How does this compare to https://github.com/arximboldi/immer (other than the C/C++ difference)?

Also, it's my understanding that, in practice, persistent data structures require a garbage collector in order to handle deallocation when used in a general-purpose way. How does your implementation handle that?

Also, have you seen https://github.com/cnuernber/ham-fisted ? I think there are a few other Java-based persistent collections as well in the overall Clojure ecosystem that also improve on Hickey's original implementation, but I can't recall them now…