BarrierSet is an abstract base class. There is no need for its ctor to be public (it can be protected instead). Its destructor should either be protected or public and virtual, with the former preferred if public not required by usage, which it better not be, given that it's presently non-virtual.

The ctor should also take an argument specifying the kind, rather than initializing _kind to Uninit and requiring derived classes to reassign _kind in their constructors.