Drake: RigidBody::body_index_ is not a good unique identifier in RBT

[amcastro-tri]

RigidBody::body_index_ changes with calls to compile() or more specifically RBT::SortTree(). We need a unique identifier for RigidBody's to, for instance, reference them properly in the KinematicsCache without creating undesired circular dependencies (see comment here).
Also we should be able to guarantee the user with a unique identifier for each RigidBody that does not change during the lifespan of RigidBodyTree.

cc'ing @jwnimmer-tri, @liangfok, @sherm1

[liangfok]

I can also imagine that the body indices will change when we add and remove parts of a tree (like when we tile in and out different parts of the world based on the player character's location).

I suggest adding a body_id that's created and applied in a manner similar to model_instance_id.

[amcastro-tri]

Implementation detail body indices will change, yes. However unique body id's will not. The PR I am working on right now introduces RigidBodyId (simply a typedef to int) and RigidBody::get_id().
That change alone is helping me removing dependencies in the cache and also helping with the template madness in RBT.

[jwnimmer-tri]

FWIW, I think semantic typedefs to PODs are really hard to maintain. If it's just an int, either call it an int, or make it a struct BodyId { int value; } so that the typechecker helps you. Having types that look like the typechecker will help but don't are frustrating for maintainers.

[sherm1]

I agree with @jwnimmer-tri that type-safe integers should be used rather than plain typedefs. I would also distinguish general Ids ("handles") from those that are suitable for use as indexes (i.e. they are guaranteed to be small integers in a range 0..n-1 or whatever).

If we use them, type safe integers should be standardized since we are likely to need lots of them. Ideally they have an implicit conversion to int, but no implicit conversion from int. I've used macros to create such classes in the past, but I think a CRTP implementation could be made to work.