Access Violation Writing in a 2 way queue

Question

I'm trying to create a two sided queue with c++. I'm using Visual Studio 2012 and keep getting:

First-chance exception at 0x00D95A29 in Console_Assignment1.exe: 0xC0000005: Access violation writing location 0x00000008.

I think i'm having a pointer issue (might be trying to dereference something i shouldn't). so far i have had zero luck finding the problem and i'd really appreciate a second look.

(The code is too long to paste so i'll just copy the functions i think are giving me the problem.) Maybe just a small overview. I have a node class that holds two pointer to node (next and previous) and an int (value). and a queue class that holds two pointers to node (first and last) and an int (size).

// enqueueBeg - adds a new node at the beginning of the queue.
void DBL_Queue::enqueueBeg(int insert_val)
{
node* new_node = new node(insert_val);  // Creates the new node.
new_node->setNext( this->getFirst() ); // Connects the new node to the first in the queue
this->getFirst()->setPrev( new_node ); // Connects the first node in the queue to the new one
this->setFirst( new_node );             // Sets the new node as the first in the queue
this->setSize ( this->get_queue_size() + 1 ); // adds 1 to the size of the list

// dequeueBeg - removes the first node of the queue.
int DBL_Queue::dequeueBeg()
{
int ret_value = this->getFirst()->getVal();
node* old_node = this->getFirst();
this->setFirst( this->getFirst()->getNext() ); // Sets the second node in the queue as the first.
this->getFirst()->setPrev( NULL ); // Removes the link between the new first new and the old one.
this->setSize( this->get_queue_size() - 1); // Removes 1 from queue size
delete old_node;  // Deletes the node that use to be first.
return ret_value; // Returns the value of the old node.

// DBL_Queue Destructor
DBL_Queue::~DBL_Queue()
{
if (this->first == NULL)   // if queue is empty do nothing
    return;
else 
{
    while (this->first->getNext() != NULL)  // go through all nodes and delete them one by one
    {
        node* deletion = this->getFirst();
        this->setFirst( this->getFirst()->getNext() );
        delete deletion;
    }
}
}

Thanks in advance for the help!

Have you tried running in the debugger? It will help you locate the where the crash is, and also let you examine variables to help you see what might have caused it. However, have you thought about what happens when you enqueue the first node, meaning there is no current first node (i.e. this->getFirst() returns NULL)? You have similar problems with the dequeueing function. — Some programmer dude
I think you are going to have to post all the code. It's quite likely you've got multiple problems. It's also quite likely that the problems are somewhere different from where you think they are. The advice is always the same, post the smallest, complete, compilable program you can manage that still has the bug you are trying to solve. In other words, do some work to cut out the irrelevant stuff, but otherwise post everything. — john

Austin Austin · Accepted Answer · 2012-11-03T20:42:10

I think this is your problem:

 while (this->first->getNext() != NULL)  // go through all nodes and delete them one by one
{
    node* deletion = this->getFirst();
    this->setFirst( this->getFirst()->getNext() );
    delete deletion;
}

When you remove the last node, you will call

this->setFirst( null );

because this->getFirst()->getNext() will be null, right? So then while(this->first->getNext() becomes null->getNext()

Why not just

while(this->first != NULL)

?

Edit: unless you really care about minimizes the destructor's run time, why not

while(this->getFirst() != NULL) {this->dequeueBeg;}

Access Violation Writing in a 2 way queue

2 Answers