CODEWITHSUNDEEP
c++pointersdata-structureslinked-listdelete-operator
Rohan
Rohan

Reputation: 1805

Memory not freed up after deleting linked list's last node

I coded the following. Insert, delete at beginning, insert at beginning and end are all working fine. The memory marked in bold is not freed up. Should cout<<temp give an error? Please comment on the correctness of this code. 

void del(node** head)
{
    node* temp = (*head)->next;
    node* prev = *head;
    while(temp ->next!= NULL)
    {
        prev = temp;
        temp = temp -> next;
    }
    cout<<endl<<temp;
    cout<<endl<<prev;

    //delete prev ->next;
    prev -> next = 0;

    delete temp;
    cout<<endl<<"temp after free"<<temp;
    cout<<endl<<prev;
}
void main()
{
    node* head = NULL;  
int x = 5;
head = insert(head,x);
insert(head,6);
insert(head,7);
insert(head,8);
print(head);
del(&head);
print(head);
getch();    }

Output:

Empty List
|5| at 00673BF0
-------------------
|6| at 00673C30
-------------------
|7| at 00673FB8
-------------------
|8| at 00673FF8
-------------------

00673FF8
00673FB8
temp after free00673FF8
00673FB8

|5| at 00673BF0
-------------------
|6| at 00673C30
-------------------
|7| at 00673FB8
-------------------

Upvotes: 0

Views: 871

Answers (3)

chanduthedev
chanduthedev

Reputation: 382

In the code its good to free the temp node first. Then assign the prev->next to NULL. So that it will be very easy to understand that temp node is not available, thats way making prev->next NULL.

Upvotes: 0

bjhend
bjhend

Reputation: 1713

As Attila already pointed out it's undefined behavior to dereference a pointer after deleting it. To prevent accidentally doing that it's good practice to assign NULL to the pointer directly after the delete. Dereferencing a NULL pointer immediately causes an error pointing you to the right cause.

Upvotes: 1

Attila
Attila

Reputation: 28762

delete does not set the value of the pointer to NULL, but the memory pointed to is no longer valid (does not contain a live node)

This means that cout << temp will print the value of the pointer it still has (and was the address of the node before the delete), but dereferencing temp (e.g. *temp or temp->next) is undefined behavior

Note: nowehere in del do you modify the pointer pointed to by head, so you either don't need the double indirection (node**) or you should assign the new head to *head.

Upvotes: 2

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