How to test for lossless double / integer conversion?

Question

I have one double, and one int64_t. I want to know if they hold exactly the same value, and if converting one type into the other does not lose any information.

My curre

Answer 1

In addition to Pascal Cuoq's elaborate answer, and given the extra context you give in comments, I would add a test for negative zeros. You should preserve negative zeros unless you have good reasons not to. You need a specific test to avoid converting them to (int64_t)0. With your current proposal, negative zeros will pass your test, get stored as int64_t and read back as positive zeros.

I am not sure what is the most efficient way to test them, maybe this:

int int64EqualsDouble(int64_t i, double d) {
    return (d >= INT64_MIN)
        && (d < INT64_MAX)
        && (round(d) == d)
        && (i == (int64_t)d
        && (!signbit(d) || d != 0.0);
}