Why is there a difference between functions of Link Class Reference and gazebo::physics::Link Class Reference ?

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As detailed on the Migration guide From Gazebo 7 to Gazebo 8, function

inline virtual const math::Pose Entity::GetWorldPose() const

was replaced with

inline virtual const ignition::math::Pose3d &Entity::WorldPose() const

When looking through the API documentation, be sure to go check if the documentation's version matches your Gazebo version. For Gazebo 7, this is the page you should be looking at.

As for converting between gazebo::math and ignition::math

ignition::math::Something ignSmth = gzSmth.Ign();
gazebo::math::Something gzSmth(ignSmth);