Deep Copy Constructor for binary tree

Question

I am trying to create a deep copy of my binary tree data structure in C++. The problem is the code I am using only seems to be giving me a shallow copy (which seems to cause problems with my deconstructor).

the code below is my binary tree copy constructor:

BinaryTreeStorage::BinaryTreeStorage(const BinaryTreeStorage &copytree):root(NULL)
{
    root = copytree.root;
    copyTree(root);
}

BinaryTreeStorage::node* BinaryTreeStorage::copyTree(node* other)
{
    //if node is empty (at bottom of binary tree)
    /*
        This creates a shallow copy which in turn causes a problem 
        with the deconstructor, could not work out how to create a 
        deep copy.
    */
    if (other == NULL)
    {
        return NULL;
    }

    node* newNode = new node;

    if (other ->nodeValue == "")
    {
        newNode ->nodeValue = "";
    }

    newNode->left = copyTree(other->left);
    newNode->right = copyTree(other->right); 

    return newNode;
}

Any help would be appreciated. Thanks

Here is the deconstructor that throws a memory exception (which i believe is because of the shallow copy i do above)

BinaryTreeStorage::~BinaryTreeStorage(void)
{
    try
    {
        destroy_tree();//Call the destroy tree method
        delete root;//delete final node
    }
    catch(int &error)
    {
        cout << "Error Message : " << error << endl;
    }
}
void BinaryTreeStorage::destroy_tree()
{
    destroy_tree(root);
}
void BinaryTreeStorage::destroy_tree(node *leaf)
{
  if(leaf!=NULL)
  {
    //Recursively work way to bottom node 
    destroy_tree(leaf->left);
    destroy_tree(leaf->right);
    //delete node
    delete leaf;
 }
}

Answer 1

You're not performing a deep copy of your root node, only its children.

Shouldn't it be:

root = copyTree(copytree.root);

?

EDIT: In addition, you destroy the root twice:

destroy_tree();//Call the destroy tree method

//once here
//remove this line
delete root;//delete final node

and

if(leaf!=NULL)
{
   //Recursively work way to bottom node 
   destroy_tree(leaf->left);
   destroy_tree(leaf->right);

   //one more time here
   delete leaf;
}

If you only do one of these fixes, the problem won't be solved.

Answer 2

Actually, I think we can directly using the copy constructor to recursively deep copy the tree. Suppose the class is defined as follows:

class TreeNode {
public:
  TreeNode() : value(), count(0), left(nullptr), right(nullptr) {}
  TreeNode(const TreeNode &);

  ~TreeNode();

  TreeNode &operator=(const TreeNode &);

  // Other members...

private:
  std::string value;
  int count;
  TreeNode *left;
  TreeNode *right;
  // Note that the valid state for the `left` and `right` pointer is either
  // `nullptr` or a subtree node. So that we must check these pointers every
  // time before dereferencing them.
};

Then the copy constructor is

TreeNode::TreeNode(const TreeNode &n)
    : value(n.value), count(n.count), left(nullptr), right(nullptr) {
  if (n.left)
    left = new TreeNode(*n.left);  // recursively call copy constructor
  if (n.right)
    right = new TreeNode(*n.right);  // recursively call copy constructor
}

The recursion will be stopped at the leaf node, since both its two children are nullptr.

And so does the destructor.

TreeNode::~TreeNode() {
  delete left;  // recursively call destructor on left subtree node
  delete right;  // recursively call destructor on right subtree node
}

When left or right is nullptr, delete will do nothing, so that the recursion will be stopped.

You can see the complete code from here.