Questions / Quant development, C++ ownership & memory
A cycle can keep objects alive
Each node holds a shared_ptr to the other. How many destructors have run at the print?
int destroyed = 0;
struct Node { std::shared_ptr<Node> next; ~Node() { ++destroyed; } };
auto a = std::make_shared<Node>(), b = std::make_shared<Node>();
a->next = b; b->next = a;
a.reset(); b.reset();
std::cout << destroyed;Answer it in the app It's free. You get hints, a second try if you miss, and a follow-up question.
Show the answer and worked solution
Answer: 0 destructor calls
- Each node starts with one outside owner and gains one owner from the other node.
- Resetting both outside handles leaves each count at 1, held by the other node.
- No count reaches zero, so no destructor runs and both nodes leak.
Interview shortcut. Cycle: each count stays at 1, nothing is destroyed.
Takeaway. A shared_ptr cycle keeps every member alive.
In the app this has a follow-up where one thing about the setup changes. Try the follow-up.