C++ pointer troubles

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);
}

"C++" + "raw pointers" = "asking for troubles". You did it, you got troubles. Now go learn about standard containers and smart pointers, and grab a book instead of trying things blindly, especially when coming from Java. — Griwes
– Griwes, Commented Sep 30, 2012 at 19:51
I disagree. If you don't get pointers, DEFINITELY use them for a while. I wouldn't recommend (owning) raw pointers in production code either, but it's extremely important to get what's actually going on (having control over that is, after all, a large part of why we use C++ in the first place). — Cubic
– Cubic, Commented Sep 30, 2012 at 20:17
@Cubic, pointers are important, right. But you need to get bigger picture before getting into them; you need to know how the language (as in "language + standard library") works; in modern C++, pointers are quite advanced topic, and should not be presented to beginners, as they have no interest in learning them - you don't use raw pointers or containers written by yourself much in C++ (if not s/much/at all...). — Griwes
– Griwes, Commented Oct 1, 2012 at 17:13

Dietmar Kühl · Accepted Answer · 2012-09-30 19:52:42Z

6

You pass sr by value and assign to it. This won't change you root. Pass it by reference instead:

void pinsert( Node*& sr, int e)

answered Sep 30, 2012 at 19:52

Dietmar Kühl

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Comments

pb2q · Accepted Answer · 2012-09-30 22:24:47Z

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.

Zdeslav Vojkovic · Accepted Answer · 2012-09-30 19:55:05Z

2

The problem is that root is always NULL, you never assign anything to it.

When you call pinsert(root, e) and then do this:

void pinsert(Node* sr, int e){
    if(sr == NULL){
        sr = new Node(e);
    }

you send a copy of root and assign to that copy, not the root which remains NULL. You need to either send the pointer by reference (void pinsert(Node** sr, int e)) - notice 2 'stars' or void pinsert(Node*& sr, int e).

answered Sep 30, 2012 at 19:55

Zdeslav Vojkovic

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