struct pointer, check if null

[GisleAune]

I am -- for practice's sake -- trying to make some sort of queue system with a simple linked list that have a queue entry structure (T value and next). I try upon adding the first element to see if there's already anyone there, but checking for a Entry* pointer existing has not proven easy.

template <class T>
class Queue
{
public:
	void add(T item)
	{
		T* value = &item;
		if(last)
		{
			last->next = new Entry();
			last->next->value = value;
			last = last->next;
		}
		else
		{
			front = new Entry;
			last = front;
		}
	}
private:
	struct Entry
	{
		T* value;
		Entry* next;
	};
	Entry* front;
	Entry* last;
	int* length;
};
int main()
{
	Queue<int> q;
	q.add(32);
	system("pause");
}

The error gotten on runtime is
Unhandled exception at 0x012414fa: 0xC00000005. Access violation writing location 0xccccccc0

I'm pretty clueless here. It seems to go on even though the front* and last* leads to nowhere.

[kooth]

You need to write a constructor for the Queue class. In there, initialize all of your pointers to NULL and that will help tremendously.

[ahura24]

you need contractor to set null to your pointer and distructor to free memory

[GisleAune]

I did so and they were added as usual, but when trying to use a simple peek function (get the next one, but not remove it) I get the magic number CDCDCDCD, which according to Wikipedia means uninitialized heap memory, do you know why? I think the problem lies within the peek() method.

template <class T>
class Queue
{
public:
	// Constructor and Destructor
	Queue()
	{
		front = NULL;
		last = NULL;
	}
	void add(T item)
	{
		T* value = &item;
		if(last)
		{
			last->next = new Entry();
			last->next->value = value;
			last = last->next;
		}
		else
		{
			front = new Entry;
			last = front;
		}
	}
	T* peek()
	{
		if(front != NULL)
		{
			return front->value;
		}
		else
		{
			return NULL;
		}
	}
private:
	struct Entry
	{
		T* value;
		Entry* next;
	};
	Entry* front;
	Entry* last;
	int* length;
};
int main()
{
	Queue<int> q;
	int v = 4;
	q.add(v);
	cout << q.peek() << endl;
	system("pause");
}

Trying to return a dereferenced value gives an exception. It's for debugging's sake that I returned the pointer.

[kooth]

When you call add() the first time, last is NULL, so your if statement allocates front, but you didn't assign a value to it.

[GisleAune]

Okay, now I assigned the value to it -- I should have noticed that really -- and now it returns a large number upon dereferncing in peek(). Why does it do that?

[kooth]

Try making add take a reference to a T instead of by value.

[GisleAune]

Thank you, it works now.