问va_copy --移植到visual C++？

EN

std::string format (const char *fmt, ...);
{
   va_list ap;
   va_start (ap, fmt);
   std::string buf = vformat (fmt, ap);
   va_end (ap);
   return buf;
}


std::string vformat (const char *fmt, va_list ap)
{
   // Allocate a buffer on the stack that's big enough for us almost
   // all the time.
   s ize_t size = 1024;
   char buf[size];

   // Try to vsnprintf into our buffer.
   va_list apcopy;
   va_copy (apcopy, ap);
   int needed = vsnprintf (&buf[0], size, fmt, ap);

   if (needed <= size) {
       // It fit fine the first time, we're done.
       return std::string (&buf[0]);
   } else {
       // vsnprintf reported that it wanted to write more characters
       // than we allotted.  So do a malloc of the right size and try again.
       // This doesn't happen very often if we chose our initial size
       // well.
       std::vector <char> buf;
       size = needed;
       buf.resize (size);
       needed = vsnprintf (&buf[0], size, fmt, apcopy);
       return std::string (&buf[0]);
   }