Dereferencing head of empty linked list for next node

Question

I was trying to implement linked list in C++, when this idea struck my mind. With standard node definition as

class node {
public:
    int data;
    node *next;
};

I created an empty list node *head; then tried this

if(head->next == nullptr)
    cout<<"Stores nullptr";
if(! head->next)
    cout<<"Returns bool values";

But there is no output, so what is stored inside head->next ?

I can't see you do initialization of `next` anywhere, so it will contain a random value. Also the conditions in your `if` statements are equivalent. — πάντα ῥεῖ, May 12 '19 at 08:25
*I created an empty list* `node *head;` - no, you've created an uninitialized pointer to node class instance. — user7860670, May 12 '19 at 08:29
Possible duplicate of [Implicit cast of null pointer to bool](https://stackoverflow.com/questions/33474487/implicit-cast-of-null-pointer-to-bool) — super, May 12 '19 at 08:30
To everyone, don't you think c++ automatically does these initializations and create default constructor, I was just exploiting that fact. Now, my question was just what is stored inside `head->next` if head is nullptr(by default)? — Snigdh, May 12 '19 at 09:19
@Snigdh _"don't you think c++ automatically does these initializations"_ No we don't think so. We **know** that it doesn't. — πάντα ῥεῖ, May 12 '19 at 09:23
It is atleast mentioned in my book and my compiler does this also. — Snigdh, May 12 '19 at 09:44
@πάνταῥεῖ Ok, so where can I read about this can you provide any reference please. — Snigdh, May 12 '19 at 09:54
@Snigdh [1](https://stackoverflow.com/questions/1910832/why-arent-pointers-initialized-with-null-by-default) [2](https://stackoverflow.com/questions/26142100/c-default-constructor-does-not-initialize-pointer-to-nullptr) — πάντα ῥεῖ, May 12 '19 at 10:00

score 0 · Answer 1 · answered May 12 '19 at 08:39

First of all, you should create some space/allocate memory for the node class in main.

Note that node *head; is only a declaration not a definition. For further details have a look at What is the difference between a definition and a declaration?.

You allocate space for the object
Initialize its values, to be more elegant define a constructor method

    node *head = new node;
    head->next = nullptr;
    head->data=0;

I would still consider this as a duplicate of Linked lists in C++

If you consider duplicates just flag the question as such, instead of writing an answer. — πάντα ῥεῖ, May 12 '19 at 08:42

score -1 · Answer 2 · answered May 12 '19 at 08:30

If you declare node *head;, head is an uninitialized pointer that contains a random address. Dereferencing it is Undefined Behavior?

You need to a) initialize head: node *head = nullptr;, and b) test for that condition: if (head == nullptr) { head = new node; node->head = nullptr; ...

Dereferencing head of empty linked list for next node

2 Answers2