Check if a pointer is NULL and it causes a runtime error

Question

Definition of a ListNode is:

 // Definition for singly-linked list.
 struct ListNode {
     int val;
     ListNode *next;
     ListNode(int x) : val(x), next(NULL) {}
 };

When I check if a list is hit the end, I used:

int v1 = h1 == NULL? h1->val:0; // h1 is defined before: ListNode* h1 = l1;

But it returns a runtime error, but if I changed it to

int v1 = h1? h1->val:0;

it is accepted.

Why is that?

Check that condition again... It might be clearer if you try to turn it into a normal if-else statement. — Some programmer dude
– Some programmer dude, Commented May 15, 2015 at 17:48

Bill Lynch · Accepted Answer · 2015-05-15 17:56:49Z

10

This line of code:

 int v1 = h1 == NULL ? h1->val : 0;

Is more or less identical to:

int v1 = 0;
if (h1 == NULL)
    v1 = h1->val;

Note that if h1 == NULL, then you will exhibit undefined behavior when you dereference h1 in the following line.

answered May 15, 2015 at 17:49

Bill Lynch

82.4k16 gold badges132 silver badges181 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ed Heal · Accepted Answer · 2015-05-15 17:49:49Z

1

You need != instead of == as Null tends to be zero or false

answered May 15, 2015 at 17:49

Ed Heal

60.3k18 gold badges91 silver badges137 bronze badges

Do you mean there is a logical error in that particular code, or is it a general statement?