Question

I am currently writing stack that is being implemented with a linked list. I am get this error:

Unhandled exception at 0x75249617 in STACK_LinkedList.exe: Microsoft C++ exception: std::bad_alloc at memory location 0x002ee8f8. 

I believe it is possibly coming from either my push() or pop() functions. I can’t find my error. I am fairly new to linked lists, so I have a little bit of a tough time finding errors.

Here is my push() function:

// Adds an item to the top of the stack
template <class S>
void Stack<S>::push(const S & e)
{
    NodePointer temp = new Node(e);

    if ( isEmpty() )
    {
        theTop = theFront = temp;
    }
    else
    {
            // Whatever is after top is stored in temp to keep track of it
        theTop->next = temp;

            // TheTop is stored in temp
        theTop = temp;  
        delete temp; 
    }
}

Here is my pop() function:

//Takes the item off the top of the stack
template <class S>
void Stack<S>::pop()
{
    if ( !isEmpty() )
    {        
                //temp node is set equal to the front
        NodePointer temp = theFront;

                //Holds the second to last node in the linked list
        NodePointer pred;

                //loops through until the node after temp is 0
        while (temp->next != 0)
        {
                        //sets the second to last as temp
            pred = temp ;

                        //basically "increments" temp to the next node
            temp = temp->next ;
        }

                //sets temp equal to the top
        temp = theTop;

                //the top is then set to its predecessor
        theTop = pred;

                //deletes what was known as the top
        delete temp;
    }

    else
        cout << "STACK IS EMPTY" << endl;

}

Thanks alot! I believe most of my logic is correct. I just am missing something small. If it’s something else please tell me and i’ll post that code.

Answer 1

You should not delete your temp in push! It’s a part of the list. So when you access this data later, you get surely an exception.

Second, you have to initialize your pred with NULL in pop(), otherwise you’ll get an undefined value assigned to theTop if the stack contains only 1 item.

Third, you should delete in pop() the Node which you allocated in push().

In general, your approach seems to be not very efficient. You should better store the pointers other way round: from stack top to the bottom items. That way you won’t need to traverse the whole stack on each pop(). Your code will be something like that:

void push(data)
{
    allocate new top
    new top's next is the old top
    store new top in the class
}

void pop()
{
    if empty, ERROR;
    new top = old top's next
    deallocate old top
}

Note that you don’t need theFront at all.