SegFaulting with a dequeue function

Question

Basically, I'm implementing a queue using a linked list to try and simulate people standing in line at a store over the course of the day and they wait until the person in front of them finishes their business. The first few people go through fine but when i get to the second call of dequeue it segfaults me. The gdb debugger says the error comes from this line head=current->next; (where current=head).

Here is my dequeue function:

    void BankQueue::dequeue()
   {
      Node* current=head;
      head=current->next;
      if(head!=NULL)
      {
            head->prev=NULL;
      }
      delete current;
   }

Here is the enqueue function(in case when enqueueing i'm causing a memory leak):

    void BankQueue::enqueue(Customer s)
    {
         Node* node= new node;
         node->data=s;
         node->next=NULL;
         if(tail==NULL)
         {
              head=node;
              tail=node;
              node->prev=NULL;
         }
         else
         {
              node->prev=tail;
              tail->next=node;;
              tail=node;
         }

Any help you guys can offer as to where the segfault could be occuring would be amazing, thanks in advance.

P.S.I can provide more information if necessary.

Answer 1

Your dequeue function is flawed. Look what happens if head were to be NULL:

void BankQueue::dequeue()
{
    // current == NULL
    Node* current = head;
    // Setting head to NULL->next
    // This will reference memory location 0x00000000 + (some offset)
    head=current->next;
    // This is undefined, but it most likely will return true
    if(head!=NULL)
    {
        // undefined
        head->prev=NULL;
    }
    // Delete NULL
    delete current;
}

Also, yes, tail needs to be updated in there too.

// After you've made sure that head is valid
if (head == tail) {
    // there is only node, so we just clear tail
    tail = NULL;
}
// Then you proceed with removal

---

Thomas, in response to your comment:

void BankQueue::dequeue()
{
    // If the queue has instances
    if (head)
    {
        // If there is only one instance
        if (head == tail)
        {
            tail = NULL;
        }

        // set the new head
        head = head->next;
        // delete the old head if it exists
        if (head->prev)
        {
            delete head->prev;
        }
        // null data
        head->prev = NULL;
    }
}