length of va_list when using variable list arguments?

Question

Is there any way to compute length of va_list? All examples I saw the number of variable parameters is given explicitly.

Answer 1

One approach that hasn't been mentioned yet is to use a pre-processor macro to call the variadict function using the va_list length as the first parameter and also forward along the arguments. This is somewhat of a "cute" solution, but does not require manually inputting the argument list length.

Assume you have the following function:

int Min(int count, ...) {
    va_list args;
    va_start(args, count);

    int min = va_arg(args, int);
    for (int i = 0; i < count-1; ++i) {
      int next = va_arg(args, int);
      min = min < next ? min : next;
    }
    va_end(args);

    return min;
}

The idea is that you have a preprocessor macro capable of counting the number of arguments by using a mask for the __VA_ARGS__. There are a few good preprocessor libraries for determining the __VA_ARGS__ length including P99 and Boost Preprocessor, but just so I don't leave holes in this answer, here's how it can be done:

#define IS_MSVC _MSC_VER && !__INTEL_COMPILER

/**
 * Define the macros to determine variadic argument lengths up to 20 arguments. The MSVC 
 * preprocessor handles variadic arguments a bit differently than the GNU preprocessor,
 * so we account for that here. 
 */
#if IS_MSVC
  #define MSVC_HACK(FUNC, ARGS) FUNC ARGS
  #define APPLY(FUNC, ...) MSVC_HACK(FUNC, (__VA_ARGS__))
  #define VA_LENGTH(...) APPLY(VA_LENGTH_, 0, ## __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#else
  #define VA_LENGTH(...) VA_LENGTH_(0, ## __VA_ARGS__, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#endif

/**
 * Strip the processed arguments to a length variable.
 */
#define VA_LENGTH_(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, N, ...) N

Note: A lot of the noise from above is work-around support for MSVC.

With the above defined, you can create a single macro to perform all length based operations:

/**
 * Use the VA_LENGTH macro to determine the length of the variadict args to
 * pass in as the first parameter, and forward along the arguments after that.
 */
#define ExecVF(Func, ...) Func(VA_LENGTH(__VA_ARGS__), __VA_ARGS__)

This macro is capable of calling any variadict function as long as it begins with the int count parameter. In short, instead of using:

int result = Min(5, 1, 2, 3, 4, 5);

You can use:

int result = ExecVF(Min, 1, 2, 3, 4, 5);

Here's a template version of Min which uses the same approach: https://gist.github.com/mbolt35/4e60da5aaec94dcd39ca