在main中访问类的模板朋友功能。

Question

I am having a hard time finding a simple solution to this. I am implementing an expression tree using the following classes, I declare a friend function of class Tree. My problem comes when I try to get it started in main.

template<class object> class Tree;
template<class object> Tree<object>::expTree(Tree<object> *&T);  // ERROR

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 Tree<object> expTree(Tree<object> *&T){
    buildTree(T.root);
  };
  void buildTree(TreeNode<object> *&tree);
  void printTree(TreeNode<object> *&tree)const;
};

In main, I get "error: expTree was not declared in this scope." I also get, " error: expected constructor, destructor, or type conversion before â;â token" on the second line of this code.. Anyone have any pointers?

Answer 1

template<class object> Tree<object>::expTree(Tree<object> *&T);  // ERROR
                     //^^^^^^^^^^^^^^ cause of the error

This function template is actually a free function, and will be a friend of the class Tree . So it should be written as:

template<class object> expTree(Tree<object> *&T); 

That is, remove Tree<object>:: from the declaration. I just realized that it is missing the return type, and I suppose you mean Tree<object> to be the return type (and Tree<object>:: is a typo in your code). If so, then write this:

template<class object> Tree<object> expTree(Tree<object> *&T); 

_{I would like to comment on your style of naming the template parameters and arguments. It is customary to use T , U , V etc for template parameters, not for function argument. So it feels good when you write the declaration as:}

 template<class T> Tree<T> expTree(Tree<T> *&object);  

_{Well I just swapped the names.}