Template Class in C++ - trouble writing begin for linked list

Question

I'm a very confused student. Any help you can give is appreciated. I'm unsure how to write this code for a template class. I keep getting some odd compiling errors that look like this:

g++ -c test_list.cpp
List.cpp: In constructor ‘cop4530::List<T>::iterator::iterator(cop4530::List<T>::Node*) [with T = int]’:
List.cpp:247:   instantiated from ‘typename cop4530::List<T>::iterator cop4530::List<T>::begin() [with T = int]’
test_list.cpp:25:   instantiated from here
List.cpp:112: error: invalid conversion from ‘cop4530::List<int>::Node*’ to ‘int’
List.cpp:112: error:   initializing argument 1 of ‘cop4530::List<T>::Node::Node(const T&, cop4530::List<T>::Node*, cop4530::List<T>::Node*) [with T = int]’
List.cpp: In constructor ‘cop4530::List<T>::iterator::iterator(cop4530::List<T>::Node*) [with T = std::basic_string<char, std::char_traits<char>, std::allocator<char> >]’:
List.cpp:247:   instantiated from ‘typename cop4530::List<T>::iterator cop4530::List<T>::begin() [with T = std::basic_string<char, std::char_traits<char>, std::allocator<char> >]’
test_list.cpp:134:   instantiated from here
List.cpp:112: error: no match for ‘operator=’ in ‘*((cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::iterator*)this)->cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::iterator::<anonymous>.cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::const_iterator::current = p’
List.h:11: note: candidates are: cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::Node& cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::Node::operator=(const cop4530::List<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::Node&)
make: *** [test_list.o] Error 1

Most importantly - Here is the exact code I am trying to fix:

template <class T>
    typename List<T>::iterator List<T>::begin()
    { // iterator to first element
      if (!empty())
        return iterator( head->next );
    }

Here is the full header:

#ifndef DL_LIST_H
#define DL_LIST_H
#include <iostream>

namespace cop4530 {

template <typename T>
class List {
 private:
    // nested Node class
    struct Node {
    T data;
    Node *prev;
    Node *next;

    Node(const T & d = T(), Node *p = NULL, Node *n = NULL) 
        : data(d), prev(p), next(n) {}
    };

 public:
    //nested const_iterator class
    class const_iterator {
    public:
    const_iterator(); // default zero parameter constructor
    const T & operator*() const; // operator*() to return element

    // increment/decrement operators
    const_iterator & operator++();
    const_iterator operator++(int);
    const_iterator & operator--();
    const_iterator operator--(int);

    // comparison operators
    bool operator==(const const_iterator &rhs) const;
    bool operator!=(const const_iterator &rhs) const;

    protected:
    Node *current; // pointer to node in List
    T & retrieve() const; // retrieve the element refers to
    const_iterator(Node *p); // protected constructor

    friend class List<T>;
    };

    // nested iterator class
    class iterator : public const_iterator {
    public:
    iterator() {}
    T & operator*();
    const T & operator*() const;

    // increment/decrement operators
    iterator & operator++();
    iterator operator++(int);
    iterator & operator--();
    iterator operator--(int);

    protected:
    iterator(Node *p);
    friend class List<T>;
    };

 public:
    // constructor, desctructor, copy constructor
    List(); // default zero parameter constructor
    List(const List &rhs); // copy constructor
    // num elements with value of val
    explicit List(int num, const T& val = T()); 
    // constructs with elements [start, end)
    List(const_iterator start, const_iterator end); 

    ~List(); // destructor

    // assignment operator
    const List& operator=(const List &rhs);

    // member functions
    int size() const; // number of elements
    bool empty() const; // check if list is empty
    void clear(); // delete all elements
    void reverse(); // reverse the order of the elements

    T &front(); // reference to the first element
    const T& front() const;
    T &back(); // reference to the last element
    const T & back() const; 

    void push_front(const T & val); // insert to the beginning
    void push_back(const T & val); // insert to the end
    void pop_front(); // delete first element
    void pop_back(); // delete last element

    void remove(const T &val); // remove all elements with value = val

    // print out all elements. ofc is deliminitor
    void print(std::ostream& os, char ofc = ' ') const; 

    iterator begin(); // iterator to first element
    const_iterator begin() const;
    iterator end(); // end marker iterator
    const_iterator end() const; 
    iterator insert(iterator itr, const T& val); // insert val ahead of itr
    iterator erase(iterator itr); // erase one element
    iterator erase(iterator start, iterator end); // erase [start, end)


private:
    int theSize; // number of elements
    Node *head; // head node
    Node *tail; // tail node

    void init(); // initialization
};

// overloading comparison operators
template <typename T>
bool operator==(const List<T> & lhs, const List<T> &rhs);

template <typename T>
bool operator!=(const List<T> & lhs, const List<T> &rhs);

// overloading output operator
template <typename T>
std::ostream & operator<<(std::ostream &os, const List<T> &l);

// include the implementation file here
#include "List.cpp"

} // end of namespace 4530

#endif

Here is most of the .cpp:

using namespace std;

// --------------------- CONST_ITERATOR --------------------- //
template <class T>
List<T>::const_iterator::const_iterator()
{ // default zero-parameter constructor. Set pointer current to NULL.
  current = NULL;
}

template <class T>
const T& List<T>::const_iterator::operator*() const
{ // returns a reference to the corresponding element in the list by calling retrieve() member function.
  return retrieve();
}

template <class T>
typename List<T>::const_iterator& List<T>::const_iterator::operator++()
{
  current = current->next;
  return *this;
}
template <class T>
typename List<T>::const_iterator List<T>::const_iterator::operator++(int)
{
  const_iterator old = *this;
  ++( *this );
  return old;
}
template <class T>
typename List<T>::const_iterator& List<T>::const_iterator::operator--()
{
  current = current->prev;
  return *this;
}
template <class T>
typename List<T>::const_iterator List<T>::const_iterator::operator--(int)
{
  const_iterator old = *this;
  --( *this );
  return old;
}

template <class T>
bool List<T>::const_iterator::operator==(const const_iterator &rhs) const
  {return current == rhs.current;}

template <class T>
bool List<T>::const_iterator::operator!=(const const_iterator &rhs) const
  {return !( *this == rhs );}

template <class T>
T& List<T>::const_iterator::retrieve() const
{ // return a reference to the corresponding element in the list.
  return current->data;
}

template <class T>
List<T>::const_iterator::const_iterator(Node *p)
{ // one-parameter constructor
  // Set pointer current to the given node pointer p.
  current = p;
}

// --------------------- ITERATOR --------------------- //

template <typename T>
T& List<T>::iterator::operator*()
{
  return List<T>::iterator::retrieve();
}

template <typename T>
const T& List<T>::iterator::operator*() const
{
  return List<T>::iterator::retrieve();
}

template <class T>
typename List<T>::iterator& List<T>::iterator::operator++()
{
  *this->current = *this->current->next;
  return *this;
}

template <class T>
typename List<T>::iterator List<T>::iterator::operator++(int)
{
  iterator old = *this;
  ++( *this );
  return old;
}

template <class T>
typename List<T>::iterator& List<T>::iterator::operator--()
{
  *this->current = *this->current->prev;
  return *this;
}

template <class T>
typename List<T>::iterator List<T>::iterator::operator--(int)
{
  iterator old = *this;
  --( *this );
  return old;
}

template <class T>
List<T>::iterator::iterator(Node *p)
{ // one-parameter constructor
  // Set current to the given node pointer p
  *this->current = p;
}

// --------------------- LIST --------------------- //

template <class T>
List<T>::List()
{ init(); }

template <class T>
List<T>::List( const List & rhs )
{ // Copy constructor
  init();
  *this = rhs;
}

template <class T>
List<T>::List(int num, const T& val)
{ //Constructs a list with num elements, all initialized with value val
  init();
  iterator itr = begin();
  for (int i = 0; i < num; ++i)
  {
    insert(itr, val);
    ++itr;
  }
}

template <class T>
List<T>::~List()
{ // Destructor
  clear();
  delete head;
  delete tail;
}

template <class T>
const typename List<T>::List& List<T>::operator=(const List &rhs)
{ // Assignment operator
  iterator ritr = rhs.first();
  iterator itr = begin();

  if( this != &rhs )
    {
      clear();
      for( ; ritr != NULL; ritr++, itr++ )
        insert( ritr.retrieve( ), itr );
    }
  return *this;
}

template <class T>
int List<T>::size() const
{ // return the number of elements in the List
  return theSize;
}

template <class T>
bool List<T>::empty() const
{ // check if list is empty
  return head->next == NULL;
}

template <class T>
void List<T>::clear()
{ // delete all elements
  while( !empty() )
    erase( begin() );
}

template <class T>
T& List<T>::front()
{ // reference to the first element
  return head->next->data;
}

template <class T>
typename List<T>::iterator List<T>::begin()
{ // iterator to first element
  if (!empty())
    return iterator( head->next );
}

template <class T>
typename List<T>::iterator List<T>::end()
{ // end marker iterator
  if (!empty())
    return iterator( tail->prev );
}

template <class T>
typename List<T>::iterator List<T>::insert( iterator itr, const T & x )
{
  Node *p = itr.current;
  theSize++;
  return iterator( p->prev = p->prev->next = new Node( x, p->prev, p) );
}

template <class T>
typename List<T>::iterator List<T>::erase( iterator itr )
{
  Node *p = itr.current;
  iterator retVal( p->next );
  p->prev->next = p->next;
  p->next->prev = p->prev;
  delete p;
  theSize--;

  return retVal;
}

template <class T>
typename List<T>::iterator List<T>::erase( iterator start, iterator end )
{
  for( iterator itr = start; itr != end; )
    itr = erase( itr );

  return end;
}

template <class T>
void List<T>::init()
{ // Initaialize the member variables of a List
  theSize = 0;
  head = new Node;
  tail = new Node;
  head->next = tail;
  tail->prev = head;
}

Let me first tell you (a) I know they are in separate files which is a big no-no for class templates, but I cannot change it for this assignment. (b) This is for homework. (c) I've tried a few variations of this code that fetches either more or less compile errors along the same vein. If you help me I will be so happy... Been at this for an hour or two.

Thanks! If you need any more info, let me know!

Answer 1

I'm not using the same compiler as you, but; I included the header in the cpp rather than the other way around, added a 'using' for your namespace in the cpp, and fixed the function signature of the implementation of operator= and that got what you've given us to build.

Answer 2

This is your problem:

return iterator itr( head->next );

You should probably be doing this:

return iterator( head->next );

The code as you have it is trying to declare an iterator named itr, which is a statement, not an expression (as required by return).