为双向链表创建类的新实例

Question

I have been provided with a starter code for creating a doubly linked list. The problem I'm having is implementing a function that inserts a newly created node at the 'head'.

A node in the linked list is the following struct:

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

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

The code for insertion at head is as under:

void List<T>::insertAtHead(T item)
{
     ListItem<int> a(item);     //create a node with the specified value

                   if(head==NULL)
                   {
                                 head=&a;   //When the list is empty, set the head
                                            //to the address of the node
                   }

                   else
                   {

                         //when the list is not empty, do the following.
                        head->prev=&a;
                        a.next=head;
                        head=&a;
                   }
}

Now the problem is, when I'm supposed to create a new class object with a different memory address whenever I insert an item. What I'm doing above updates the same memory location. I need to know how to create a new class object.

Answer 1

What you are doing is wrong, and potentially dangerous (using pointers to local variables). You need to allocate a new node with the new expression:

ListItem<int>* a = new ListItem<int>(item);

Of course, when done with the list, you have to remember to free the memory with delete .