Eigen passing Quaternionf, discards qualifiers - c++

Question

i am trying to use Eigen::Quaternionf. But i am getting when i just want to asign one Quaternion an error.
oldQuat = pos;
both are Eigen::Quaternionf, the following error is given. Is must be because the Methods are declared as constant (see: http://eigen.tuxfamily.org/dox/classEigen_1_1Quaternion.html)
Fehler: passing 'const Quaternionf {aka const Eigen::Quaternion<float>}' as 'this' argument of 'Eigen::Quaternion<Scalar, Options>& Eigen::Quaternion<Scalar, Options>::operator=(const Eigen::Quaternion<Scalar, Options>&) [with _Scalar = float, int _Options = 0, Eigen::Quaternion<Scalar, Options> = Eigen::Quaternion<float>]' discards qualifiers [-fpermissive]
I have to idea on how to get past this error. Thanks in advance
edit:
for (itCanon = canonicalValues.begin(), itTraj = exampleTraj.begin(); itCanon != canonicalValues.end(); ++itCanon, itTraj++)
{
const SampledTrajectoryV2::TrajData& state = *itTraj;
Eigen::Vector3f axis;
axis << itTraj->getPosition(1), itTraj->getPosition(2), itTraj->getPosition(3);
Eigen::AngleAxisf angleAxis(itTraj->getPosition(0), axis);
Eigen::Quaternionf pos(angleAxis);
pos.normalize();
//Error in both these lines! Same error both times.
Eigen::Vector3f vecVel = calcAngularVelocity(oldQuat, pos);
oldQuat = pos;
// D0 element R3x3 nicht kompatibel mit quat
result[*itCanon] = - A_Z*(B_Z*2*log(pos) - TAU * vecVel);
}
oldQuat is declared in the header. as are A_z, B_Z and TAU.

Answer 1

Eigen::Quaternionf<S, O>::operator= is declared as non const, but oldQuat is a const object (I suspect). Change it to non-const.