CODEWITHSUNDEEP
c++visual-c++macros
ceztko
ceztko

Reputation: 15227

Argument counting macro with zero arguments for VisualStudio

gcc does support argument counting macros with zero arguments with the ## __VA_ARGS__ convention. The following works compiled with gcc:

#include <stdio.h>

#define NARGS(...) __NARGS(0, ## __VA_ARGS__, 5,4,3,2,1,0)
#define __NARGS(_0,_1,_2,_3,_4,_5,N,...) N

int main()
{
  printf("%d\n", NARGS());     // prints 0
  printf("%d\n", NARGS(1));    // prints 1
  printf("%d\n", NARGS(1, 2)); // prints 2
  return 0;
}

Is there an equivalent for VisualC++ that will work with zero arguments macros? Non standard extensions or tricks accepted.

EDIT: Fixed the example to work with GCC extensions and C++ compiler.

Upvotes: 4

Views: 2967

Answers (4)

Captain&#39;Flam
Captain&#39;Flam

Reputation: 537

Here is some code that works with gcc (5.3) AND VisualStudio2010 :

#include <stdio.h>

#define expand(x)                          x
#define prefix(...)                        0,##__VA_ARGS__
#define lastof10(a,b,c,d,e,f,g,h,i,j,...)  j
#define sub_nbarg(...)                     expand(lastof10(__VA_ARGS__,8,7,6,5,4,3,2,1,0))
#define nbarg(...)                         sub_nbarg(prefix(__VA_ARGS__))

#define test(...) printf("(%s) ---> %d\n",""#__VA_ARGS__,nbarg(__VA_ARGS__))

int main () 
    {
    test() ;              // () ---> 0
    test(1) ;             // (1) ---> 1
    test(1,2) ;           // (1,2) ---> 2
    test(1,2,3) ;         // (1,2,3) ---> 3
    test(1,2,3,4) ;       // (1,2,3,4) ---> 4
    test(1,2,3,4,5) ;     // (1,2,3,4,5) ---> 5
    test(1,2,3,4,5,6) ;   // (1,2,3,4,5,6) ---> 6
    return 0 ;
    }

Upvotes: 2

ceztko
ceztko

Reputation: 15227

The following example works fine in VisualStudio 2010 and newer, gcc and clang with non standard extensions enabled. In Microsoft compilers it assumes the trailing comma in the AUGMENTER macro will be removed by the preprocessor when arguments count is zero. This is non standard and it has been also reported elsewere. In gcc and clang it uses the widely known ## __VA_ARGS__ non standard extension. 

#include <stdio.h>

#ifdef _MSC_VER // Microsoft compilers

#define EXPAND(x) x
#define __NARGS(_1, _2, _3, _4, _5, VAL, ...) VAL
#define NARGS_1(...) EXPAND(__NARGS(__VA_ARGS__, 4, 3, 2, 1, 0))

#define AUGMENTER(...) unused, __VA_ARGS__
#define NARGS(...) NARGS_1(AUGMENTER(__VA_ARGS__))

#else // Others

#define NARGS(...) __NARGS(0, ## __VA_ARGS__, 5,4,3,2,1,0)
#define __NARGS(_0,_1,_2,_3,_4,_5,N,...) N

#endif

int main()
{
  // NARGS
  printf("%d\n", NARGS());          // Prints 0
  printf("%d\n", NARGS(1));         // Prints 1
  printf("%d\n", NARGS(1, 2));      // Prints 2
  fflush(stdout);

#ifdef _MSC_VER
  // NARGS minus 1
  printf("\n");
  printf("%d\n", NARGS_1(1));       // Prints 0
  printf("%d\n", NARGS_1(1, 2));    // Prints 1
  printf("%d\n", NARGS_1(1, 2, 3)); // Prints 2
#endif

  return 0;
}

Macros were tested with real compilers, Wandbox and Webcompiler

Upvotes: 16

BLUEPIXY
BLUEPIXY

Reputation: 40145

Using the idea of @Jarod42 and BOOST_PP_VARIADIC_SIZE of boost can be written as follows.

#include <stdio.h>
#include <boost/preprocessor/variadic/size.hpp>

#define FOO(...) (sizeof(""#__VA_ARGS__) == sizeof("") ? 0 : BOOST_PP_VARIADIC_SIZE(__VA_ARGS__))

int main()
{
  printf("%d\n", FOO());     // prints 0
  printf("%d\n", FOO(1));    // prints 1
  printf("%d\n", FOO(1, 2)); // prints 2
  return 0;
}

Upvotes: -1

Jarod42
Jarod42

Reputation: 218148

Could you try:

#define __NARGS(_1, _2, _3, _4, _5, VAL, ...) VAL
#define NARGS(...) (sizeof(#__VA_ARGS__) == sizeof("") ? 0 : __NARGS(__VA_ARGS__, 5, 4, 3, 2, 1))

That works with g++ (Demo).

Upvotes: 0

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