template program crashing during execution

Question

I'm implementing Doubly Linked List in C++, using templates. As I'm trying my hands on templates.

template <class T>
class DList{
    Node<T> *head;
    Node<T> *tail;
    public:
        Dlist(){
            head = tail = nullptr;
        }       
        void addToHead(T el){
            Node<T> *newNode = new Node<T>(el);
            if(head == nullptr){
                head = tail = newNode;
            }
            else{
                head -> prev = newNode;
                newNode -> next = head;
                head = newNode;
            }
        } };

When using this addToHead(), the if condition is not executing. And the program crashes.

Why are so many people here trying to write templated classes and functions when they obviously could not write the non-templated versions? It's just adding complexity that gets in the way of learning C++. — , Nov 25 '17 at 17:18
How do you know, that _the comparison for if is not executing_? — , Nov 25 '17 at 17:20
_when the if is changed to_ that's exactly the same as in the snippet before. — , Nov 25 '17 at 17:22
Yeah @OliverCharlesworth, I did some debugging using cout statements. — Shadow King, Nov 25 '17 at 17:35
Here's how I did it - `cout<<"\nHere before if"; if(head == 0){ cout<<"\nHere in in"; head = tail = newNode; }` — Shadow King, Nov 25 '17 at 17:36
Unrelated: prefer to use `nullptr` over 0 when parking a pointer at NULL. It makes your code a bit easier to read. — user4581301, Nov 25 '17 at 18:27
The code shown is fine, so the problem has to be in code you haven't shown. Please provide a [mcve] showing exactly how you are using the list. — Remy Lebeau, Nov 25 '17 at 18:41
You thought it must be 0, but according to your code it isn’t. Use a debugger. — , Nov 25 '17 at 18:50
Thank you @manni66, found my mistake. Rectified it, now its working fine. — Shadow King, Nov 26 '17 at 04:11
@NeilButterworth I'm learning how to use templates in C++, I see nothing wrong in it. — Shadow King, Dec 03 '17 at 09:51

score 0 · Accepted Answer · answered Dec 03 '17 at 09:49

They mistake was with the constructor function, it was just an typo. Because of which the head and tail pointers were not initializing and when the head pointer was compared against the nullptr in the if condition of the addToHead(), program was crashing.

template program crashing during execution

1 Answers1