Eigen library (http://goo.gl/cV5LY), which is used extensively in ROS and PCL.
Robotics Stack Exchange is a question and answer site for professional robotic engineers, hobbyists, researchers and students. It only takes a minute to sign up.Sign up to join this community
Welcome neel, please take the time to look around and contribute to other questions.
Eigen has a Quaternion constructor that takes a rotation matrix.