Transform quaternion orientation to vector

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Hi,

I have a geometry_message::Pose message. This message has a position and an orientation in quaternions. I would like to obtain a vector that represents the orientation. Just like in rviz you visualize a pose with an arrow. The arrow is given by the position and a direction (orientation).

How can I transform the quaternion into a unit vector that represents the direction? In CPP.

Thanks in advance!