In a document tree where each child also points back at its parent, which of those two edges should be non-owning?

## The rule in one line A reference cycle becomes reclaimable again the moment **exactly one** of its edges stops contributing to a count. Such an edge can still be stored and still be followed; it simply does not keep its target alive. Breaking one edge is enough, because reclamation only needs some member of the ring to be able to reach zero — once one does, its destruction decrements the next, and the cascade runs the rest of the way around. The interesting part of the question is not *whether* to weaken an edge but *which*, because that choice decides lifetimes, and getting it backwards replaces a leak with a much worse bug. ## Walking the four possibilities | what owns | the section's fate | the paragraphs' fate | verdict | |---|---|---|---| | both edges own | never freed | never freed | the cycle: nothing is reclaimed | | child list owns, back-pointer does not | freed when the last outside handle drops | freed with the section | correct | | back-pointer owns, child list does not | survives while an outside handle holds it | destroyed as soon as the creating handle drops | children vanish early | | neither edge owns | freed when its handle drops | destroyed at once | the structure has no owner at all | Only the second row gives the lifetimes the document actually needs: the section is the container, so it must be the thing whose survival implies its contents survive. ## Three tests for the direction When the objects are not obviously parent and child, these three questions usually agree, and when they disagree the design is telling you something: 1. **Containment.** Which object's lifetime is conceptually inside the other's? The container owns; the reference pointing back out of it does not. 2. **Tolerance of absence.** Which side can carry on sensibly if its target has already gone? A paragraph asking about inherited formatting can handle "no section any more" — the section cannot handle half its paragraphs disappearing. 3. **Structure versus convenience.** Which edge is part of the data, and which one exists to save a lookup? A back-pointer is very often an optimisation over walking down from the root, and an optimisation should not extend a lifetime. ## What the non-owning side has to pay Making the back-pointer non-owning is not free; it moves the cost from memory to the call site. - Every use of the edge must **check first** and have a branch for the target being gone, so the type that expresses the edge has to be able to say "absent". - Some designs offer an unchecked non-owning edge instead, which costs nothing per use and turns the mistake into reading freed memory rather than seeing an absent value; that trade belongs to whoever owns the invariant. - The check has to happen at the moment of use, not once at startup, because the target can be destroyed between two uses. - Two count updates per link disappear, so the linking path actually gets slightly cheaper. ## Where the convention stops holding A hand-placed non-owning edge is a promise about a run-time shape made at authoring time, and it holds exactly as long as someone is keeping it. - **Graphs whose shape is data.** A user-built document with arbitrary cross-links, a plugin that registers an object holding the host, a cache that memoises objects holding the cache — nobody wrote the ring, so nobody placed the non-owning edge. - **Peers with no containment relation.** When neither object is naturally inside the other, all three tests come back silent, and forcing an answer invents an invariant that no reader can check locally. - **Growth.** The rule lives in a review comment or a style guide, and a compiler in most settings will not enforce it; every new contributor is a fresh chance to add an owning back-edge. - **Rings longer than two.** Three or four objects can close a loop across modules that each look correct on their own. When those apply, the answer is not a better convention: it is machinery — a cycle collector that searches for rings, or an occasional reachability pass — with the convention kept as the cheap first line for the shapes you do control. ## What an interviewer is listening for A candidate who just says "make the back-pointer weak" has memorised the shape. The stronger answer justifies the direction from containment, then says what the wrong direction does — paragraphs destroyed while the section is still on screen — and finally admits the cost: every read of the upward edge now needs a guard, and the rule itself is unenforced.