array. The size of this array will depend on the number of arguments.
• Grab the address of the compound literal, to get an array pointer type.
• Let _Generic check which type you got, then call the proper function based on that.

This is 100% standard C and also type safe.

---

Demo:

#include 


#define add(...) _Generic ( &(int[]){__VA_ARGS__}, \
                            int(*)[2]: add2,       \
                            int(*)[3]: add3) (__VA_ARGS__)

int add2 (int a, int b);
int add3 (int a, int b, int c);

int main (void)
{
  printf("%d\n", add(1, 2));
  printf("%d\n", add(1, 2, 3));
//printf("%d\n", add(1, 2, 3, 4)); Compiler error for this.
}


int add2 (int a, int b)
{
  return a + b;
}

int add3 (int a, int b, int c)
{
  return a + b + c;
}