“-Weverything” yielding “Comparing floating point with == or != is unsafe”

Question

I have a string that I convert to a double like this:

double d = [string doubleValue];

The documentation for doubleValue tells

Answer 1

Probably not necessary for this very simple use case, but here is an example of how you would account for the potential discrepancy if you were trying check if a number is equal to -1:

#include 
#include 

int main(void) {
  float sales = -1;

  // basically if sales == -1
  if (fabs(1 + sales) < FLT_EPSILON) {
    return 0;
  }
}