__VA_ARGS__に含まれる引数の数を数えるマクロ。

num_va_args.cpp

// Getting the number of variable arguments contained in __VA_ARGS__
// Original idea is: http://groups.google.com/group/comp.std.c/browse_thread/thread/77ee8c8f92e4a3fb/346fc464319b1ee5

#include <cstdio>
#include <iostream>

#define PP_NARG(...) PP_NARG2(0, ##__VA_ARGS__, PP_RSEQ_N)
#define PP_NARG2(...) PP_ARG_N(__VA_ARGS__)
#define PP_ARG_N(__0, __1, __2, __3, __4, __5, __6, __7, __8, __9,	\
		 _10, _11, _12, _13, _14, _15, _16, _17, _18, _19,	\
		 _20, _21, _22, _23, _24, _25, _26, _27, _28, _29,	\
		 _30, _31, _32, _33, _34, _35, _36, _37, _38, _39,	\
		 _40, _41, _42, _43, _44, _45, _46, _47, _48, _49,	\
		 _50, _51, _52, _53, _54, _55, _56, _57, _58, _59,	\
		 _60, _61, _62, _63, N, ...) (N)
#define PP_RSEQ_N 63, 62, 61, 60,		\
    59, 58, 57, 56, 55, 54, 53, 52, 51, 50,	\
    49, 48, 47, 46, 45, 44, 43, 42, 41, 40,	\
    39, 38, 37, 36, 35, 34, 33, 32, 31, 30,	\
    29, 28, 27, 26, 25, 24, 23, 22, 21, 20,	\
    19, 18, 17, 16, 15, 14, 13, 12, 11, 10,	\
    9,  8,  7,  6,  5,  4,  3,  2,  1,  0

int main()
{
  std::cout << PP_NARG() << std::endl;	   // => 0
  std::cout << PP_NARG(A) << std::endl;	   // => 1
  std::cout << PP_NARG(A, B) << std::endl; // => 2
  return 0;
}