Question

I’m defining a binary search tree class like this:

template <class T>
class BSTNode
{
  public:
    BSTNode(){
      right = left = 0;
    }

    BSTNode(const T& el, BSTNode * l = 0, BSTNode * r = 0)
    {
    val = el; left = l; right = r;
    }   

    T val;
    BSTNode * right;
    BSTNode * left;
};

template <class T>
class BST
{
  public:
    BST(){ root = 0;}           
    void Insert(const T& el);
  private:
    BSTNode<T> * root;
};

and I implement the Insert() function like this:

template <class T>
void BST<T>::Insert(const T& el)
{
  bool bEqual = false;
  BSTNode<T> * p = root;

  while(p)
  {
    if(el == p->val)
    {
      bEqual = true;
      return;
    }
    if(el < p->val)
    {
      // go left
      p = p->left;
    }
    if(el > p->val)
    {
      // go right
      p = p->right;
    }
  }

  if(!p)
  {
    std::cout << "creating new node " << el << "\n";
    p = new BSTNode<T>(el);
  }  
}

Why does the root pointer variable stay at 0, and not the address of the new object?

Answer 1

You never do root = ?; in your code; also, p = new BSTNode<T>(el); leaks memory.

My guess is that you wanted to p to be a reference to pointer, so you could change the original pointer.

 BSTNode<T> *& p = root; // watch out, it won't solve anything

But, in such case, p is not-reassignable. You may want to check if pointer you assign to p is null and just insert the new value in correct place (eg. p->left = new BSTNode<T>(el);) and reassign p only when given pointer is not null.

I mean:

template <class T>
void BST<T>::Insert(const T& el)
{
  bool bEqual = false;
  BSTNode<T> * p = root;

  if (p == 0)
  {
     root = new BSTNode<T>(el);
     return;
  }

  while(true)
  {
    if(el == p->val)
    {
      bEqual = true;
      return;
    }
    if(el < p->val)
    {
      if (p->left == 0)
      {
        p->left = new BSTNode<T>(el);
        return;
      }
      p = p->left;
    }
    if(el > p->val)
    {
      if (p->right == 0)
      {
        p->right = new BSTNode<T>(el);
        return;
      }
      p = p->right;
    }
  }
}