how to access class private data members in method parameter of class , i get an error invalid use of non static data members

i have two classes Node and AvlTree , i will make other methods of AvlTree later but i can't access root data member of AvlTree in it's own class , how can i access root in AvlTree class inOrder method.

My code is following here

class Node {
    public:
        int key;
        Node* left;
        Node* right;
        int height;

        Node(int key) : key(key) , left(nullptr) , right(nullptr) , height(1) {};
};

class AvlTree {
    private:
        Node* root;
    public:
        AvlTree() : root(nullptr) {};

        int height(Node* ptr) {
            
        }

        int getBalanceFactor(Node* ptr) {
            
        }

        void inOrder(Node* itr = root) {      // <--- i get an error here

        }
};

I tried with this->root but that also does't work , what i am do wrong here , also can i not access like this in it's own class. I got an error like

09_avl_tree.cpp:36:34: error: invalid use of non-static data member ‘AvlTree::root’
   36 |         void inOrder(Node* itr = root) {
      |                                  ^~~~
09_avl_tree.cpp:15:15: note: declared here
   15 |         Node* root;
      |               ^~~~

I don't want to make root as static data member. because i want multiple instances of AvlTree.

Solution

The short answer, as the compiler is telling you, is that you can't do that as a default value of an argument.

The simplest approach would be to overload the inOrder() function, for example (within the definition of AvlTree)

  void inOrder(Node *itr)
  {
        // whatever
  }

  void inOrder()
  {
       inOrder(root);
  }

Also, unrelated to your question, the shadowing of member names in Nodes constructor (e.g. an argument named key used to initialise a member named key) is not a good idea, since it is easy to mislead human readers about what the code does. It is therefore often considered preferable to name the argument differently from the member.