Question

I am having trouble with the following code. I am a java developer trying to teach myself c++. Mainly my code in main() is not inserting anything to the root Node. Can anyone help me. I am sure there is something off with my pointers.

class Node{
    public:
        Node* left;
        Node* right;
        int data;

    Node(int n){
        data = n;
        right = NULL;
        left = NULL;
    }
};

class BST{

    Node* root;

    public:

    BST(){
        root = NULL;
    }

    void insert(int e){
        pinsert(root, e);
    }

    void pinsert(Node* sr, int e){
        if(sr == NULL){
            sr = new Node(e);
        }
        else{
            if((sr->data) > e ){
                pinsert(sr->left, e);
            }
            else{
                pinsert(sr->right, e);
            }
        }
    }
};


int main(){
    BST tree;
    tree.insert(6);
    tree.insert(7);
}

Answer 1

The problem with your insert is when you begin with an empty list. When you pass a Node * to your pinsert function, the argument is passed by value, so your function ends up with a copy of the pointer. When the pointer is NULL, you change the pointer to point to your new node. Since you only have a copy of the pointer in pinsert, this only changes the copy of the pointer. When the function returns, the pointer, and the list, is unchanged.

Pass a reference to the pointer to get around this:

void pinsert(Node *&sr, int e)

Now the rest of your code doesn’t need to change, and your function can change the pointer.

This problem is analogous to passing object references in java: you can change the contents of the object, by setting fields and calling methods, but you can’t change the argument to a new object, or to null.