Remove() function in Queue using a doubly linked list

Question

I have written the program below and overall it seems to work fine... Except that the remove() function keeps deleting all of the values in the queue without even being called...

#include <iostream>
#include <string>

using namespace std;

//Create a node struct
struct Node {
  int data;
  Node *next;
  Node *prev;
};

class Queue {
private:
  Node *head;
  Node *tail;
  int size;
public:
  Queue();
  ~Queue();
  void add(int d);
  int remove();
  bool isEmpty();
  void printQueue(bool o);
};

//set to NULL
Queue::Queue() {

  head = tail = NULL;
  size = 0;

}

//destructor
//call remove until empty
Queue::~Queue() {

  while (!isEmpty())
    remove();
}

//adds a node with the given data at the back of the queue
void Queue::add(int d) {

  Node *temp = new Node;
  temp->data = d;
  temp->next = NULL;
  temp->prev = tail;

  if (isEmpty()) {

    //add to head
    head = temp;
    tail = temp;

    cout << "Added: " << tail->data << endl;

  } else {

    //append
    tail->next = temp;
    tail = temp;

    cout << "Added: " << tail->data << endl;
  }
  size++;
}

//removes the node at the head of the queue and returns its data
int Queue::remove() {
//TODO DOESNT WORK PROPERLY

  if (isEmpty()) {

    tail = NULL;

    cout << "The queue is empty." << endl;

    return 0;

  } else {

    Node *temp = head;
    int value = head->data;

    cout << "Removed: " << head->data << endl;

    //moves pointer to next node
    head = head->next;

    if (head)
      head->prev = NULL;

    size--;
    delete temp;
    return value;
  }
}

//determines if the queue is empty
bool Queue::isEmpty() {
  return (size == 0);
}

//prints the contents of the queue from front to back, or front
//to back, depending on the value of the parameter
void Queue::printQueue(bool o) {

  if (isEmpty()) {

    cout << "The queue is empty." << endl;

  } else {

    Node *p;

    if (o == true) {

      p = head;

      cout << "Printing front to back:" << endl;

      //print front to back
      while(p != NULL) {
        cout << p->data << " ";
        p = p->next;
      }

      cout << endl;

    } else if (o == false) {

      p = tail;

      cout << "Printing back to front:" << endl;

      //print back to front
      while (p != NULL) {
        cout << p->data << " ";
        p = p->prev;
      }

      cout << endl;
    }
  }
}

int main() {

  Queue q;

  q.add(9);
  q.add(10);
  q.add(11);
  q.add(12);
  q.add(13);
  q.add(14);
  q.add(15);
  q.add(16);

  q.printQueue(true);
  q.printQueue(false);

  return 0;
}

In the main, remove() isn't even used, but the output is

Added: 9
Added: 10
Added: 11
Added: 12
Added: 13
Added: 14
Added: 15
Added: 16
Printing front to back:
9 10 11 12 13 14 15 16 
Printing back to front:
16 15 14 13 12 11 10 9 
Removed: 9
Removed: 10
Removed: 11
Removed: 12
Removed: 13
Removed: 14
Removed: 15
Removed: 16

Why does this keep happening? What can I do to fix it? Help, please :(

Thanks in advance!

Answer 1

The problem is probably not in the code actually, at a glance it behaves normally.

You declare Queue q; as a local variable in your main. This means that when the main finishes it's execution, q goes out of scope, and the destructor is called.

Hope this sheds some light on the matter....