To get a value inside [-pi,pi], you can add pi, do fmod, and subtract pi again. Here is one way that works:

Scalar rotationBetween(Scalar from, Scalar to)
{
    if (from > to)
        return -pi + std::fmod(from - to + pi, pi * 2);
    else
        return pi - std::fmod(to - from + pi, pi * 2);
}

If you just want short code and don’t care about performance, this can be interesting:

Scalar rotationBetween(Scalar from, Scalar to)
{
    return 2 * std::atan(std::tan((from - to) / 2);
}

To get a value inside [-pi,pi], you can add pi, do fmod, and subtract pi again. Here is one way that works. Unfortunately you still need to somehow test whether from - to is positive or negative before calling fmod:

Scalar rotationBetween(Scalar from, Scalar to)
{
    return (from > to) ? -pi + std::fmod(from - to + pi, pi * 2)
                       :  pi - std::fmod(to - from + pi, pi * 2);
}

If you are absolutely sure that from and to are in [-2pi,2pi], then this simpler version will work:

Scalar rotationBetween(Scalar from, Scalar to)
{
    std::fmod(from - to + 5 * pi, pi * 2) - pi;
}

Finally, if you just want short code and don’t care about performance, this can be interesting:

Scalar rotationBetween(Scalar from, Scalar to)
{
    return 2 * std::atan(std::tan((from - to) / 2);
}