1 00:00:00,410 --> 00:00:03,980 ‫So now we're going to write the destructor for our linked list. 2 00:00:04,280 --> 00:00:11,150 ‫And a couple of videos ago we wrote this constructor and you always know what function is the constructor 3 00:00:11,150 --> 00:00:16,820 ‫because it has the same name as the class and the same is true for the destructor. 4 00:00:16,820 --> 00:00:22,190 ‫It also has the same name as the class, except it has this tilde at the front. 5 00:00:22,580 --> 00:00:29,390 ‫One of the things that's confusing about just structures is that some classes like the Linked List class, 6 00:00:29,390 --> 00:00:34,430 ‫has a written destructor, but the node class does not. 7 00:00:34,730 --> 00:00:40,970 ‫So if you don't actually write a destructor for a class, it is the default destructor that will be 8 00:00:40,970 --> 00:00:45,080 ‫run when you delete an instance of that class. 9 00:00:45,440 --> 00:00:50,210 ‫So when we create a new linked list, it's going to look something like this. 10 00:00:50,420 --> 00:00:56,120 ‫We have this new keyword and that means that we're going to run the constructor for that class. 11 00:00:56,120 --> 00:01:01,250 ‫And in this case we're going to call this instance of Linked List, my linked list. 12 00:01:01,670 --> 00:01:07,010 ‫So then if you're going to delete this linked list, you would do it like this and that is going to 13 00:01:07,010 --> 00:01:08,720 ‫run the destructor. 14 00:01:08,720 --> 00:01:14,960 ‫Or if you don't have code written for a destructor, it will run the default destructor to explain the 15 00:01:14,960 --> 00:01:16,340 ‫default destructor list. 16 00:01:16,340 --> 00:01:22,160 ‫Look at what would happen if we ran the default destructor on a linked list. 17 00:01:22,520 --> 00:01:28,520 ‫So this linked list is head and tail and all of these nodes and then also the length. 18 00:01:28,790 --> 00:01:33,230 ‫But there are two different classes that make up this linked list. 19 00:01:33,230 --> 00:01:37,100 ‫There's the linked List class and there is the node class. 20 00:01:37,280 --> 00:01:43,790 ‫And because these are two different classes, the linked List class, if you run the default destructor 21 00:01:44,030 --> 00:01:48,860 ‫is just going to delete these items head, tail and length. 22 00:01:49,190 --> 00:01:55,760 ‫Those nodes are created by a whole other class and the linked list class doesn't know how to delete 23 00:01:55,760 --> 00:01:56,210 ‫them. 24 00:01:56,600 --> 00:02:03,380 ‫So if you just run the default destructor for the linked list, these will all be deleted, but all 25 00:02:03,380 --> 00:02:06,290 ‫of these nodes will remain in memory. 26 00:02:06,620 --> 00:02:12,110 ‫So that is why we have to actually write a destructor for the linked list class. 27 00:02:12,620 --> 00:02:18,860 ‫So now let's create the destructor I'm going to bring back Head and we'll start our code out like this. 28 00:02:19,370 --> 00:02:26,750 ‫So we'll create a variable temp and we'll point it at head and then we need to loop through and individually 29 00:02:26,750 --> 00:02:28,880 ‫delete each one of these nodes. 30 00:02:28,880 --> 00:02:31,580 ‫So our loop is going to be while head. 31 00:02:31,760 --> 00:02:36,770 ‫In other words, while head is pointing at a node, that conditional will be true. 32 00:02:37,040 --> 00:02:41,630 ‫You could also write this as while head is not equal to null pointer. 33 00:02:41,990 --> 00:02:48,830 ‫So I'm going to focus just in on this while loop and the first thing we'll do is move head over by setting 34 00:02:48,830 --> 00:02:51,650 ‫it to be equal to head next. 35 00:02:51,650 --> 00:02:53,510 ‫And that moves that over. 36 00:02:54,150 --> 00:02:56,490 ‫Then we'll say delete temp. 37 00:02:56,700 --> 00:03:01,020 ‫And this is what actually removes that L1 node from memory. 38 00:03:01,020 --> 00:03:03,690 ‫So I'm just going to make that disappear. 39 00:03:03,960 --> 00:03:09,300 ‫And then we need to move temp over and have it point to that same node that head is pointing to. 40 00:03:09,690 --> 00:03:11,910 ‫So we'll do this again while head. 41 00:03:11,910 --> 00:03:17,550 ‫And that is true, it is pointing to a node and that will set head to be equal to head. 42 00:03:17,550 --> 00:03:21,960 ‫Next we're going to delete temp and move temp over. 43 00:03:22,440 --> 00:03:30,990 ‫Then we'll do this again, we'll move head over, will delete temp and move temp over and we'll do this 44 00:03:30,990 --> 00:03:32,640 ‫a couple more times here. 45 00:03:33,690 --> 00:03:37,920 ‫And then move temp over and then we'll do this this one last time. 46 00:03:38,040 --> 00:03:45,810 ‫Now, when we set head to be equal to head next head next as null pointer and that does this, then 47 00:03:45,810 --> 00:03:48,930 ‫we're going to delete temp, we're going to move temp over. 48 00:03:48,930 --> 00:03:54,630 ‫But now what do we come up here on the while loop head is pointing to null pointer that breaks us out 49 00:03:54,630 --> 00:03:59,040 ‫of the while loop and we have now deleted all of the nodes. 50 00:03:59,340 --> 00:04:05,730 ‫So let's bring back these two attributes for the linked list and bring in the rest of our code here. 51 00:04:05,940 --> 00:04:11,460 ‫And notice that none of this code does anything to delete these three items. 52 00:04:11,670 --> 00:04:18,540 ‫As I mentioned at the beginning of the video, these are all deleted with the default destructor, so 53 00:04:18,540 --> 00:04:22,500 ‫you don't have to do anything extra to make sure those get deleted. 54 00:04:22,770 --> 00:04:28,650 ‫So there will be a few other data structures in the course that have a written destructor, But this 55 00:04:28,650 --> 00:04:35,460 ‫is the only one I'll actually walk through because all of the data structures that have a written destructor 56 00:04:35,460 --> 00:04:37,020 ‫have one thing in common. 57 00:04:37,020 --> 00:04:38,730 ‫They all have nodes. 58 00:04:39,060 --> 00:04:43,560 ‫So that's why this is the only time I'll actually walk through this. 59 00:04:44,070 --> 00:04:47,460 ‫And that is our linked list destructor.