Passing variable number of arguments around

In magnificent C++11 you could use variadic templates:

template <typename... Ts>
void format_string(char *fmt, Ts ... ts) {}

template <typename... Ts>
void debug_print(int dbg_lvl, char *fmt, Ts... ts)
  format_string(fmt, ts...);

To pass the ellipses on, you initialize a va_list as usual and simply pass it to your second function. You don't use va_arg(). Specifically;

void format_string(char *fmt,va_list argptr, char *formatted_string);

void debug_print(int dbg_lvl, char *fmt, ...) 
 char formatted_string[MAX_FMT_SIZE];

 va_list argptr;
 format_string(fmt, argptr, formatted_string);
 fprintf(stdout, "%s",formatted_string);

There's no way of calling (eg) printf without knowing how many arguments you're passing to it, unless you want to get into naughty and non-portable tricks.

The generally used solution is to always provide an alternate form of vararg functions, so printf has vprintf which takes a va_list in place of the .... The ... versions are just wrappers around the va_list versions.

Variadic Functions can be dangerous. Here's a safer trick:

   void func(type* values) {
        while(*values) {
            x = *values++;
            /* do whatever with x */