Why is std::atan2 returning -0.0?

Question

Really? Negative zero?!

double Vector2D::GetFacingAngle(const Vector2D& target, const Vector2D& source) {
    a2de::Vector2D facingVec(Vector2D(source) - Vector2D(target));

    //Negating 'y' argument to account for flipped nature of 'y' screen coordinates.
    double result = std::atan2(-facingVec.GetY(), facingVec.GetX());

    //Which causes -0.0 as a possible result when source is right of target at the same 'y' coordinate.
    if(a2de::Math::IsEqual(result, -0.0)) return 0.0;
    return result;
}

Is this a common behavior? (Similar to subtracting 'y' coordinates for 'up' movement in screen coordinates contrary to Cartesian coordinates.)

Answer 1

What you're seeing is a signed zero, which is a possibility in certain systems of number representation, including the IEEE standard used for C++ floating point representation, even though it isn't really a thing in "ordinary" mathematics.

The link contains some notes on behavior and handling of the occurrence, although beyond that it's not entirely clear to me what you're asking, specifically -- it's not a bug with atan2 though. In fact, the wiki article on the function notes that the relevant Intel assembly instruction (FPATAN), which you're likely generating here, will produce a negative zero when first parameter in negative zero but the second is positive zero. Your explicit negation of your vector's Y component could easily produce such a scenario.