round() for float in C++

Question

I need a simple floating point rounding function, thus:

double round(double);

round(0.1) = 0
round(-0.1) = 0
round(-0.9) = -1

I can find

Answer 1

If you ultimately want to convert the double output of your round() function to an int, then the accepted solutions of this question will look something like:

int roundint(double r) {
  return (int)((r > 0.0) ? floor(r + 0.5) : ceil(r - 0.5));
}

This clocks in at around 8.88 ns on my machine when passed in uniformly random values.

The below is functionally equivalent, as far as I can tell, but clocks in at 2.48 ns on my machine, for a significant performance advantage:

int roundint (double r) {
  int tmp = static_cast (r);
  tmp += (r-tmp>=.5) - (r-tmp<=-.5);
  return tmp;
}

Among the reasons for the better performance is the skipped branching.