Precision loss when passing arguments using va_list and printf - c++

Question

I have a logging function which works like this:
// public function:
void ConsoleUI::log(const std::string& format, ...) {
va_list args;
va_start(args, format);
log(format, args);
va_end(args);
}
// overloaded private function:
void ConsoleUI::log(const std::string& format, va_list args) {
//wprintw( outWin, format.c_str(), args);
//wprintw( outWin, "\n");
printf( format.c_str(), args);
printf( "\n");
}
(Side note: This should do the exact same thing as just a normal printf, so it's pretty useless in this state. That's because this is a minumal working example. In the end, this should work with ncurses - see commented section.)
I then create an instance of the ConsoleUI class, called ui:
ConsoleUI ui;
And later I use it to log stuff, more precisely, a time_duration in microseconds:
now = boost::posix_time::microsec_clock::local_time();
boost::posix_time::time_duration delta = now - lastTime;
double dt = 1e-6*(double)delta.total_microseconds();
lastTime = now;
ui.log( "logged dt: %f", dt );
ui.log( "logged dt 2: %lu", delta.total_microseconds());
printf( "dt: %f\n", dt);
printf( "dt 2: %lu\n", delta.total_microseconds());
The output I get:
logged dt: 0.000000
logged dt 2: 140736013247624
dt: 0.018739
dt 2: 18739
What I expected:
logged dt: 0.018739
logged dt 2: 18739
dt: 0.018739
dt 2: 18739
Note that over multiple calls of this, the values in the last two lines change slightly (as is to be expected of a delta time) and the first two values don't change - which looks like there's something wrong with the format.
So, bottom line: Calling printf directly works, but passing it down the logger and then calling printf doesn't work...

Answer 1

Since you can't actually pass your arguments from log to printf, you are passing a va_list - this is the right solution, but you can't pass a va_list to printf.
This line:
printf( format.c_str(), args);
prints args with the format in format. Given that args is of the type va_list (typically implemented in the comiler ABI as a void pointer) it will not represent the data that you expect in the format string. If you use
vprintf( format.c_str(), args);
it should work just fine, that's what vprintf is for.
(Note that it's nothing to do with "loss of precision" - it's passing incorrect type of argument to printf - and since it's a programmaticly formulated string, even enabling warnings is not going to work).