CODEWITHSUNDEEP
c++pointersnull
Mark Minkoff
Mark Minkoff

Reputation: 83

How could you set a pointer a pointer is pointing to, to nullptr in C++?

I created a tree data structure class that allocates memory for a node implementation. The remove function is successful in finding and deleting the allocated memory for the tree node, but I have trouble setting the actual node to nullptr. My remove function creates a tree node pointer and sets it to the address of the node to be deleted. I delete that pointer and set it to nullptr, but the tree node is only affected by delete and is not set to nullptr. Therefore, it becomes junk and messes up my program.

The problem starts at case 2 where my remove function checks if the node is a leaf. 

template <class DType>
bool Tree<DType>::remove(DType toDelete)
{
    if (head == nullptr) {
        return false;
    } 

    if (!check_existence(toDelete)) // CASE 1: Node does not exist.
    {
        std::cout << "Remove Error: " << toDelete << " does not exist." << std::endl;
        return false;
    }

    TreeNode<DType>* nodeToDelete = find(toDelete); // Address of node to be deleted.
    TreeNode<DType>* nodeToDelete_left = nodeToDelete->get_left();
    TreeNode<DType>* nodeToDelete_right = nodeToDelete->get_right();

    if (!(nodeToDelete_left) && !(nodeToDelete_right)) // CASE 2: Node is a leaf.
    {
        nodeToDelete->delete_nodes(nodeToDelete);
        nodeToDelete = nullptr;
        nodeCount--;
        return true;
    }

    TreeNode<DType>* temp = nullptr;

    if ((nodeToDelete_left != nullptr) ^ (nodeToDelete_right != nullptr)) // CASE 3: Node has one child.
    {
        temp = nodeToDelete;

        if (nodeToDelete_left) // 3A: Left child exists
        {
            nodeToDelete = nodeToDelete_left;
        }
        if (nodeToDelete_right) // 3B: Right child exists.
        {
            nodeToDelete = nodeToDelete_right;
        }
        delete temp;
        temp = nullptr;
        nodeCount--;
        return true;
    }
    // CASE 4: Node has two children
    temp = find_min(nodeToDelete_right);
    nodeToDelete->set_data(temp->get_data());
    delete temp;
    temp = nullptr;
    nodeCount--;
    return true;
}

Upvotes: 0

Views: 381

Answers (2)

Ted Lyngmo
Ted Lyngmo

Reputation: 117178

DType x;
DType* ptr = &x;
DType** ptrptr = &ptr;

std::cout << ptr << '\n'; // some address will be printed

// setting a pointer a pointer is pointing to, to nullptr by dereferencing the
// pointer-pointer
*ptrptr = nullptr;

std::cout << ptr << '\n'; // nullptr representation (0) printed

Upvotes: 1

melk
melk

Reputation: 530

You're taking a copy of the pointer returned by find. Setting it to nullptr won't change the value of the original (you can, however, delete the object pointed to by the original).
Make sure find returns a reference to pointer (e.g. TreeNode<DType>*&), and that your variables are declared accordingly (e.g. TreeNode<DType>*& nodeToDelete).

Upvotes: 0

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