1 00:00:01,400 --> 00:00:08,900 OK, so welcome back to another one of these lectures about pointers and some dynamic memory stuff in 2 00:00:08,900 --> 00:00:17,090 this lecture, we're going to talk about kind of some weird C++ behavioral stuff that you need to know 3 00:00:17,090 --> 00:00:23,420 about to avoid kind of shooting yourself in the foot with this language, because C++ lets you do a 4 00:00:23,420 --> 00:00:27,140 lot of granular level stuff and it doesn't really protect you. 5 00:00:27,170 --> 00:00:33,050 Like other languages, it'll let you do some things that really just aren't OK, and it shouldn't let 6 00:00:33,050 --> 00:00:33,530 you do. 7 00:00:33,530 --> 00:00:35,750 But it does that to let you have control. 8 00:00:35,990 --> 00:00:40,730 So you can really kind of create sticky situations for yourself, especially with these pointers and 9 00:00:40,730 --> 00:00:41,070 stuff. 10 00:00:41,120 --> 00:00:49,880 So what I'm going to do is I'm going to actually comment out some of this stuff down here, so I'll 11 00:00:49,880 --> 00:00:50,750 just go. 12 00:00:53,340 --> 00:00:55,680 Right here and just kind of block comment this out. 13 00:00:56,370 --> 00:00:58,620 And what I'm going to do is make another function. 14 00:00:59,010 --> 00:01:01,710 What it's going to do is return an end point here. 15 00:01:02,730 --> 00:01:05,700 So I'm going to go ahead and do that. 16 00:01:05,700 --> 00:01:09,780 I'm just going to call this my other function. 17 00:01:10,950 --> 00:01:15,840 And what I'm going to do, though, is I'm going to make a local variable in here and I'm just going 18 00:01:15,840 --> 00:01:16,950 to call this like. 19 00:01:19,160 --> 00:01:23,820 I'll just call, yes, like in local bar five. 20 00:01:25,040 --> 00:01:32,460 And what I'll do is I will return the address. 21 00:01:32,880 --> 00:01:33,220 All right. 22 00:01:33,280 --> 00:01:39,350 So I'm putting this ampersand like we were just talking about the address of the local bar right here. 23 00:01:40,460 --> 00:01:46,280 And then what I'm going to do is I'm going to make another pointer in here and the integer pointer, 24 00:01:46,310 --> 00:01:55,880 and I'm just going to call it main pointer equals my other function because what's going to happen is 25 00:01:55,880 --> 00:02:01,160 it's going to return this address and it's going to take that address and store it in this pointer. 26 00:02:01,910 --> 00:02:10,790 The real problem here, though, there is a problem and that is that or make it a local variable inside 27 00:02:10,790 --> 00:02:15,170 this function and returning the address of that variable. 28 00:02:16,010 --> 00:02:21,440 So the reason that that is a problem. 29 00:02:22,340 --> 00:02:31,350 Is that the stat region of the memory is where this local variable is being created, it is stored there. 30 00:02:31,370 --> 00:02:36,920 You remember in that lecture when we were talking about the virtual address space, the most recent 31 00:02:37,160 --> 00:02:43,670 virtual address space lecture where we showed the stack growing downwards towards lower memory addresses 32 00:02:43,670 --> 00:02:47,980 and we had those green boxes right, and each one was a stack frame. 33 00:02:48,710 --> 00:02:55,640 Whenever a function gets called, it makes a new stack frame and those frames pile up like plates, 34 00:02:55,640 --> 00:02:59,900 you know, stacks of plates growing towards lower memory and the stack is growing down. 35 00:03:01,790 --> 00:03:06,620 Well, there was a stack frame for this, my other function, when it gets cold, it has a new stack 36 00:03:06,620 --> 00:03:06,920 frame. 37 00:03:06,920 --> 00:03:10,910 So what you can imagine is that there's all those houses on the street, right? 38 00:03:12,380 --> 00:03:18,560 Before a house is even built, there still is an address for the empty lot, right? 39 00:03:20,040 --> 00:03:27,170 So it's it's kind of like there was an empty lot and, you know, maybe it had some other stuff there 40 00:03:27,180 --> 00:03:31,350 you don't even know, maybe there was another house there, but what happened was got rid of one house, 41 00:03:31,350 --> 00:03:36,210 got rid of whatever it was there, bulldozed it, built a new house for this function, right? 42 00:03:36,390 --> 00:03:39,900 New house went ahead and stored some things in that house. 43 00:03:39,900 --> 00:03:41,130 Maybe, you know, there's like. 44 00:03:43,370 --> 00:03:44,150 You stored. 45 00:03:44,240 --> 00:03:49,990 I don't know, there's like a car in the garage, and that is this integer, right? 46 00:03:50,000 --> 00:03:52,160 It's a local piece of data. 47 00:03:52,160 --> 00:03:53,180 It's a local variable. 48 00:03:53,180 --> 00:03:55,730 It's stored inside that house in a stack frame. 49 00:03:56,570 --> 00:04:05,090 When the function is done, that stack frame, there's a good chance that stack frame gets torn down 50 00:04:05,450 --> 00:04:08,690 because the operating system is managing that memory. 51 00:04:09,200 --> 00:04:10,670 It does a lot of stuff, right? 52 00:04:11,330 --> 00:04:13,040 It's just a complicated zone. 53 00:04:13,040 --> 00:04:18,620 It's like a really busy street, which is houses going up and going down and just cars flying in and 54 00:04:18,620 --> 00:04:22,090 out like all over the place, data going everywhere. 55 00:04:22,100 --> 00:04:27,350 It's a real, messy zone in the operating system has like a lot of different rules that it uses to manage 56 00:04:27,350 --> 00:04:28,010 that area. 57 00:04:29,630 --> 00:04:35,270 So what that means is that you should always consider when the function is done. 58 00:04:36,220 --> 00:04:45,070 You don't necessarily have access to this exact variable in memory anymore because it was created in 59 00:04:45,070 --> 00:04:46,450 that stack frame. 60 00:04:46,690 --> 00:04:52,480 And now that stack frame is not going to be the same way it was when you were inside this function. 61 00:04:53,650 --> 00:04:59,890 So when the execution is flowing through this function, your program is being ran in, it's done with 62 00:04:59,890 --> 00:05:00,670 that function. 63 00:05:01,090 --> 00:05:03,520 This might just disappear. 64 00:05:04,270 --> 00:05:06,690 It might still be there or might still disappear. 65 00:05:06,700 --> 00:05:12,370 You don't know that's up to the operating system to decide, but the thing is that you shouldn't rely 66 00:05:12,370 --> 00:05:12,790 on it. 67 00:05:12,970 --> 00:05:17,350 And so what happens when we want to use this? 68 00:05:17,350 --> 00:05:20,980 So it returns the address here that goes right here, right? 69 00:05:20,980 --> 00:05:26,650 Because we called this function and it got returns down to here this address of local VAR. 70 00:05:27,950 --> 00:05:32,630 That address then got saved into this point, and now we have a pointer with that address and then what 71 00:05:32,630 --> 00:05:33,920 do we try and do with it? 72 00:05:33,930 --> 00:05:37,430 Maybe we try and print it out, right? 73 00:05:38,660 --> 00:05:40,400 Main main point here. 74 00:05:40,760 --> 00:05:50,990 Print out, main pointer in line, you know, and then maybe we do something like, you know, main 75 00:05:50,990 --> 00:05:57,530 pointer equals 50 or 540, I don't know. 76 00:05:57,530 --> 00:06:00,770 And then we go ahead and print out mean pointer again now. 77 00:06:04,490 --> 00:06:06,230 So let me go ahead and save this. 78 00:06:07,640 --> 00:06:10,490 And then run this hour compiled and one, compile it. 79 00:06:11,540 --> 00:06:14,030 And then what happens when we compile it so. 80 00:06:15,150 --> 00:06:16,140 Let's check this out. 81 00:06:17,680 --> 00:06:26,500 So what it does is it doesn't give us in error, but it gives us a warning and it's giving us a warning 82 00:06:26,500 --> 00:06:28,600 about doing this right here. 83 00:06:30,430 --> 00:06:35,290 So it says address of local variable returned. 84 00:06:35,830 --> 00:06:42,760 So it's talking about Line 10, this local variable, which referring to my 10, but it knows that, 85 00:06:42,760 --> 00:06:44,590 you know, that's going to be a problem. 86 00:06:44,710 --> 00:06:49,050 But like I said, C++ is going to go ahead and let you do it. 87 00:06:49,060 --> 00:06:50,230 This is just a warning. 88 00:06:50,230 --> 00:06:51,100 It's not an error. 89 00:06:51,370 --> 00:07:00,430 This program still compiled successfully so we can still run this executable, even though this is a 90 00:07:00,430 --> 00:07:01,490 problem, you know? 91 00:07:01,990 --> 00:07:05,470 This may or may not exist when we run it, this function. 92 00:07:06,790 --> 00:07:10,840 When it's getting ran, you know, all this stuff will be in a stacked frame and we get torn down, 93 00:07:10,840 --> 00:07:15,730 and then we're thinking that we still have access to this zone of memory expecting us to be able to 94 00:07:15,730 --> 00:07:16,330 print it out. 95 00:07:17,350 --> 00:07:19,990 So let's go ahead and show that we can run this. 96 00:07:22,230 --> 00:07:22,930 I run it. 97 00:07:22,950 --> 00:07:24,930 Nothing prints out, right? 98 00:07:25,170 --> 00:07:27,270 And this might not be the case for you. 99 00:07:27,510 --> 00:07:28,860 Something might print out. 100 00:07:29,100 --> 00:07:30,600 You might see some numbers. 101 00:07:30,990 --> 00:07:32,130 I don't really know. 102 00:07:32,580 --> 00:07:34,770 Like I said, it's undefined behavior. 103 00:07:34,770 --> 00:07:39,420 So what should we do if this is bad? 104 00:07:40,140 --> 00:07:45,990 Then what can we do if we want to make some sort of local variable in here in this function? 105 00:07:46,560 --> 00:07:48,440 But we want to be able to return it. 106 00:07:48,450 --> 00:07:49,380 What would we do? 107 00:07:49,830 --> 00:07:54,870 Well, like I said, the heap is a much less busy area, right? 108 00:07:54,900 --> 00:07:58,380 It's not managed quite as much by the operating system. 109 00:07:58,380 --> 00:07:59,580 It's a little more free. 110 00:08:00,060 --> 00:08:06,650 So what we could do is return that value instead on a heap, right? 111 00:08:06,660 --> 00:08:09,600 We can put this on the heap instead of putting it on the stack. 112 00:08:11,830 --> 00:08:18,250 So what I could do is comment this out and kind of do a similar thing, but make it on the heap so I 113 00:08:18,250 --> 00:08:22,540 could say and and then make a pointer and I could just say. 114 00:08:26,150 --> 00:08:37,540 You know, I'll just call this local bar and I can say equals new and like this, and then I could say, 115 00:08:37,540 --> 00:08:42,700 you know, a local bar, star, local bar I.D. reference ID, I say it's five. 116 00:08:43,480 --> 00:08:49,450 And then what I could do is I could just return local bar because it's already a pointer, right? 117 00:08:50,470 --> 00:08:52,000 So I could do something like this. 118 00:08:52,510 --> 00:08:55,120 I would like to also show you some new syntax right here. 119 00:08:55,120 --> 00:09:01,000 So if you don't want to do something like this, you can all on one line make it initialized to five. 120 00:09:01,510 --> 00:09:04,330 By doing that with putting some parentheses there. 121 00:09:06,010 --> 00:09:11,770 So instead of doing this extra line if you want, you can both declare this variable and then do the 122 00:09:11,770 --> 00:09:17,530 new int and initialize it with the value five by putting in parentheses after the end. 123 00:09:20,010 --> 00:09:25,170 So this changes stuff up, this will make it OK and won't give a warning for us, so let's go ahead 124 00:09:25,170 --> 00:09:26,130 and compile this. 125 00:09:26,640 --> 00:09:30,270 We notice we don't get any warning here and then let's go ahead and run it. 126 00:09:30,930 --> 00:09:33,690 You notice it prints out what we expected this time. 127 00:09:34,170 --> 00:09:38,420 We initialized it with five in here and we went ahead and returned this local bar. 128 00:09:38,890 --> 00:09:44,060 We got the pointer and got saved in this pointer, right? 129 00:09:44,100 --> 00:09:46,950 As we returned an integer pointer, it got saved in this pointer. 130 00:09:47,550 --> 00:09:53,610 Remember that when we had an ampersand here before we were returning an address, but also remember 131 00:09:53,610 --> 00:09:55,230 that pointers are just addresses. 132 00:09:55,260 --> 00:10:00,120 That's what I said in the last tutorial, the last video I was just talking about the fact that it's 133 00:10:00,120 --> 00:10:01,350 really just doing an address. 134 00:10:01,980 --> 00:10:06,120 Now we're doing the same thing, but it's just we're returning explicitly the pointer, right? 135 00:10:06,300 --> 00:10:07,140 We made a pointer. 136 00:10:07,140 --> 00:10:08,280 We return a pointer. 137 00:10:08,520 --> 00:10:10,890 The pointer is getting saved into this other pointer. 138 00:10:10,890 --> 00:10:12,210 They're all the same data types. 139 00:10:12,210 --> 00:10:13,500 So this is all gravy. 140 00:10:13,500 --> 00:10:14,130 It's OK. 141 00:10:14,730 --> 00:10:16,410 We go ahead and we print that out. 142 00:10:16,410 --> 00:10:17,550 It was five, right? 143 00:10:17,550 --> 00:10:20,070 So it's printing out what it was initialized to here. 144 00:10:20,070 --> 00:10:21,300 We d reference it. 145 00:10:21,660 --> 00:10:23,430 We change it to five hundred forty. 146 00:10:23,700 --> 00:10:27,120 Then we print it out and we see this expected result here. 147 00:10:28,500 --> 00:10:29,670 Why is this OK? 148 00:10:29,790 --> 00:10:31,740 Well, it's because it's on the heap. 149 00:10:32,040 --> 00:10:33,960 The heap is a lot more free. 150 00:10:34,230 --> 00:10:34,800 It's a lot. 151 00:10:34,950 --> 00:10:42,210 I would call it a more chill area, I guess, to be able to go ahead and do that and declare some variables 152 00:10:42,210 --> 00:10:46,610 there without us having the problem of, like, I don't know, Stack Frame got torn down. 153 00:10:46,620 --> 00:10:49,560 Who knows whether we can access this local variable anymore? 154 00:10:50,550 --> 00:10:50,910 All right. 155 00:10:50,910 --> 00:10:58,260 So an important point to know about using functions, returning addresses, making local variables. 156 00:10:58,770 --> 00:11:02,520 C++ sometimes lets you do things that you really shouldn't be doing. 157 00:11:03,220 --> 00:11:06,060 I just want to let you know that and point that out. 158 00:11:07,020 --> 00:11:09,620 Another thing that we need to do is what well, we did. 159 00:11:09,840 --> 00:11:13,350 We allocated this memory right? 160 00:11:14,750 --> 00:11:16,100 And you may be thinking well. 161 00:11:17,300 --> 00:11:20,130 It's in a local function, how do we allocate it? 162 00:11:20,150 --> 00:11:21,320 We don't know where it is. 163 00:11:21,350 --> 00:11:22,250 No, it's fine. 164 00:11:22,250 --> 00:11:22,980 It's on the heap. 165 00:11:22,980 --> 00:11:25,670 We're returning a pointer, which is just an address. 166 00:11:26,240 --> 00:11:27,770 This address gets returned. 167 00:11:27,770 --> 00:11:33,560 We're still able to refer to that address, except that actual memory that's at that address. 168 00:11:34,460 --> 00:11:41,630 It's way over and he planned, and we still have access to that zone of memory so we can go ahead and 169 00:11:41,630 --> 00:11:47,600 do a delete and we can go ahead and delete main pointer. 170 00:11:49,290 --> 00:11:51,780 We didn't declare main point, right? 171 00:11:53,970 --> 00:11:56,670 That's not what we declared, we declared this pointer. 172 00:11:57,930 --> 00:12:04,500 And it was pointing to this piece of memory return, that pointer, that pointer got saved in here. 173 00:12:07,120 --> 00:12:08,200 We printed it out. 174 00:12:09,270 --> 00:12:14,280 And you may be thinking, well, we've done all this stuff with it, but why don't we have to delete 175 00:12:15,150 --> 00:12:16,650 local law, Poyner? 176 00:12:17,160 --> 00:12:18,690 Isn't that what we have to delete? 177 00:12:19,740 --> 00:12:26,340 These things main pointer and local law pointer, they're having the same address in it, and the thing 178 00:12:26,340 --> 00:12:33,030 that really matters to delete is what's at that address, so we can still follow main pointer to the 179 00:12:33,030 --> 00:12:37,650 same address as local VAR, right? 180 00:12:38,430 --> 00:12:42,690 And that's going to contain they're both pointing to the same region of memory. 181 00:12:42,720 --> 00:12:46,050 So here, when we delete main pointer, we still solve that problem. 182 00:12:46,050 --> 00:12:50,790 We still deleted the memory, the memories, the thing that we need to free up, right? 183 00:12:50,790 --> 00:12:52,260 We need to unreservedly. 184 00:12:53,130 --> 00:12:54,990 Right, so that can be used still. 185 00:12:55,470 --> 00:12:58,010 So this is totally valid and fine to do. 186 00:13:00,350 --> 00:13:00,890 The. 187 00:13:01,890 --> 00:13:07,080 Local stuff in the function, like the address, you know, it can be created on the stack, but it's 188 00:13:07,080 --> 00:13:11,970 really just a pointer that memory that we're referring to is actually over on the heap, right? 189 00:13:12,960 --> 00:13:18,300 So just want to like really make sure that you understand the different locations that data is going 190 00:13:18,300 --> 00:13:24,240 into, you know, we here, we're storing some addresses somewhere in those addresses point to other 191 00:13:24,240 --> 00:13:27,060 regions of memory that store the actual data that we care about. 192 00:13:27,570 --> 00:13:27,930 Right. 193 00:13:27,930 --> 00:13:30,870 And since we did it this way, we didn't run into any warning. 194 00:13:30,870 --> 00:13:32,640 So we don't have issues, right? 195 00:13:34,310 --> 00:13:38,780 OK, so this is kind of a complex thing to wrap your head around, like I said, I'm breaking these 196 00:13:38,780 --> 00:13:46,970 into kind of smaller bite sized lectures, so it's easier to, I guess, digest or ingest whatever you 197 00:13:47,540 --> 00:13:51,590 will, you know, but I'm going to go ahead and in that lecture here. 198 00:13:52,130 --> 00:13:57,710 So the next lecture, I can start up on another topic and I don't cluster too much complicated stuff 199 00:13:57,710 --> 00:13:58,280 together. 200 00:13:58,970 --> 00:13:59,240 All right. 201 00:13:59,250 --> 00:14:01,340 So with that, I will see you in the next lecture.