Question

Trying to teach myself C++ (I normally use Python) and wrote this code.

#include <iostream>
using namespace std;


class TreeNode {
    public:
        TreeNode *left;
        TreeNode *right;
        int value;

        TreeNode(int _value);
        void add_node(TreeNode node);
};

TreeNode::TreeNode(int _value) {
    left = 0;
    right = 0;
    value = _value;
    cout << "Creating node with value: " << value << endl;
}

void TreeNode::add_node(TreeNode node) {
    cout << "Adding " << node.value << " to " << value << endl;
    if (node.value < value) {
        cout << node.value << " < " << value << endl;
        if (left) {
            cout << "Left node of " << value << " exists " << left->value << endl;
            left->add_node(node);
        } else {
            cout << "Left node of " << value << " does not exist" << endl;
            left = &node;
        }
    }
    if (node.value > value) {
        cout << node.value << " > " << value << endl;
        if (right) {
            cout << "Right node of " << value << " exists " << right->value << endl;
            right->add_node(node);
        } else {
            cout << "Right node of " << value << " does not exist" << endl;
            right = &node;
        }
    }
}

int main ()
{
    TreeNode root(25);
    TreeNode n1(15);
    TreeNode n2(30);
    TreeNode n3(20);

    root.add_node(n1);
    root.add_node(n2);
    root.add_node(n3);

    cout << root.left->value << endl;
    cout << root.right->value << endl;

    return 0;
}

The program compiles but runs with results that I don’t understand.

Creating node with value: 25
Creating node with value: 15
Creating node with value: 30
Creating node with value: 20
Adding 15 to 25
15 < 25
Left node of 25 does not exist
Adding 30 to 25
30 > 25
Right node of 25 does not exist
Adding 20 to 25
20 < 25
Left node of 25 exists 20
Adding 20 to 20
20
20

I was expecting the last bit to be different.

Adding 20 to 25
20 < 25
Left node of 25 exists 15
Adding 20 to 15
20 > 15
Right node of 15 does not exist
15
30

Can someone explain what is happening here?

Answer 1

You are copying the address of a copy of TreeNode, not the address of the TreeNode in main.
Take note of the commented out function calls, and read this:
How to pass objects to functions in C++?

//Output with TreeNode node as arg
//Creating node with value: 25
//Creating node with value: 15
//Creating node with value: 30
//Creating node with value: 20
//Adding 15 to 25
//15 < 25
//Left node of 25 does not exist
//Adding 30 to 25
//30 > 25
//Right node of 25 does not exist
//Adding 20 to 25
//20 < 25
//Left node of 25 exists 20
//Adding 20 to 20
//20
//20

//Output with TreeNode & node as arg
//Creating node with value: 25
//Creating node with value: 15
//Creating node with value: 30
//Creating node with value: 20
//Adding 15 to 25
//15 < 25
//Left node of 25 does not exist
//Adding 30 to 25
//30 > 25
//Right node of 25 does not exist
//Adding 20 to 25
//20 < 25
//Left node of 25 exists 15
//Adding 20 to 15
//20 > 15
//Right node of 15 does not exist
//15
//30

#include <iostream>
using namespace std;


class TreeNode {
    public:
        TreeNode *left;
        TreeNode *right;
        int value;

        TreeNode(int _value);
        //void add_node(TreeNode  node);
        void add_node(TreeNode & node);
};

TreeNode::TreeNode(int _value) {
    left = 0;
    right = 0;
    value = _value;
    cout << "Creating node with value: " << value << endl;
}

//void TreeNode::add_node(TreeNode node) {
void TreeNode::add_node(TreeNode & node) {
    cout << "Adding " << node.value << " to " << value << endl;
    if (node.value < value) {
        cout << node.value << " < " << value << endl;
        if (left) {
            cout << "Left node of " << value << " exists " << left->value << endl;
            left->add_node(node);
        } else {
            cout << "Left node of " << value << " does not exist" << endl;
            left = &node;
        }
    }
    if (node.value > value) {
        cout << node.value << " > " << value << endl;
        if (right) {
            cout << "Right node of " << value << " exists " << right->value << endl;
            right->add_node(node);
        } else {
            cout << "Right node of " << value << " does not exist" << endl;
            right = &node;
        }
    }
}

int main ()
{
    TreeNode root(25);
    TreeNode n1(15);
    TreeNode n2(30);
    TreeNode n3(20);

    root.add_node(n1);
    root.add_node(n2);
    root.add_node(n3);

    cout << root.left->value << endl;
    cout << root.right->value << endl;

    return 0;
}