C++ pointers or memory

Question

My code seems like it should be working given the algorithm, but I'm new to C++ and it seems that these pointers are overwriting themselves when I call insert multiple times. For example, if I call insert with values 1, 3, 5, then the root will be 1 (as expected), but 3 will be overwritten and the right child of the root will have value 5 instead of 3.

virtual bool insert(const Data& item) {
if(root == NULL){
  BSTNode<Data> newNode (item);
  root = &newNode;
  isize++;
  return true;
}
BSTNode<Data>* nextNode = root;
BSTNode<Data>* prevNode = NULL;
bool isLeft;
while(nextNode!=NULL) {
  if (item < nextNode->data) {
    prevNode = nextNode;
    nextNode = nextNode->left;
    //std::cout << prevNode->data;
    isLeft = true;
  }
  else {
    prevNode = nextNode;
    nextNode = nextNode->right;
    //std::cout << prevNode->data;
    isLeft = false;
  }
}
BSTNode<Data> createNode (item);
createNode.parent = prevNode;
if (isLeft) prevNode->left = &createNode;
else prevNode->right = &createNode;
isize++;
return true;
}

Answer 1

You have an invalid pointer due to pointing to a local object which is going to destroy:

BSTNode<Data> newNode (item);
root = &newNode;

Object newNode is a local object within method insert after returning from this method (it goes out of scope), pointer root will point to a destroyed object. A naive possibility to solve the problem is allocating newNode in heap by new :

root = new BSTNode<Data>(item);

but you must delete it somewhere, and also same issue for createNode .

---

As recommended by many, you should use smart-points such as unique_ptr and shared_ptr .

Answer 2

Assuming that root is a member variable, you need to allocate it on the heap and not on the stack:

if(root == NULL){
  root = new BSTNode<Data>(item);
  isize++;
  return true;
}

Otherwise the node is destroyed once it goes out of scope at the closing brace of the if body. The same goes for

if (isLeft) prevNode->left = new BSTNode<Data>(item);
else prevNode->right = new BSTNode<Data>(item);

I haven't checked your actual insertion logic though. You should should post the entire code, there is too little here to check it. Is it a binary tree?

Edit: Don't forget to delete the nodes if you create them on the heap, as per MM's answer.