Compiler not understanding recursive function to print tree level-order C++ - c++

Question

I'm currently attempting to solve the following problem:
Write a function void TreeNode::levelOrder(int n) with wrapper void Tree::levelOrder() to print a tree level-order using recursion
This is the class header:
#ifndef TREE_H
#define TREE_H
#include <algorithm>
class TreeNode {
private:
int key;
TreeNode *left;
TreeNode *right;
public:
TreeNode(int key);
void insert(int key);
void display(TreeNode *p, int lvl);
void preOrder();
void inOrder();
void postOrder();
int size();
int height();
};
class Tree {
private:
TreeNode *root;
public:
Tree();
Tree(int *a, int n);
void insert(int key);
void preOrder();
void inOrder();
void postOrder();
void display(TreeNode *p);
int size();
int height();
};
void TreeNode::display(TreeNode *p, int lvl){
if (p!=NULL){
if (p->left) display(p->left, lvl + 4);
if (p->right) display(p->right, lvl + 4);
if (lvl) {
std::cout << std::setw(lvl) << ' ';
}
cout << p->key << "\n";
}
}
void Tree::display(TreeNode *p){
if (p!=NULL){
return display(p,0);
}
With these two functions, I am still getting the following error:
"./Tree.h:139:20: error: too many arguments to function call, expected single
argument 'p', have 2 arguments
return display(p,3);
~~~~~~~ ^
./Tree.h:137:1: note: 'display' declared here
void Tree::display(TreeNode *p){ "
I don't understand why the compiler can't recognize to call the function from TreeNode class with 2 arguments. Instead, the compiler is telling me the recursive function should only expect one argument.
I know I'm most likely not understanding the bigger picture here so could anyone try and clear this issue up for me? Thanks alot.

Answer 1

The issue is in this method:
void Tree::display(TreeNode *p){
if (p!=NULL){
return display(p,0);
}
return display(p,0); means return this->display(p,0); and the compiler is right.
What you probably meant is p->display(p, 0);.
As a side note, the display() methods in both classes seem to be static methods (and they don't have any reason to be); they don't use the properties of the object used to invoke them.
In order to be correct OOP, you should rewrite them as follows:
void TreeNode::display(int lvl)
{
if (left) {
left->display(lvl + 4);
}
if (right) {
right->display(lvl + 4);
}
if (lvl) {
std::cout << std::setw(lvl) << ' ';
}
cout << key << "\n";
}
void Tree::display(){
if (root) {
root->display(0);
}
}