Why is joint_index_ signed in JointModel?

[tylerjw]

Description

Why is the joint_index_ in JointModel a signed integer?

Here is where you get it: https://github.com/ros-planning/moveit2/blob/52ffbfdbd4ffee3267371ab8d43d14a30082f547/moveit_core/robot_model/include/moveit/robot_model/joint_model.h#L215

Here is one of many examples of it being used to index into an array: https://github.com/ros-planning/moveit2/blob/52ffbfdbd4ffee3267371ab8d43d14a30082f547/moveit_core/robot_model/src/robot_model.cpp#L184

I can't find an example of it being set to a negative value even though the interface would permit that. And if you did set it to a negative value it would cause all these operator[] calls that use it to access memory from before the array.

It is initialized to -1 here: https://github.com/ros-planning/moveit2/blob/52ffbfdbd4ffee3267371ab8d43d14a30082f547/moveit_core/robot_model/src/joint_model.cpp#L58

Would it hurt anything if I made it an unsigned value and initialized it to 0? Am I misunderstanding something about the interface here? Is there an implicit contract that this value will be initialized (correctly) before it is used and is therefore safe?