Why am I getting an "a.exe has stopped working" error on Visual Studio Code for my linked list program?

Question

I'm playing around with a linked list, and I'm getting an error when I try to print out the value of the tail and the address of the next value:

struct Node {
    int n;
    Node *next;
};

class LinkedList {
    public:
        Node *head = NULL;
        Node *tail = NULL;
};

int main() {
    LinkedList L;
    L.head = NULL;
    L.tail = NULL;

    Node *new_node = new Node();
    new_node->n = 1;
    new_node->next = NULL;
    
    L.tail->next = new_node;
    L.tail = new_node;

    cout << L.tail << endl;
    cout << L.tail->next << endl;
}

🔎🐞Did run your code in a debugger to see where that error occurs, then run it again with a breakpoint near that failure so you can step carefully ahead and watch what happens leading up to that point? — tadman
– tadman, Commented Feb 27, 2023 at 22:24
PSA: In C++ use nullptr in preference to C's typeless NULL. — tadman
– tadman, Commented Feb 27, 2023 at 22:24
L.tail->next is set null, so you're asking it to print an invalid Node pointer. — tadman
– tadman, Commented Feb 27, 2023 at 22:25
L.tail->next = new_node; is dereferencing a null pointer. I would expect this to go splat. — teapot418
– teapot418, Commented Feb 27, 2023 at 22:26

Remy Lebeau · Accepted Answer · 2023-02-27 22:29:28Z

3

Consider these statements:

L.tail = NULL;
//...
L.tail->next = new_node;

You are trying to use a null pointer to access memory. That invokes undefined behavior.

Also, these assignments:

LinkedList L;
L.head = NULL;
L.tail = NULL;

are redundant, due to the class's default initialization of its members:

class LinkedList {
    public:
        Node *head = NULL;
        Node *tail = NULL;
};

edited Feb 27, 2023 at 22:29

Remy Lebeau

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answered Feb 27, 2023 at 22:27

Vlad from Moscow

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