Question

I am having problems making a copy constructor.
Consider the code below:

In List.h

template <class T>
struct ListNode
{
    T value;
    ListNode<T> *next;
    ListNode<T> *prev;

    ListNode(T theVal)
    {
        this->value = theVal;
        this->next = NULL;
        this->prev = NULL;
    }
};

template <class T>
class List
{
    ListNode<T> *head;

public:
    List();
    List(const List<T>& otherList); // Copy Constructor.
    ~List();
    };

In list.cpp

template <class T>
List<T>::List()
{
    head=NULL;
}
template <class T>
List<T>::~List()
{
}
template <class T>
List<T>::List(const List<T>& otherList)
{
}

// I have google the problem. Concept is simple. Make a new head and assign its node the //values of old List node.
// So for I have tried the following.

ListNode<T> *old = head; // pointer to old list.
ListNode<T> *new;// pointer to new head.

while (old->next!=NULL){
new->value = old->value;
old = old->next; 
}

// The only problem is how to make a new head that would point to my new Copied list.

Answer 1

This whole question is unclear, and there are a number of problems with the code, including a useless destructor, and not copy assignment operator.

You can’t generally define templates in a .cpp file, the entire template definition must be visible to all users of the code, which generally means defining the entire template in header files.

ListNode<T> *new;// pointer to new head.

new is a keyword in C++, you can’t use it for variable names.

// The only problem is how to make a new head that would point to my new Copied list.

What copied list? You haven’t actually copied anything or created any new nodes.

You need to create copies of the nodes in otherList

template <class T>
List<T>::List(const List<T>& otherList)
{
  ListNode<T>* node = otherList.head;
  ListNode<T>** tail = &head;
  while (node)
  {
    *tail = new ListNode<T>(node->value);
    tail = &tail->next;
    node = node->next;
  }
}