Why does this variadic argument count macro fail with VC++?

Question

I got the following implementation to get the number of arguments in a variadic macro (currently limited to 16 args). However, for VS2010 the output is always 1

Answer 1

Does the following work-around help?

#define EXPAND(x) x
#define PP_NARGS(...) \
    EXPAND(_xPP_NARGS_IMPL(__VA_ARGS__,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0))

I think your macro isn't wrong in particular, but MSVC's __VA_ARGS__ expansion seems to behave differently from C99.

Answer 2

The problem seems that Visual Studio is expanding __VA_ARGS__ after passing it into the subsequent macro, while gcc expands it before passing.

In your case, PP_NARGS(A,V,C,X,Y,Z) binds A,V,C,X,Y,Z to __VA_ARGS__, and then passes it as a whole to _xPP_NARGS_IMPL.

As a test, run:

#define PP_NARGS(...) \
    _xPP_NARGS_IMPL(__VA_ARGS__,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0)

#define _xPP_NARGS_IMPL(x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x11,x12,x13,x14,x15,N,...) \
  (std::cout << #x1 << std::endl, N) 

int main() {
  int i = PP_NARGS(A, V, C, X, Y, Z);
  std::cout << i;
  return 0;
}

You will see A, V, C, X, Y, Z printed on the screen, and not just A as you would probably expect.

---

A possible solution, as suggested by Ise Wisteria already, is to force the expansion via:

#define EXPAND(x) x
#define PP_NARGS(...) \
    EXPAND(_xPP_NARGS_IMPL(__VA_ARGS__,15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0))