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.

I added the declaration before the class Tree.
The function is implemented as well.

I am getting error: friend declaration âvoid buildTree(TreeNode<object>*&)â declares a non-template function
in the friend declaration that is in bold in the code above.
When implementing, I wrote:

template<class object>
void buildTree(blahblah)

is that correct since it is a friend?
Or should it be
template<class object>
void Tree<object>::buildTree(blah)

I have tried several different ways and keep getting friend-related errors.

Yes that is what I currently have.

Code:

template<class object>
void buildTree(TreeNode<object> *&tree);

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;
  void expTree(Tree<object> &expression);
  friend void buildTree(TreeNode<object> *&tree); // line 54
  void printTree(TreeNode<object> *&tree)const;
};

template<class object>
void buildTree(TreeNode<object> *&tree)
{
  // code..
}

The error I get is:
warning: friend declaration âvoid buildTree(TreeNode<object>*&)â declares a non-template function
302hw4code2.cpp:54: warning: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here) -Wno-non-template-friend disables this warning

Maybe it is a bug, but what Elysia posted, essentially the following program, fails to compile.

Code:

template<typename T> void foo();
 
template<typename T>
class Bar
{
    T t;
    friend void foo();
};
 
template<typename T> void foo()
{
    Bar<int> b;
    b.t = 6;
}
int main()
{
    foo<int>();
    return 0;
}

I will try in the morning. Thanks!

So my solution to rid my code of friend-related errors was to implement the function in the class itself. My updated class is as follows:

Code:

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

BUT, now I am getting error: 224: error: âexpTreeâ was not declared in this scope

Here is what that error refers to:

Code:

  
int main()
{
  Tree<string> *T;
  cout << "Enter postfix expression to convert:" << endl;
  expTree(*&T);  // line 224
  return 0;
}

Is this part of the same problem that I just have to play around with?

Code:

expTree(*&T);

That looks pretty redundant and I wouldn't name a variable T, since you're already using T as a template parameter.

Code:

Tree<string> *mytree;
expTree(mytree);

That should compile. I think the argument is the right type.

edit - of course, Elysia ninja posts. Darn!

@whiteflags Yes. I remember posting this code in frustration, I've fixed the type long ago when I had a brief moment of clarity. Haha

So, It still doesnt compile for me. Maybe my compiler doesn't like it.
Here is the only error I am getting:

In function `main':
302hw4code2.cpp.text+0xae): undefined reference to `void expTree<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >(Tree<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >*&)'
collect2: ld returned 1 exit status


the only piece of code that's relevant I have already posted. but for this error, these are the classes I currently have:

Code:

#include<iostream>
#include<string>
using namespace std;

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

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

and the main, even with little tweaks.

Code:

int main()
{
  Tree<std::string> *T;
  std::cout << "Enter postfix expression to convert:" << std::endl;
  expTree(T);
  return 0;
}

My solution for now is i've written a new code with only one struct, and no templates. haha. It compiles. Sad code but ive gotten no stubborn errors. Working code is better than noncompiling code!

Yea, I've posted in 3 different forums and haven't been able to get it to compile. I know my implementation logic stuff is correct, because when I rewrote it without the mess it compiled fine.
g++ doesn't like me.

Thank you forum people.