#include <cassert>
#include <memory>

struct Node {
    static int alive;
    std::shared_ptr<Node> child;
    std::weak_ptr<Node> parent;
    Node() { ++alive; }
    ~Node() { --alive; }
};
int Node::alive = 0;

int main() {
    auto parent = std::make_shared<Node>();
    auto child = std::make_shared<Node>();
    parent->child = child;
    child->parent = parent;
    {
        auto locked = child->parent.lock();
        assert(locked && locked->child == child);
    }
    parent.reset();
    assert(Node::alive == 1);
    assert(child->parent.expired());
    assert(!child->parent.lock());
    child.reset();
    assert(Node::alive == 0);
}