Best way to get the Fractional Part of a Number

Question

Given const auto foo = 13.42 I would like to get the numbers fractional part, so .42

I'm leaning toward just using fmod like: fmod(foo, 1.0)

But I could also just do foo / static_cast<int>(foo) - 1.0 Or perhaps some other mystical method.

Would there be a motivation for me to not simply use fmod?

Answer 1

Two ways I can think of, either a cast or rounding down with std::floor

int main()
{    
    const auto foo = 13.53;

    auto firstWay = foo - static_cast<long long>(foo);  // Truncating via cast and subtracting

    auto otherWay = foo - std::floor(foo); // Rounding down and subtracting

    return 0;
}

Quick Bench Result shows the fmod approach as the slowest option by far, and the cast as the fastest: QuickBench