second parameter of `va_start' not last named argument

[Antonio Coralles <noche.suapie@xxxxxxxxx>]

is there a way to prevent this gcc warning?
in a code similar to

void foo(const std::string& format, ...);

i determine the neccessary number of arguments by counting the 
occurences of "%s" in format,
similar to printf. therefore, there is no need for the caller to pass 
the number of arguments as a second
argument.

thanks
            Antonio