Cannot Insert into Ordered Linked List - c++

Question

I am writing a three file C++ program for my class. This program is ordered linked list. The program compiles but crashes when I attempt to insert (Run the program, select choice press enter, type an int to insert and press enter). Any help would be greatly appreciated.
Driver File:
#include "SortedLinkedList.h"
#include <iostream>
using namespace std;
int displayMenu();
void proccessChoice(int, SortedLinkedList&);
int main()
{
SortedLinkedList sSList;
int choice = displayMenu();
do
{
if (choice != 3)
{
proccessChoice(choice, sSList);
}
} while (choice != 3);
return 0;
}
void proccessChoice(int input, SortedLinkedList& l)
{
switch(input)
{
case 1:
int num;
cout << "Please enter a int: ";
cin >> num;
l.addItem(num);
break;
case 2:
l.popFirst();
break;
}
}
int displayMenu()
{
int choice;
cout << "menu" << endl;
cout << "===========" << endl;
cout << "1. add an int" << endl;
cout << "2. Show Sorted Linked List" << endl;
cout << "3. Exit" << endl;
cin >> choice;
cin.ignore();
return choice;
}
Declaration File:
struct sslNode
{
sslNode* next;
int item;
};
class SortedLinkedList
{
private:
sslNode* head;
bool isEmpty ();
public:
SortedLinkedList();
~SortedLinkedList();
void addItem(int);
int popFirst();
};
Implementation File:
#include <iostream>
using namespace std;
#include "SortedLinkedList.h"
SortedLinkedList::SortedLinkedList()
{
head = NULL;
}
SortedLinkedList::~SortedLinkedList()
{
sslNode *temp, *nextLink;
nextLink = head;
while(nextLink != NULL)
{
temp = nextLink->next;
delete nextLink;
nextLink = temp;
}
}
bool SortedLinkedList::isEmpty()
{
return (head == NULL);
}
void SortedLinkedList::addItem(int itemToInsert)
{
sslNode* cur;
sslNode* prev;
sslNode* newNode = new sslNode();
newNode->item = itemToInsert;
newNode->next = NULL;
cur = head;
prev = NULL;
bool moreToSearch (cur != NULL);
while (moreToSearch) //Find insertion point
{
if (cur->item > newNode->item) // while current location has a greater value then what needs to be inserted move pointers forward.
{
prev = cur;
cur = cur->next;
moreToSearch = (cur != NULL);
}
else // if current loacation and what is to be inserted are equal or less then we have found the point of insertion
{
moreToSearch = false;
}
}
if (prev = NULL)
{
newNode->next = head->next;
head = newNode;
}
else
{
prev->next = newNode;
newNode->next = cur;
}
//Insert as only item in list
//Insert in found location
}
int SortedLinkedList::popFirst()
{
sslNode* first;
first = head->next;
head = head->next;
int item = first->item;
return item;
}

Answer 1

Your problem is you forgot an =
if (prev = NULL)
{
newNode->next = head->next;
head = newNode;
}
else
{
prev->next = newNode;
newNode->next = cur;
}
if(prev = NULL)
should be
if(prev == NULL)
right now this is always false because prev becomes null which evaluates to false
and then it fails at
prev->next = newNode;
because your are dereferencing the null pointer.
You'll also want to treat the case where head == NULL before trying to insert anything. Basically if head == NULL, head = newNode;

Answer 2

It crashes because head is initialized to NULL. You probably want to make a dummy head node, depending on your design, or check if its NULL before using it in addItem().
This is how things go down:
SortedLinkedLis
t::SortedLinkedList() // ctor is called
...
head = NULL
SortedLinkedList::addItem(int)
sslNode* cur;
...
cur = head;
...
bool moreToSearch (cur != NULL) // this is surely false
...
if (prev = NULL)
{
newNode->next = head->next;
...//BUT head == NULL ! crash!