Circular Linked List Program in C++

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdbool>
using namespace std;
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}
int length(){
   int length = 0;
   
   //if list is empty
   if(head == NULL) {
      return 0;
   }
   current = head->next;
   while(current != head) {
      length++;
      current = current->next;
   }
   return length;
}

//insert link at the first location
void insertFirst(int key, int data){
   
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {
   
      //point it to old first node
      link->next = head;
      
      //point first to new first node
      head = link;
   }
}

//delete first item
struct node * deleteFirst(){
   
   //save reference to first link
   struct node *tempLink = head;
   if(head->next == head) {
      head = NULL;
      return tempLink;
   }
   
   //mark next to first link as first
   head = head->next;
   
   //return the deleted link
   return tempLink;
}

//display the list
void printList(){
   struct node *ptr = head;
   cout << "\n[ ";
   
   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         cout << "(" << ptr->key << "," << ptr->data << ") ";
         ptr = ptr->next;
      }
   }
   cout << " ]";
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   cout << "Original List: ";
   
   //print list
   printList();
   while(!isEmpty()) {
      struct node *temp = deleteFirst();
      cout << "\n Deleted value:";
      cout << "(" << temp->key << "," << temp->data << ") ";
   }
   cout << "\n List after deleting all items: ";
   printList();
   return 0;
}