The easiest way to implement these is with Weak references in one of the directions. This requires some checking and/or locks, but the fact it is not trivial to

The easiest way to implement these is with Weak references in one of the directions. This requires some checking and/or locks, but the fact it is not trivial to do is kinda the point: the integrity of a cyclic data structure depends on managing multiple pointers in multiple structs simultaneously, and with that comes concurrency dragons.