WEBVTT 00:00.270 --> 00:07.800 I want to show you some of the pitfalls that you should avoid when using dynamic memory. 00:07.830 --> 00:10.350 So here's a simple one. 00:10.540 --> 00:20.710 Say add and pointer and it points to this new integer and we say each winter this this is OK. 00:22.180 --> 00:27.310 Then you say lead again. 00:27.800 --> 00:32.250 So double delete a pointer to memory. 00:32.480 --> 00:36.980 Remember that once you delete the memory the memory is gone back to the heap. 00:36.980 --> 00:42.770 So your program doesn't really control it anymore and it's part of the heap now. 00:42.980 --> 00:47.830 So once you delete it again really only bad things will happen. 00:48.630 --> 00:59.840 So instead what you should do is always say this to set it to null pointer and so this is good. 01:01.300 --> 01:16.700 So Never leave something in another example something to not do is say we had pointer of 10 integers 01:16.790 --> 01:18.180 and it doesn't have to be. 01:18.180 --> 01:23.460 Could could still we could do it this way too but it's going to do it for a difference. 01:23.870 --> 01:32.690 And let's say we had a different integer an actual integer set to five year. 01:32.730 --> 01:41.050 So this point your points to the head of these 10 contiguous integers in memory. 01:41.050 --> 01:41.420 Right. 01:41.470 --> 01:45.440 So this array of 10 integers sets went to the head. 01:45.450 --> 01:46.740 But what if you did this. 01:46.770 --> 01:50.390 So point it to something else. 01:50.400 --> 01:51.240 OK. 01:51.600 --> 01:54.330 So this is very bad. 01:54.420 --> 01:55.040 OK. 01:55.350 --> 02:04.770 This is what's called a memory leak because there's no way to reclaim where it regionally pointed to. 02:04.850 --> 02:05.840 Right. 02:05.960 --> 02:11.560 Like we pointed originally to the head of some memory which gave some address back right. 02:11.870 --> 02:16.090 But now we're pointing somewhere else and our owning pointer. 02:16.250 --> 02:18.830 Originally this was the owning pointer. 02:19.010 --> 02:22.750 Right now kind of points somewhere else. 02:24.520 --> 02:28.410 Which is very bad so never ever ever do this. 02:28.750 --> 02:29.750 OK. 02:29.830 --> 02:32.950 So in a similar way 02:35.650 --> 02:40.030 here can say something like this. 02:43.340 --> 02:48.450 Just put the address 02:51.710 --> 02:52.460 winter 02:55.830 --> 02:59.360 this. 02:59.620 --> 03:04.470 And then remember we could do this you could increment the pointer right. 03:04.870 --> 03:10.810 And that would give you the address of the next one. 03:10.900 --> 03:17.310 The next item in the array right. 03:17.360 --> 03:23.610 So if we did that and ran it of course you'd get. 03:24.090 --> 03:26.910 You know it's four bytes more. 03:26.920 --> 03:28.200 Right. 03:28.300 --> 03:38.250 So the next one which is fine but what you've really done here is now you don't point to the head anymore. 03:38.510 --> 03:45.160 OK point to the initial address. 03:45.390 --> 03:45.770 Right. 03:45.810 --> 03:48.680 So you're kind of pointing to the next one instead. 03:49.410 --> 04:01.790 OK so you should never you should always leave the initial pointer owning pointer alone and access it 04:01.790 --> 04:07.630 through some other means like making a new pointer and pointing it to the memory. 04:07.650 --> 04:17.500 So any owning pointer should always point exactly to where pointed to when you knew the memory. 04:17.510 --> 04:18.130 OK. 04:18.910 --> 04:22.140 And it would be even worse if you did something like this. 04:22.850 --> 04:23.370 This would have. 04:23.370 --> 04:27.320 Now you said delete point here and you kind of forgot that you incremented it. 04:27.540 --> 04:29.040 Well see what happens. 04:31.640 --> 04:33.240 Oh not good. 04:33.240 --> 04:33.820 Right. 04:34.040 --> 04:44.160 So it says pointer being freed it's say was not allocated. 04:44.160 --> 04:47.580 It set a breakpoint and all this all this stuff right. 04:47.580 --> 04:51.220 So it's really saying hey you're deleting something. 04:51.340 --> 04:53.110 In fact we even do it wrong. 04:53.130 --> 04:53.460 Right. 04:53.460 --> 04:54.280 It should be like this. 04:54.280 --> 04:59.890 Even OK you're deleting something but it wasn't. 04:59.890 --> 05:03.380 It's not to the same address as it should be. 05:03.390 --> 05:03.780 Right. 05:03.790 --> 05:07.510 It's not pointing to the original addresses really what this is saying. 05:07.920 --> 05:08.530 OK. 05:08.680 --> 05:19.120 So this is very very bad to do so always just keep the same or if it's an owning pointer just keep it 05:19.270 --> 05:22.400 pointing to where it originally pointed to. 05:22.690 --> 05:23.140 OK. 05:23.140 --> 05:25.850 Access it through some other pointer. 05:25.900 --> 05:30.010 So if you really wanted to do this we should say. 05:30.100 --> 05:32.260 Pointer to right. 05:33.780 --> 05:36.590 Hey goes to pointer. 05:37.170 --> 05:42.710 And then we use pointer to where here. 05:43.860 --> 05:46.230 Are two parts plus K. 05:46.400 --> 05:57.720 And then we could delete the memory OK of the owning pointer of your and this should be OK. 05:58.000 --> 06:06.960 Jay so always access it through a different pointer and keep the owning pointer the same 06:11.770 --> 06:16.750 so other pitfalls again deleting pointers you don't own right. 06:16.780 --> 06:18.140 So never do that. 06:18.190 --> 06:22.940 Not assigning the null pointer after the memory was ready. 06:23.260 --> 06:29.070 Here's a nother example let's say you had 06:34.940 --> 06:40.340 her course new integer 10 06:42.690 --> 06:47.580 integers and never do this. 06:47.590 --> 06:49.830 OK you're here sort of. 06:50.190 --> 06:56.190 This is the same as with the arrays you're kind of are assigning. 06:56.190 --> 06:59.190 Say so you're signing outside 07:01.800 --> 07:04.670 your index range right. 07:05.760 --> 07:09.230 Ok it's very bad to do this. 07:09.270 --> 07:11.160 We don't know what will happen. 07:12.100 --> 07:15.670 If you do this it might crash it might not crash. 07:15.670 --> 07:20.070 Hopefully it will crash because at least then you'll see where is crashing. 07:20.090 --> 07:20.950 Right. 07:21.190 --> 07:24.000 So that that would this is bad to do. 07:26.880 --> 07:41.520 And of course if you do something like this say it and then you delete that with the array image mismatch 07:41.560 --> 07:42.100 right. 07:42.790 --> 07:44.360 Her mismatch. 07:44.740 --> 07:51.430 So you knew that with only the regular new but you're deleting it with the array delete. 07:51.490 --> 07:53.350 So don't do this. 07:53.350 --> 07:55.600 This is also very bad. 07:55.660 --> 07:57.220 And of course vice versa. 07:57.220 --> 07:57.700 Right. 07:59.170 --> 08:07.350 So these are some of the pitfalls if you follow the guidelines outlined by this time you should. 08:07.340 --> 08:09.110 You should usually be OK. 08:09.190 --> 08:19.380 I tend not to use dynamic memory so much but it's useful in some search situations and in the next section 08:20.370 --> 08:25.280 when we do our hangman game you'll see just how useful it can be.