Thread: Queues, Nodes and passing instances of a class

I've got a question concering how to pass instances of a class through nodes/queues.

My question deals with arrays. When I push an instance of class on to my queue and then proceed to pop that same object off of my queue and call the function to return this instance of classes array of values, I can index the first value (ie array[0]) and recieve the first value fine. When I try to index any other value (array[1] for instance) I get these funny numbers like -10958358 .

Now I did not instatiate this classes array like this, why does this happen?

If source code is needed, I can post, but it is quite lengthy.

What Zach asked for would definitely not hurt. But I think I understand what you are asking enough to help. It sounds to me like you are not allocating new data in your queue. And specifically show the lines of code where you add data and where you try to access data by an index.

Allright, here is my code.

My customer/data class (basically the class that is encapsulated by the node)

Code:

Customer :: Customer (int id, int items, int seconds)
{
	customerValues[0] = id;
	customerValues[1] = items;
	customerValues[2] = seconds;
	customerValues[3] = items;
	}

int* Customer :: getCustomerValues()
{
	return customerValues;
}

My Node class

Code:

Node :: Node ()
{
	link = NULL;
}

Customer* Node :: getCustomer ()
{	
		return current;
}

void Node :: setNextLink (Node* next)
{
	link = next;
}

void Node :: setCustomer(Customer* incoming)
{
	current = incoming;
}

Node* Node :: getLink()
{
	return link;	
}

My queue (now, my pop function doesn't really pop it off yet, I want to return the correct values from my customer before I start popping off values, you'll see what I mean...)

Code:

Queue :: Queue (string type)
{
	queueSize = 0;
	frontNode = new Node;
	backNode = frontNode;
	frontNode->setNextLink(NULL);
	queueType = type;
}
		
bool Queue :: isEmpty()
{
	return frontNode == backNode;
}

void Queue :: push(Customer incoming)
{
		backNode->setCustomer(&incoming);
		backNode->setNextLink(new Node);
		backNode=backNode->getLink();
		backNode->setNextLink(NULL);
		queueSize++;

}

Customer Queue :: pop()
{
//	Node *nodeHolder;
	
	if (isEmpty())
		cout << "Queue is Empty!";
	else
	{	
		Customer frontCustomer = *frontNode->getCustomer();
cout << frontCustomer.getCustomerValues()[0];// if I index it with zero, I get the correct value
		cout << (frontCustomer.getCustomerValues()[2]); //now if I try doing this, I get funny values
		return frontCustomer;
		/*nodeHolder = frontNode;
		frontNode = frontNode->getLink();
		delete nodeHolder;
		queueSize--;
		*/	
	}

	
	
}

int Queue :: size()
{
	return queueSize;
}

and finnally my driver for it, just for testing, it doesn't really do much yet

Code:

main()
{
	string y = "express";
	Customer one (100,100,100);
	Queue lane(y);
	lane.push(one);
	 lane.pop();


	
	
	
}

I think my problem deals with how I'm referencing my array, I dunno.

Show the declaration of customerValues in Customer.

Also show the declaration of Node, to see how you represent Customer in Node.

Hmmm something is not within the code that you are showing. If it isn't a big deal just upload the cpp file.

Everything is within the code above. I did not include the header files because I thought that may be redundant. The declaration of customerValues is in the header and I declare it as an array: int customerValues[4] .

I represent Customer in node by having a pointer point to the incoming customer reference. I return that pointer with Node.getCustomer();

Originally Posted by xshapirox

Allright, here is my code.

Code:

void Queue :: push(Customer incoming)
{
		backNode->setCustomer(&incoming);
		backNode->setNextLink(new Node);
		backNode=backNode->getLink();
		backNode->setNextLink(NULL);
		queueSize++;

}

incoming is a local copy. When push returns the new Node has a pointer to an object that has had it's destructor called and has gone out of scope, hilarity insues.

The normal way to avoid this is to have node contain a Customer, rather than a pointer to a Customer. There is no need for backNode to be constructed without a customer. We can also simplify things by removing a lot of your getLink() functions and just making Queue a friend of Node. Remember the main reason we want to keep as much private as possible is to avoid having to "chase" our changes. As Queue is closely related to Node it makes sense to have to change Queue extensively if we change Node, but we don't want anyone else to have to know about the details.

Code:

class Node {
    Customer cust;  // <= copy
    Node *next;
    Node(const Customer &c) : cust(c), next(0) {}
//                             copy ctor ^^^^  
    friend class Queue;
};
...
void Queue::Push(const Customer &c) { // <= not a copy
    if(tail) {
        tail->next = new Node(c); // <= copy made
        tail = tail->next;
    }  else { // empty Queue
        head = new Node(c); // <= copy made
        tail = head;
    }
}
// Access, undefined if empty.
Customer & Queue::Top() const {return head->cust;}
... //Useage
Queue q;
q.Push(Customer(0,1,3));
Customer c = q.Top();
q.Push(c);
q.Push(q.Top());

Allright, I have a little problem though:

When I try to declare a Customer in my Node header file, I get this error:

Code:

In method `Node::Node(const Customer &)':
Node.cpp:5: no matching function for call to `Customer::Customer ()'
Customer.h:9: candidates are: Customer::Customer(int, int, int)
Customer.h:14:                 Customer::Customer(const Customer &)

I don't know why though...

You have to use the initialization list like grib showed you because your Customer class doesn't have any constructors that take no arguments. You want to copy construct the Customer member of the Node in the initialization list.

Code:

    Node(const Customer &c) : cust(c), next(0) {}
//                    copy ctor ^^^^  

When I try the above and try to compile I get this error:

Code:

Node.cpp: In method `Node::Node(const Customer &)':
Node.cpp:4: type `Customer' is not an immediate basetype for `Node'

This is in my Node source file, not the header.

Thanks, I think I got it. Forget the above message.

I understand now.

I really appreciate ya'lls help. If there was anything I could do to show my appreciation I would (I got a bottle of brandy and a bag of grass, but I doubt anyone lives near me)