Trouble getting C++ program to print list using Iterator and Operator Overloading

Hello All, First time here. I was hoping you could help me with my program. I'm trying to use the given Iterators to overload my = operator to print my list to screen. I keep getting Link2019 errer though and have narrowed the problem down to my print operator. I'm wondering if it has anything to do with the fact that my operator is in private part of class? I'm new to this concept.

Code:

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#include    "List.h"

// methods for Node all
//ME

Node::Node( const string &s, Node * p, Node * z) : word( s ), next( p ), prev(z)                //constructor
{
    word = s;              // init. word data with a copy of s
    next = p;              // next pointer points to p
        prev = z;                                //prev pointer points to z
}

const string &Node::get_word( ) const        // get a const reference to word
{
    return word;                                                //returns curent objects data
}
 
Node *Node::get_next( ) const        // get a Node * (value of next)
{
        return next;                                //returns current objects pointer to next
}

                           
Node *Node::set_next( Node * p )        // set next to a new value
{
    next = p;              // next points to p
    return next;
}

Node *Node::get_prev( ) const        // get a Node * (value of next)
{
        return prev;                                //returns current objects pointer to next
}

Node *Node::set_prev( Node * z )        // get a Node * (value of next)
{
        prev = z;
        return prev;                                //returns current objects pointer to next
}




//****************************************************************************8
// methods for List


List::List( )              // constructor: init. head and tail
{
    cout << "List::List( )\n";

    head = tail = 0;
}


List::List( const List &rhs )        //Copy Constructor
{
        copy_list( rhs );
}


List::~List( )              // destructor: deallocate the list
{
    cout << "List::~List( )\n";
        delete_list( );
}


void List::copy_list( const List &rhs )                //Given
{
    head = tail = 0;
                            // copy rhs' list into this
    for( Node *p = rhs.head; p; p = p->get_next( ) )
    {
        push_back( p->get_word( ) );
    }
}


void List::push_back( const string &s )
{
                            // p points to a new node
    Node *p = new Node( s, 0, tail );

    if( tail == 0 )        // tail not pointing to a node yet?
    {
        head = tail = p;    // head & tail point to new node in the list
    }
    else
    {                      // tail->next points to new node
        tail->set_next( p );
        tail = p;          // tail points to last node in the list
    }
}


void List::pop_back( )
{
        if( tail )                                // tail points to a node?
        {
                Node *tmp = tail;
                tail      = tail->get_prev( );

                delete tmp;                        // delete node to be removed

                if( tail == 0 )                // no more elements in the list?
                {
                        head = 0;
                }
                else
                {
                        tail->set_next( 0 );
                }
        }
}

// other methods for List...

void List::delete_list( )        //ME
{

    for( Node *p = head; p; )
    {
        Node *tmp = p;      // remember current pointer
        p = p->get_next( ); // advance p to the next Node
        delete tmp;        // deallocate tmp
        cout << "Deleted\t" << tmp << "\tnext is\t" << p << '\n';
    }
}

void List::push_front( const string &s )        //ME
{
Node *p = new Node(s,0,head);

    if( head == 0 )        // head not pointing to a node yet?
    {
        head = tail = p;    // head & tail point to new node in the list
    }
    else
    {                      // head->next points to new node
               
                p -> set_next(head);
                head = p;

    }
}

void List::pop_front( )                //ME
{
 Node* temp; 
    if(head == NULL) 
    { 
        cout<<"\nLinked list is empty"; 
        return; 
    } 
    if(head->get_next() == NULL)      //to check if only one node is present 
    { 
        temp = head; 
        head = NULL; 
        //cout<<"\nDeleted node: "<<temp->data; 
        free(temp); 
      } 
    else            //If more than one nodes are present 
    { 
        temp = head; 
        head = head->get_next(); 
        //cout<<"\nDeleted node: "<<temp->data; 
        free(temp); 
      } 
}

List::Iterator List::begin( ) const        // beginning of a linked list ME
{
    return List::Iterator( head );
}

List::Iterator List::end( ) const        // end of a linked list ME
{
    return List::Iterator( NULL );
}




// methods for Iterator
                            // constructor: init. an Iterator
List::Iterator::Iterator( Node *p )
{
    current = p;   
       
}

                            // get a const ref to word
const string &List::Iterator::operator *( ) const
{
    return current->get_word( );
}

                            // get a const ref to word
void List::Iterator::operator ++( )
{
    if( current )
    {
        current = current->get_next( );
    }
}

                            // current != p
bool List::Iterator::operator !=( const Iterator &iter ) const
{
    return current != iter.current;
}

List &List::operator =( const List &s )
{
        List::Iterator iter;

    for( iter = s.begin( ); iter != s.end( ); ++iter )
    {
        cout << "List *iter = " << *iter << "\n";
    }
        return  (*this);
       
}

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Here is my header file, I'm not allowed to change it though.

Code:

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


class Node
{
  public:
    Node( const string &, Node *, Node * );

    const string &get_word( ) const;// get a const reference to word
    Node  *get_next( ) const;      // get a Node * (value of next)
    Node  *set_next( Node * );    // set next to a new value
    Node  *get_prev( ) const;      // get a Node * (value of prev)
    Node  *set_prev( Node * );    // set prev to a new value

  private:
    string  word;
    Node    *next;
    Node    *prev;
};


class List
{
  public:
    List( );                        // constructor
    List( const List & );          // copy constructor
    ~List( );                      // destructor
                                    // push a node to the back of list
    void push_back( const string & );
                                    // push a node to the front of list
    void push_front( const string & );
                                    // pop  a node from the back  of list
    void pop_back( );
                                    // pop  a node from the front of list
    void pop_front( );

    class Iterator
    {
      public:
        Iterator( Node * = 0 );               
        const string &operator *( ) const;
        void operator ++( );
        bool operator !=( const Iterator & ) const;

      private:
        Node *current;
    };

    Iterator begin( ) const;        // pointer to beginning of the list
    Iterator end( )  const;        // pointer to end      of the list

  private:

    Node    *head;
    Node    *tail;

    void copy_list( const List &  );// copy a linked list
    void delete_list( );            // delete a linked list
                                    // do NOT allow copy assign. operator
    List &operator =( const List & );
};

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And finally my main, something else I can't change. I commented everything out except cout statement.

Code:

---

#include    "List.h"

// implement operator <<( ) using Iterators
ostream &operator <<( ostream &, const List & );


int main( )
{
    List la;                    // create list la
       
        la.push_front( "mom" );
    la.push_back( "please" );
    la.push_back( "send" );
    la.push_back( "money" );
    la.push_front( "hi" );
       
    cout << "\nla contains:\n" << la << '\n';
        /*
    List lb( la );              // copy list la to lb
        /*
    cout << "lb contains:\n" << lb << '\n';
       
    lb.pop_front( );
    lb.pop_front( );
    lb.push_front( "mother" );
    lb.push_front( "dear" );
    lb.push_back( "Bubba" );
        /*
    cout << "lb contains:\n" << lb << '\n';
        /*
    List lc;                    // create list lc
    lc.push_back( "money" );
    lc.push_front( "send" );

    cout << "\nlc contains:\n" << lc << '\n';

    List ld;                    // create list ld
    cout << "\nld contains nothing:\n" << ld << '\n';

    ld.push_front( "hi" );
    cout << "ld contains:\n" << ld << '\n';

    ld.pop_front( );
    cout << "ld contains nothing:\n" << ld << '\n';

    ld.push_back( "hello" );
    ld.push_back( "Bubba" );
    cout << "ld contains:\n" << ld << '\n';

    ld.pop_front( );
    cout << "ld contains:\n" << ld << '\n';

    ld.pop_front( );
    cout << "ld contains nothing:\n" << ld << '\n';

    List le( ld );
    cout << "le contains nothing:\n" << le << '\n';

    le.push_back( "last" );
    cout << "le contains:\n" << le << '\n';
        */
        system("pause");
    return 0;
}

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Code:

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// implement operator <<( ) using Iterators
ostream &operator <<( ostream &, const List & );

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This is a function declaration, not a definition. You need to provide a function definition with a body.