How do you create a debug only function that takes a variable argument list? Like printf()

I still do it the old way, by defining a macro (XTRACE, below) which correlates to either a no-op or a function call with a variable argument list. Internally, call vsnprintf so you can keep the printf syntax:

#include <stdio.h>

void XTrace0(LPCTSTR lpszText)
{
   ::OutputDebugString(lpszText);
}

void XTrace(LPCTSTR lpszFormat, ...)
{
    va_list args;
    va_start(args, lpszFormat);
    int nBuf;
    TCHAR szBuffer[512]; // get rid of this hard-coded buffer
    nBuf = _vsnprintf(szBuffer, 511, lpszFormat, args);
    ::OutputDebugString(szBuffer);
    va_end(args);
}

Then a typical #ifdef switch:

#ifdef _DEBUG
#define XTRACE XTrace
#else
#define XTRACE
#endif

Well that can be cleaned up quite a bit but it's the basic idea.

This is how I do debug print outs in C++. Define 'dout' (debug out) like this:

#ifdef DEBUG
#define dout cout
#else
#define dout 0 && cout
#endif

In the code I use 'dout' just like 'cout'.

dout << "in foobar with x= " << x << " and y= " << y << '\n';

If the preprocessor replaces 'dout' with '0 && cout' note that << has higher precedence than && and short-circuit evaluation of && makes the whole line evaluate to 0. Since the 0 is not used the compiler generates no code at all for that line.

Here's something that I do in C/C++. First off, you write a function that uses the varargs stuff (see the link in Stu's posting). Then do something like this:

 int debug_printf( const char *fmt, ... );
 #if defined( DEBUG )
  #define DEBUG_PRINTF(x) debug_printf x
 #else
   #define DEBUG_PRINTF(x)
 #endif

 DEBUG_PRINTF(( "Format string that takes %s %s\n", "any number", "of args" ));

All you have to remember is to use double-parens when calling the debug function, and the whole line will get removed in non-DEBUG code.