Breaking Circular References With Weak Pointers

As we saw in the previous section, circular references occur when two (or more) shared pointers reference each other in a loop, creating a deadly cycle. This interdependence prevents the destructor of the object from being called, as the reference counts never reach zero. The result is a very common memory leak which became the primary reason why weak pointers were introduced.

Breaking the cycle with weak pointers

Weak pointers are purpose-built to break these cycles. By replacing a shared pointer in a circular reference with a weak pointer, we can ensure that one of the objects does not artificially extend the lifetime of another in a manner that would prevent either from being destroyed.

To understand this concept better, let’s rewrite the linked-list circular reference problem we discussed in the previous section using a weak pointer.

main


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#include <iostream>
#include <memory>

class Node {
public:
    // A weak pointer to the next element prevents circular ownership.
    std::weak_ptr<Node> next; 

    // The constructor.
    Node() {
        std::cout << "Node constructed." << std::endl;
    }

    // The destructor.
    ~Node() {
        std::cout << "Node destructed." << std::endl;
    }
};

int main() {
    // Creating three Node objects.
    std::shared_ptr<Node> node1 = std::make_shared<Node>();
    std::shared_ptr<Node> node2 = std::make_shared<Node>();
    std::shared_ptr<Node> node3 = std::make_shared<Node>();

    // Creating a list where the last node's next is a weak pointer,
    // thus preventing a circular reference.
    node1->next = node2;
    node2->next = node3;
    node3->next = node1;

    // Now when node1, node2, and node3 go out of scope, their destructors will be called,
    // and the memory will be properly deallocated.
}

In the above code, we fixed our circular reference problem by changing next from being a shared pointer to a weak pointer. Follow the comments in the above code for better understanding. And don’t forget to run the code to see the destructors being called now. No memory leaks!

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