Why does this program break with delete command? - c++

Question

I want to delete a certain element from a linked list (it's a list of numbers).
When i find the right number, i check if the previous element is NULL. If it is, it's the head of the list, and i just move that pointer, and if it isn't i re-link elements, so that the previous element points to the next element of the element to be deleted.
Now, this works fine, unless i uncoment the following command:
delete old;
Now, old is a pointer that points to the element that needs to be deleted.
I want to delete the element, not just re-link the list.
// zag.h - header file
#ifndef _zag_h_
#define _zag_h_
#include <iostream>
using namespace std;
struct Elem {
int n;
Elem* next;
Elem(int bbr,Elem* nex = NULL){n = bbr; next = nex;}
~Elem(){delete next;}
};
class Lista {
Elem* head;
public:
Lista(){
head=NULL;
}
~Lista(){
}
void put(int broj){
Elem* temp = new Elem(broj);
Elem* n0 = NULL;
Elem* n1 = head;
while(n1!=NULL && temp->n >= n1->n){
n0 = n1;
n1 = n1->next;
}
if(n0 == NULL){
temp->next=head;
head = temp;
}
else {
n0->next = temp;
temp->next = n1;
//if(n1==NULL)tail=temp;
}
//cout << head->n << endl;
}
void remove(int num){
Elem* n1 = head;
Elem* n0 = NULL;
while(n1!=NULL && n1->n!= num){
n0 = n1;
n1 = n1->next;
}
if(n0 == NULL){
Elem* old = n1;
head = head->next;
n1 = n1->next;
delete old;
}
else {
Elem* old = n1;
n1 = n1->next;
n0->next = n1;
cout << old->n;
delete(old);
}
}//remove
void write(){
Elem* temp = head;
while(temp){
cout << temp->n << " ";
temp = temp->next;
}
cout<<endl;
} //ispisi
};
#endif
// main.cpp file
#include "zaglavlje.h"
#include <iostream>
using namespace std;
void main(){
cout << "Welcome " << endl;
Lista* l = new Lista();
l->put(4);
l->put(2);
l->put(8);
l->put(7);
l->put(6);
l->put(9);
l->put(11);
l->put(15);
l->put(17);
l->put(2);
l->put(1);
l->write();
l->remove(11);
//l->remove(2);
//l->remove(2);
//l->remove(11);
//l->remove(15);
cout << "ispisujemo elemente liste nakon brisanja" << endl;
l->ispisi();
}
So, i insert some elements in the list, write the list elements (which all seems to work fine), and then i call a function to remove one element. After that, when i try to write the list (to check if the element is really removed) i get the following error:
An unhandled win32 exception occurred in test.exe
and the debugger points the line
cout << temp->n << " ";
in the write function.
Without calling the delete old command it all works fine.

Answer 1

When your list is empty, you are attempting to access the "first" element anyway:
if (n0 == NULL && n1 == NULL)
{
// empty list, do nothing
}
else if (n0 == NULL && n1 != NULL)
{
Elem* old = n1;
head = head->next;
n1 = n1->next;
delete old;
}
else
{
Elem* old = n1;
n1 = n1->next;
n0->next = n1;
delete old;
}
When you remove the delete's in your current code, you are keeping the memory allocated, so you won't get an access violation when you access it incorrectly. When you add them back in, you are accessing memory that is no longer allocated when the list is empty.
This will also be a problem when you first run the program is head is not initialized.