why circular queue is required code example

Example: circular queue

// Circular Queue implementation in C++

#include <iostream>
#define SIZE 5 /* Size of Circular Queue */

using namespace std;

class Queue {
   private:
  int items[SIZE], front, rear;

   public:
  Queue() {
    front = -1;
    rear = -1;
  }
  // Check if the queue is full
  bool isFull() {
    if (front == 0 && rear == SIZE - 1) {
      return true;
    }
    if (front == rear + 1) {
      return true;
    }
    return false;
  }
  // Check if the queue is empty
  bool isEmpty() {
    if (front == -1)
      return true;
    else
      return false;
  }
  // Adding an element
  void enQueue(int element) {
    if (isFull()) {
      cout << "Queue is full";
    } else {
      if (front == -1) front = 0;
      rear = (rear + 1) % SIZE;
      items[rear] = element;
      cout << endl
         << "Inserted " << element << endl;
    }
  }
  // Removing an element
  int deQueue() {
    int element;
    if (isEmpty()) {
      cout << "Queue is empty" << endl;
      return (-1);
    } else {
      element = items[front];
      if (front == rear) {
        front = -1;
        rear = -1;
      }
      // Q has only one element,
      // so we reset the queue after deleting it.
      else {
        front = (front + 1) % SIZE;
      }
      return (element);
    }
  }

  void display() {
    // Function to display status of Circular Queue
    int i;
    if (isEmpty()) {
      cout << endl
         << "Empty Queue" << endl;
    } else {
      cout << "Front -> " << front;
      cout << endl
         << "Items -> ";
      for (i = front; i != rear; i = (i + 1) % SIZE)
        cout << items[i];
      cout << items[i];
      cout << endl
         << "Rear -> " << rear;
    }
  }
};

int main() {
  Queue q;

  // Fails because front = -1
  q.deQueue();

  q.enQueue(1);
  q.enQueue(2);
  q.enQueue(3);
  q.enQueue(4);
  q.enQueue(5);

  // Fails to enqueue because front == 0 && rear == SIZE - 1
  q.enQueue(6);

  q.display();

  int elem = q.deQueue();

  if (elem != -1)
    cout << endl
       << "Deleted Element is " << elem;

  q.display();

  q.enQueue(7);

  q.display();

  // Fails to enqueue because front == rear + 1
  q.enQueue(8);

  return 0;
}