Thread: Struggling with making circular linked list: compilation error from friend function?

I've been getting the following error while trying to display my circular linked list and I have no idea how to fix the problem:

prog.cpp:33:18: error: template-id 'operator<< <>' for 'std:: ostream& operator<<(std:: ostream&, const List<int>&)' does not match any template declaration
friend ostream& operator << <>(ostream&, const List<T>&);



here is my code:

Code:

 

#include <iostream>


using namespace std; 




template <class T>
class List{


private:
    class Node {
        public:
        T data;
        Node *next;
    };
public:


    List();
    List(const List &);
    ~List();


    int getSize() const;


    bool isEmpty() const;
    bool insert(const T&, const int &);
    bool remove(const int &);


    T runJosephusAlgorithm();


    const List& operator=(const List&);


    void display(ostream&) const;
    
    friend ostream& operator << <>(ostream&, const List<T>&);


private:
    Node *first; // pointer to the first element of the linked list






};


template<class T>
List<T>::List()
{
    first = NULL; 
}




template <class T>               
List<T>::List(const List & Original_list)          // copy constructor creates copy of original list in body
{
    
    Node *newNodeptr = NULL; 
    first  = Original_list.head; //original pointer
    
    do
    {
        newNodeptr = new Node;                              // do while 
        newNodeptr -> data = first ->data; 
        newNodeptr = newNodeptr ->next; 
        first = first ->next; 
        
    }while(first ->next!= Original_list.head);
    
}






template <class T>


List<T>::~List()                                         
{
   
}


template<class T> 
bool List<T>::insert(const T&, const int & count)
{
    int n = 1; 
    Node *currPtr; 
    Node *lastPtr; 
    Node *newNode; 
    
    newNode = new Node; 
    
    if (count == 1) 
    {
        newNode->next = first;
        first = newNode;
    }
    else
    {
        lastPtr = first; 
        currPtr = first; 
        
        while((currPtr !=NULL)&& (n < count))
        {
            n++;
            lastPtr = currPtr; 
            currPtr = currPtr ->next; 
        }
        lastPtr ->next = newNode; 
        currPtr = currPtr->next; 
    }
    
    
}






template <class T>
void List<T>::display(ostream &out) const
{
    Node *current = first;
    while (current != NULL) {
        out << current->data << endl;
        current = current->next;
    }
    
}
template <class T> 
ostream & operator<<(ostream & o, const List<T> &output_elements)
{
   o << output_elements.data; 
   return o; 
}




int main()
{
    
    List<int> cll_1; 
    cll_1.insert(12,1);
    cll_1.insert(24,2);
    
    
    
}

This code works for me as is with Visual Studio 2013, I didn't check with other versions.

It also works with clang 3.5 though I get a link error with the destructor.

This code doesn't work with gcc though. Moving the templete for ostream & operator<< before display doesn't cut it either.

I couldn't figure out how to do the copy constructor, so I decided to work on the insert function so I can be able to insert a node into the list, and be able to output the list and go from there. Everything seems to compile now, however I can't figure out how to output the linked list. The header file was given to me by my professor and I just don't understand how to print out the list with the code she has given me, it's incredibly frustrating. Here's what I made so far:


For the header file:

Code:

 

#include <iostream>


using namespace std;




template <class T>
class List {


private:
	class Node {
	public:
		T data;
		Node *next;
	};
public:


	List();
	List(const List &);
	~List();


	int getSize() const;


	bool isEmpty() const;
	bool insert(const T&, const int &);
	bool remove(const int &);


	T runJosephusAlgorithm();


	const List& operator=(const List&);


	void display(ostream&) const;


	friend ostream& operator << <>(ostream&, const List<T>&);


private:
	Node *first; // pointer to the first element of the linked list






};


template<class T>
List<T>::List()
{
	first = NULL;
}




template <class T>
List<T>::List(const List & Original_list)          // copy constructor creates copy of original list in body
{
	
}






template <class T>


List<T>::~List()
{


}


template<class T>
bool List<T>::insert(const T& data, const int & count)
{
	int n = 1; 
	Node *lastPtr;
	Node *currPtr; 
	Node *newNode = new Node; 
	
	if (count == 1)
	{
		
		newNode->next = first;
		first = newNode;
		return true; 
	}
	
	if (count > 1 && count != 0)
	{
	
		currPtr = first;
		lastPtr = first;


		while ((currPtr != NULL) && (n < count)) 
		{
			
			n++;
			lastPtr = currPtr;
			currPtr = currPtr->next;
		}
		lastPtr->next = newNode;
		newNode->next = currPtr;


		return true;


	}
	else
		return false; 




}






template <class T>
void List<T>::display(ostream &out) const
{
	Node *current = first;
	while (current != NULL) {
		out << current->data << endl;
		current = current->next;
	}


}
template <class T>
ostream & operator<<(ostream & o, const List<T> &output_elements)
{
	o << output_elements.data;
	return o;
}

and the main function:

Code:

#include<iostream>
#include"circularlist.h"








int main()
{
	List<int> cl_1; 
	cl_1.insert(12, 1);


	cl_1.display();      // No idea how to print this out????
	
}

For the insert function parameters, you might want to consider the count for nodes starting at zero instead of one. So insert(data, 0) would create a new node for the start of a list, similar to push_front(). Also you might want to consider keeping track of the number of nodes in list, similar to std::list::size().

Looking at the insert function, I don't see where it handles first == NULL. Since this is a circular list, then the last node of the list has a next pointer == first, not NULL.