4 ms·

Backpointers can be more than one step away, for example with a circular linked list, or with a directed (not-acyclic) graph. It's harder to see how to enforce

• Animats 7y ago
  That's where the language designer has to say "no". Trying to make ownership semantics work for arbitrary graphs is too much trouble. I had to deal with this once writing a 3D collision detection system. Objects were convex hulls, with vertices, edges, and faces, all pointing to each other. I had an implementation in C to look at, and it was a mess. Ownership was so tangled that they couldn't delete objects. So I rewrote it in C++, with each object having collections of vertices, edges, and faces. All links between them were indices, not pointers. Lots of asserts to validate all the consistency rules. Easy to delete, and much better cache coherence. Chasing pointers to tiny objects all over memory belongs to the era when cache misses didn't dominate performance. So that's the answer. Some data structures are too complex for ownership semantics. So do them another way and have all the parts be owned by some master object.