Thread: I am having trouble with friend functions

I have the following classes to implement building an expression tree. I have a function that I am having trouble placing: the buildTree function in class Tree. I am getting an undefined reference error. How can I declare it so that I may call the function in main but still access what i need to access from the classes?

Code:

template<class object>
struct Node
{
  object info;
  Node *next;
  Node<object>():info(0), next(NULL) {}
  Node<object>(const object &element, Node *n = NULL):
    info(element), next(n){}
};
template<class object>
class Stack
{
public:
  Stack();
  ~Stack();
  void makestackempty();
  bool stackEmpty() const;
  void push(object &item);
  void pop (object &item);
  void printStack() const;
private:
  Node<object> *top;
};
template<class object>
struct TreeNode
{
  object info;
  TreeNode *right;
  TreeNode *left;
  TreeNode()
  {}
};

template<class object>
class Tree
{
private:
  TreeNode<object> *root;
public:
  Tree();
  ~Tree();
  Tree(const Tree<object> &rhs);  // copy
  void operator=(const Tree<object> &rhs);
  void copyconst(TreeNode<object> *orig, TreeNode<object> *&rhs);
  void makeEmpty(TreeNode<object> *&tree);
  bool isEmpty()const;
  friend void buildTree(TreeNode<object> *&tree);
  void printTree(TreeNode<object> *&tree)const;
};

In main i currently have:

Code:

  Tree<string> T;
  cout << "Enter postfix expression to convert:" << endl;
  buildTree(T);

Any insight would be appreciated.