The initialization of ListNode

Question

I have a question about the initialization of ListNode, if I just announce a ListNode pointer, why can't I assign its next value like showed in the code.

struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(nullptr) {}
};

ListNode* tmp;
tmp->next = nullptr;// This is wrong, why is that?

ListNode* tmp2 = new ListNode(1);
tmp2->next = nullptr;// This is right, what cause that?

I just try to announce a ListNode pointer and assign its next value to nullptr. But after I announce a pointer with new function, it goes right. Why?

Answer 1

A pointer is a box that can hold the address of an object

 ListNode* tmp;
 tmp->next = nullptr;// This is wrong, why is that?

Here you created the box (tmp) but did not put the address of an object in it. The second line says - "at offset 4 from the address stored in tmp please write 0", well there is no valid address in the box so this fails.

In the second example

ListNode* tmp2 = new ListNode(1);
tmp2->next = nullptr;// This is right, what cause that?

You say

"Please make a new ListNode object"
"Put its address in the box called tmp2"
"at offset 4 from the address stored in tmp2 please write 0"

That can work, becuase the box tmp2 points somewhere valid