va_copy -- porting to visual C++?

Question

A previous question showed a nice way of printing to a string. The answer involved va_copy:

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]);
   }

}

The problem I'm having is that the above code doesn't port to Visual C++ because it doesn't provide va_copy (or even __va_copy). So, does anyone know how to safely port the above code? Presumably, I need to do a va_copy copy because vsnprintf destructively modifies the passed va_list.

Answer 1

Varargs is not a nice way to print a string. In C++, it was the least terrible solution there was.

The recommended way is to use the C++ iostreams library. If you absolutely want printf-like formatting and syntax, you can use boost::format with it, which preserves type safety and works with C++ types.

And finally, if you insist on throwing away type safety and limiting yourself to C types, you already have the printf family of functions in the language, and there is no need to reinvent them.

Answer 2

I've implemented similar things in VC++ and have never needed to use va_copy(). What happens when you try it without using the copy?

Answer 3

You should be able to get away with just doing a regular assignment:

va_list apcopy = ap;

It's technically non-portable and undefined behavior, but it will work with most compilers and architectures. In the x86 calling convention, va_lists are just pointers into the stack and are safe to copy.

Answer 4

One thing you can do is if you do not otherwise need the vformat() function, move its implementation into the format() function (untested):

#include <stdarg.h>
#include <string.h>
#include <assert.h>
#include <string>
#include <vector>


std::string format(const char *fmt, ...)
{
   va_list ap;

   enum {size = 1024};

   // if you want a buffer on the stack for the 99% of the time case 
   //   for efficiency or whatever), I suggest something like
   //   STLSoft's auto_buffer<> template.
   //
   //   http://www.synesis.com.au/software/stlsoft/doc-1.9/classstlsoft_1_1auto__buffer.html
   //
   std::vector<char> buf( size);

   //
   // where you get a proper vsnprintf() for MSVC is another problem
   // maybe look at http://www.jhweiss.de/software/snprintf.html
   //

   // note that vsnprintf() might use the passed ap with the 
   //   va_arg() macro.  This would invalidate ap here, so we 
   //   we va_end() it here, and have to redo the va_start()
   //   if we want to use it again. From the C standard:
   //
   //       The object ap may be passed as an argument to
   //       another function; if that function invokes the 
   //       va_arg macro with parameter ap, the value of ap 
   //       in the calling function is indeterminate and 
   //       shall be passed to the va_end macro prior to 
   //       any further reference to ap.   
   //
   //    Thanks to Rob Kennedy for pointing that out.
   //
   va_start (ap, fmt);
   int needed = vsnprintf (&buf[0], buf.size(), fmt, ap);
   va_end( ap);

   if (needed >= size) {
       // 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.
       buf.resize( needed + 1);

       va_start (ap, fmt);
       needed = vsnprintf (&buf[0], buf.size(), fmt, ap);
       va_end( ap);

       assert( needed < buf.size());
   }

   return std::string( &buf[0]);
}