cpp core · high

shared_ptr Cycles & weak_ptr

Two shared_ptrs pointing at each other never reach refcount 0 (a leak); break the cycle with weak_ptr on the back-reference.

The code

struct Node {
    std::shared_ptr<Node> next; // forward link
    std::shared_ptr<Node> prev; // back link  -> CYCLE
};
auto a = std::make_shared<Node>();
auto b = std::make_shared<Node>();
a->next = b;
b->prev = a; // a <-> b reference cycle

// Fix: make the back-link weak
std::weak_ptr<Node> prev; // observes, does not own

What this lesson walks through

1. 01A and B own each other
2. 02External owner goes away
3. 03Cycle -> neither hits 0 -> LEAK
4. 04Fix: make the back-link weak_ptr
5. 05External owner goes away (fixed)
6. 06A destroyed -> releases B -> B freed
7. 07The rule

Node A holds a shared_ptr to B (next) and B holds one back to A (prev). An external sp also owns A, so A's strong count is 2 and B's is 1.