Access violation for a specific function - c++

Question

I have many functions in my CustomerList.cpp file, the only one of which that doesn't work is shown below (and the break point is marked with a comment). NOTE: Store class is correct, and m_pHead is a CustomerList private variable (but that shouldn't matter).
bool CustomerList::removeStore(int ID)
{
Store *back, *temp;
if(m_pHead = NULL)
{
cout << "\nError! Store " << ID << " not found in the List!\n";
system("pause");
return false; // nothing to delete
}
// Search for the item to delete
back = NULL;
temp = m_pHead;
while((temp != NULL) && (temp->getStoreID() != ID))
{
back = temp;
temp = temp->m_pNext;
}
if(back == NULL) // Delete the first item in the list
{
m_pHead = temp->m_pNext; // THE FUNCTION BREAKS HERE
delete temp;
cout << "\nSuccess! Store " << ID << " added to List!\n";
system("pause");
return true;
}
else if(temp != NULL) // Delete from middle or end of list
{
back->m_pNext = temp->m_pNext;
delete temp;
cout << "\nSuccess! Store " << ID << " added to List!\n";
system("pause");
return true;
}
else
{
cout << "\nError! Store " << ID << " not found in the List!\n";
system("pause");
return false; // Didn't find the item to delete
}
}
Every time I make a call to this function, it breaks, even if the ID of the Store is not on the list (it shouldn't make it that far in the function).
Here is an example of a call:
// Creating a new Customer List
CustomerList *newList = new CustomerList();
newList->removeStore(3);
What in the world am I doing wrong?

Answer 1

There are some logic errors in your code. Most notably, this line;
if(m_pHead = NULL)
Is assigning NULL to m_pHead before then comparing it. Thus, temp is NULL, and back remains NULL, which is why your code reaches the spot you commented and crashes.
You need to use the == comparison operator, not the = assignment operator (your compiler should have warned you about that):
if(m_pHead == NULL)
Or safer:
if(!m_pHead)
Now, with that said, you can simplify the rest of the code to the following:
bool CustomerList::removeStore(int ID)
{
Store *temp, *previous;
// Search for the item to delete
previous = NULL;
temp = m_pHead;
while (temp != NULL)
{
if (temp->getStoreID() == ID)
{
if (m_pHead == temp)
{
// Deleting the first item in the list
m_pHead = temp->m_pNext;
}
if (previous != NULL)
{
// Deleting from middle or end of list
previous->m_pNext = temp->m_pNext;
}
delete temp;
cout << "\nSuccess! Store " << ID << " removed from List!\n";
system("pause");
return true;
}
previous = temp;
temp = temp->m_pNext;
}
cout << "\nError! Store " << ID << " not found in the List!\n";
system("pause");
return false; // nothing to delete
}
Or, if you use a standard C++ container, such as std::list, instead of making your own manual linked-list, you can do this instead:
struct isStoreID
{
int m_id;
isStoreID(int id) : m_id(id) {}
bool operator()(const Store &store) { return (store.getStoreID() == m_id); }
};
bool CustomerList::removeStore(int ID)
{
// Search for the item to delete
// where m_list is a std::list<Store>...
std::list<Store>::iterator iter = std::find_if(m_list.begin(), m_list.end(), isStoreID(ID));
bool bWasFound = (iter != m_list.end());
if (bWasFound)
{
m_list.erase(iter);
cout << "\nSuccess! Store " << ID << " removed from List!\n";
}
else
cout << "\nError! Store " << ID << " not found in the List!\n";
system("pause");
return bWasFound;
}