Dereferencing null pointer warning and code run stuck infinitely

Question

So i am trying to solve a problem to reverse a linked list and following is what i came up with.

class Solution
{
public:

   /* void insertList(struct Node* head, int data)
    {
        Node* temp{ head };
        while (temp->next != nullptr)
        {
            temp = temp->next;
        }
        Node* nextNode{ new Node(data) };
        temp->next = nextNode;
    }*/
    void insertPrev(struct Node* revHead, int data)
    {
        Node* temp{ new Node(data) };
        temp->next = revHead;
        revHead = temp;
    }
    //Function to reverse a linked list.
    struct Node* reverseList(struct Node* head)
    {
        if (head == NULL)
            return head;
        Node* revHead{ new Node(head->data) };
        Node* temp{ head->next };
        while (temp != NULL);//this line
        {
            insertPrev(revHead, temp->data);//this line 2
            temp = temp->next;
        }
        return revHead;
    }
};

when i am trying to run this code to reverse a linked list. it gets stuck infinitely on this line which i have commented as this line. Moreover, ide throws warning also that i may be potentially derefercing a null pointer in this line 2. I don't know how is that possible when i just checked above that temp should not be null for the while loop to run. Help please!

[What is a debugger and how can it help me diagnose problems?](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — Jesper Juhl, Jun 07 '23 at 09:00
To reverse a linked list (like `std::list`), just use [std::reverse](https://en.cppreference.com/w/cpp/algorithm/reverse) - one line of code and problem solved. — Jesper Juhl, Jun 07 '23 at 10:11
You should always post code as a [Minimal, Reproducible Example](https://stackoverflow.com/help/minimal-reproducible-example) that people can cut'n'paste, compile and run. The shown code fails in this regard (for example; there's no `main` function). — Jesper Juhl, Jun 07 '23 at 10:23
Voting to close as a typo. The `while(temp!= NULL)` is immediately followed by a `;`. So, if `temp` is not equal to `NULL`, that is an infinite loop - and the `{` on the next line is never reached. That would also explain the warning. If the `insertPrev()` call is reached, `temp` is `NULL` and evaluating `temp->data` (and `temp->next` on the next line) gives undefined behaviour. — Peter, Jun 07 '23 at 11:26

score 1 · Answer 1 · answered Jun 07 '23 at 08:29

You have two issues in your code:

while (temp != NULL);//this line represents an infinite loop. The semicolon represents an empty body for this while construct. Remove it.
insertPrev modifies a local variable revHead: the caller's variable with the same name will not be modified by it. To fix this, define the parameter as pass-by-reference: void insertPrev(struct Node* &revHead, int data) (add the &).

This will fix the issue.

Other remarks

As this is C++ code:

Don't declare variables with struct Node*, but just Node*.
Don't use a mix of NULL and nullptr: only use nullptr

Depending on what was asked in the challenge, you could solve this by reversing the linked list in place, without needing to create new nodes.

Dereferencing null pointer warning and code run stuck infinitely

1 Answers1

Other remarks