Destructor for circular linked list in c++? - c++

Question

when the destructor of 'class LL' ~LL() gets called for this circular singly linked-list, the program crashes instead of freeing up the heap space of the pointer. How can I solve this problem?
class Node {
public:
int data;
Node *next;
};
class LL {
private:
Node *head, *tail;
public:
LL() {
head = NULL;
tail = NULL;
}
// destructor
~LL() {
Node *p = head;
while (p->next != head) {
p = p->next;
}
while (p != head) {
p->next = head->next;
delete head;
head = p->next;
}
if (p == head) {
delete head;
head = nullptr;
}
}
// circular singly Linked list
void createLL() {
int n, x;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> x;
Node *t = new Node;
t->data = x;
t->next = NULL;
if (head == NULL) {
head = tail = t;
} else {
tail->next = t;
tail = t;
}
}
tail->next = head;
}

Answer 1

There are a few issues with the linked list.
The linked list's destructor assumes that head isn't null, when there is a possibility that it could be. Make sure to check that head isn't null before trying to clean up memory. Once that is done, it looks like your original destructor should work.
The function createLL will invoke undefined behavior if the user enters a size less than or equal to 0.
Specifically this line tail->next = head;
TreateLL is a misnomer as it doesn't actually 'create' a new list in the expected sense. The contents aren't cleared, and thus n elements are appended to the end of the current list.
Also, a circularly linked list can be created with just a single tail pointer.
However, getting your implementation of a circular linked list to work looks like this
#include <iostream>
using namespace std;
class Node {
public:
int data;
Node* next;
};
class LL {
private:
Node* head, * tail;
public:
LL() : head(nullptr),
tail(nullptr) {
}
~LL() {
if (head) {
Node* p = tail;
while (p != head) {
p->next = head->next;
delete head;
head = p->next;
}
if (p == head) {
delete head;
head = nullptr;
}
}
}
void storeUserInput() {
int n, x;
cin >> n;
if (n <= 0) {
return; //no input to retrieve.
}
for (int i = 0; i < n; i++) {
cin >> x;
Node* t = new Node;
t->data = x;
t->next = nullptr;
if (head == nullptr) {
head = tail = t;
}
else {
tail->next = t;
tail = t;
}
}
tail->next = head;
}
};
int main() {
LL l;
l.storeUserInput();
char response;
std::cin >> response;
}
It seems that you have access C++ 11 or above compiler, if so then you should be using nullptr in place of NULL as it is a definitive pointer type. See more here

Answer 2

You can do it in two steps:
Make the list non-circular. This has two sub-steps:
Detect the loop. There are published algorithms to do this. Edit: Your list has a tail pointer, so there is no need to search for it in your case.
Point the back referencing node to null (or sentinel)
Delete the list which is now non-circular in a loop. This is trivial.

Answer 3

While trying to delete in a loop your circular reference will lead to deleted memory and will have undefined behavior. So first consider breaking the circulatiry:
tail->next = 0;
Then delete in a loop
Node* p = head;
while(p)
{
Node* temp = p;
p = p->next;
delete temp;
}
By the way. tail->next will always point to the head. So you always will have both, the head and the tail in the same pointer. So you can clean the memory like this:
Node* p = tail->next; //this is head
tail->next = 0;
while(p)
{
Node* temp = p;
p = p->next;
delete temp;
}