1 00:00:04,350 --> 00:00:09,720 ‫So in this lecture, we're going to be talking all about the virtual method in C++. 2 00:00:09,720 --> 00:00:13,170 ‫We've seen it before, we've used it, but have we fully understood it? 3 00:00:13,170 --> 00:00:14,100 ‫Let's see. 4 00:00:14,100 --> 00:00:20,460 ‫So the idea of the virtual method is that it allows us to override functions in a base class. 5 00:00:20,460 --> 00:00:27,210 ‫We've seen this with things like begin, play and tic in Unreal, but let's get further into this and 6 00:00:27,210 --> 00:00:29,880 ‫understand why they're marked as virtual. 7 00:00:29,880 --> 00:00:38,340 ‫It allows us to change functionality in that base class, and it does something called Dynamic Dispatch, 8 00:00:38,340 --> 00:00:44,550 ‫which we're going to talk about in a second to kind of understand what this really means and why virtual 9 00:00:44,550 --> 00:00:50,820 ‫methods aren't the default functionality in C++ as they are in many other languages, such as C Sharp. 10 00:00:50,820 --> 00:00:52,320 ‫So let's have a look at this. 11 00:00:52,440 --> 00:00:59,910 ‫I'm going to be doing this in the CPP dot S-H website so that we're not in Unreal here at all. 12 00:00:59,910 --> 00:01:05,160 ‫We're just looking at the raw C++ functionality so that you can understand this language feature and 13 00:01:05,160 --> 00:01:06,090 ‫how it works. 14 00:01:06,150 --> 00:01:10,680 ‫Now, the first thing I'm going to do here is just go ahead and clear out the main method. 15 00:01:10,680 --> 00:01:18,120 ‫And in our use case, we're going to think about a class for guns, seeing as this is a very common 16 00:01:18,120 --> 00:01:19,680 ‫use case for games. 17 00:01:19,680 --> 00:01:26,760 ‫So let's have a look at a class called it gun fairly straightforwardly and then we're going to have 18 00:01:26,760 --> 00:01:31,860 ‫the body of this class be public and a void shoot method. 19 00:01:32,520 --> 00:01:38,460 ‫And I'm just going to go ahead and declare the body of this shoot method right here in line to keep 20 00:01:38,460 --> 00:01:39,310 ‫things super simple. 21 00:01:39,330 --> 00:01:44,550 ‫By the way, you can do this if you haven't seen this before, but generally speaking, we want to do 22 00:01:44,550 --> 00:01:49,470 ‫things in a separate C++ file to keep things separate, to make the compilation times quicker and so 23 00:01:49,470 --> 00:01:49,770 ‫on. 24 00:01:49,770 --> 00:01:55,290 ‫But here where it's just a toy example, this is fine, so we're going to do an STD C out. 25 00:01:55,290 --> 00:02:03,390 ‫So using C++ as output string stuff and we're going to go bang basically when we call this method and 26 00:02:03,390 --> 00:02:04,980 ‫then we're going to end line. 27 00:02:05,570 --> 00:02:07,850 ‫And that's their body of our shoot method. 28 00:02:07,850 --> 00:02:11,720 ‫And we can cap off the class with the curly brace and semicolon. 29 00:02:11,960 --> 00:02:14,750 ‫So let's go ahead and actually use this class. 30 00:02:14,750 --> 00:02:23,510 ‫I'm going to create a my gun variable in main and I am going to call my gun dot shoot and let's see 31 00:02:23,510 --> 00:02:25,910 ‫if everything is working here as it should. 32 00:02:25,940 --> 00:02:28,070 ‫Hit the run button and bang. 33 00:02:28,160 --> 00:02:30,530 ‫It is calling the method as we'd expect. 34 00:02:30,980 --> 00:02:39,470 ‫Now what happens if I want to extend my gun and perhaps make a rifle or pistol? 35 00:02:39,470 --> 00:02:40,420 ‫Let's call it a pistol. 36 00:02:40,430 --> 00:02:47,180 ‫So class pistol is a inherited from gun, which would do like this. 37 00:02:47,180 --> 00:02:51,530 ‫And if you haven't seen this before because you probably haven't written this necessarily before, you'd 38 00:02:51,530 --> 00:02:56,330 ‫do a public inheritance from guns so that other classes can see that you inherit from gun. 39 00:02:56,600 --> 00:03:04,280 ‫And then we do in the public section, we want to maybe create our own shoot method for the pistol that 40 00:03:04,280 --> 00:03:05,540 ‫overrides this functionality. 41 00:03:05,540 --> 00:03:13,220 ‫And instead of printing bang at the end here we want to print pill because it's a smaller gun, it makes 42 00:03:13,220 --> 00:03:17,420 ‫a smaller sound and that's our shoot method there. 43 00:03:17,420 --> 00:03:20,150 ‫And let's cap off the class there. 44 00:03:20,360 --> 00:03:23,300 ‫And what we want to do is create ourselves a pistol. 45 00:03:23,300 --> 00:03:28,010 ‫So pistol my pistol in the main method here. 46 00:03:28,670 --> 00:03:34,010 ‫And because these are just existing here, right on the main method, we're not using pointers to them. 47 00:03:34,010 --> 00:03:38,450 ‫That's why we're using the shot operator here instead of the arrow. 48 00:03:38,750 --> 00:03:42,260 ‫So the my pistol not shoot. 49 00:03:42,260 --> 00:03:43,460 ‫Let's see what that does. 50 00:03:43,610 --> 00:03:47,280 ‫I'll expect it to print PPU, which it does. 51 00:03:47,300 --> 00:03:47,930 ‫Cool. 52 00:03:48,260 --> 00:03:51,530 ‫Okay, so let's remove our methods that are calling. 53 00:03:51,530 --> 00:03:57,050 ‫Shoot directly on these and let's see how things would behave with a pointer. 54 00:03:57,290 --> 00:04:01,880 ‫So I'm going to create a gun pointer called Gun. 55 00:04:02,660 --> 00:04:03,640 ‫Peter. 56 00:04:04,570 --> 00:04:06,910 ‫And on the next line I could do it on the same line. 57 00:04:06,910 --> 00:04:08,890 ‫But for simplicity's sake, I'm going to do it on the next line. 58 00:04:08,890 --> 00:04:15,790 ‫I'm going to set that gun pointer to point to the address of that's the address of operator the address 59 00:04:15,790 --> 00:04:18,310 ‫of the my gun variable. 60 00:04:18,580 --> 00:04:23,830 ‫And then because this is a pointer, we need to use the arrow operator to call the shoot method. 61 00:04:23,830 --> 00:04:30,400 ‫So I'm going to call shoot on my gun, let's click run and we get bang as the result there. 62 00:04:30,400 --> 00:04:31,120 ‫Cool. 63 00:04:31,210 --> 00:04:34,450 ‫Now what happens if I do the same thing? 64 00:04:34,450 --> 00:04:40,840 ‫But instead of setting the gun pointer to point to my gun, I set it to point to my pistol. 65 00:04:40,990 --> 00:04:42,550 ‫Let's go ahead and hit run now. 66 00:04:42,880 --> 00:04:43,240 ‫Okay. 67 00:04:43,240 --> 00:04:45,190 ‫That's an interesting behavior there. 68 00:04:45,190 --> 00:04:51,880 ‫What's going on here is it is calling this first method in both instances. 69 00:04:51,880 --> 00:04:55,510 ‫So why hasn't this second shoot method been called? 70 00:04:55,510 --> 00:04:58,030 ‫We've got a pistol here as the pointer. 71 00:04:58,300 --> 00:05:05,500 ‫Well, what happens if you don't have a virtual method is that by default, C++ will use the type of 72 00:05:05,500 --> 00:05:07,960 ‫your pointer, which this is a gun pointer. 73 00:05:07,960 --> 00:05:11,890 ‫Remember, it will use that type to find the method to call. 74 00:05:11,890 --> 00:05:18,130 ‫So it will go and call this shoot method at the top rather than the overridden method in pistol. 75 00:05:18,130 --> 00:05:23,530 ‫In order for it to call the overridden and method in pistol, it would have to call it would have to 76 00:05:23,530 --> 00:05:25,630 ‫have a pointer of type pistol. 77 00:05:25,960 --> 00:05:27,520 ‫Now that's fine. 78 00:05:27,520 --> 00:05:29,350 ‫And you can see why they do this. 79 00:05:29,350 --> 00:05:30,430 ‫It's for efficiency. 80 00:05:30,430 --> 00:05:32,410 ‫C++ is all about efficiency. 81 00:05:32,410 --> 00:05:39,250 ‫If something is going to cost more when it's compiled, when it runs, then C++ won't do it by default 82 00:05:39,250 --> 00:05:40,600 ‫unless you ask it to. 83 00:05:40,630 --> 00:05:46,270 ‫So it's very easy for C++ to make the look up and go, Well, this is a gun pointer, so I'll call this 84 00:05:46,270 --> 00:05:47,620 ‫gun pointer method. 85 00:05:47,620 --> 00:05:50,080 ‫And this is where Dynamic Dispatch comes in. 86 00:05:50,080 --> 00:05:54,070 ‫This fancy term basically means that we don't want it to do that. 87 00:05:54,070 --> 00:05:59,950 ‫What we want it to do is say, Hang on a second, what's actually stored in that gun pointer? 88 00:05:59,950 --> 00:06:02,140 ‫What's the real type there? 89 00:06:02,380 --> 00:06:06,250 ‫And the real type we realize is it's a type pistol. 90 00:06:06,250 --> 00:06:12,520 ‫And so we've got to ask ourselves, is there a method for that type that's overridden? 91 00:06:12,520 --> 00:06:13,600 ‫And yes, there is. 92 00:06:13,600 --> 00:06:20,230 ‫So how do we give C++ the permission to be just that little bit less efficient and give us the ability 93 00:06:20,230 --> 00:06:26,110 ‫to override without having to change that pointer type, which isn't always necessarily possible. 94 00:06:26,110 --> 00:06:31,690 ‫In fact, in many cases this isn't possible, as is the case when you're having your own custom actor 95 00:06:31,690 --> 00:06:32,470 ‫class. 96 00:06:32,590 --> 00:06:38,290 ‫Unreal doesn't have the pointer set up to point to your particular act class that would require them 97 00:06:38,290 --> 00:06:41,290 ‫to change the whole engine every time you introduce a new class. 98 00:06:41,290 --> 00:06:43,120 ‫No, that's just not possible. 99 00:06:43,120 --> 00:06:46,360 ‫They have a pointer to something of type actor star. 100 00:06:46,360 --> 00:06:51,010 ‫So when they call begin play, you want it to call your begin play method. 101 00:06:51,010 --> 00:06:52,630 ‫So it has to be virtual. 102 00:06:52,630 --> 00:06:54,340 ‫And that is the key here. 103 00:06:54,340 --> 00:07:02,500 ‫If we stick virtual in front of the method in the parent class, this now signals to C++ that hang on 104 00:07:02,500 --> 00:07:10,120 ‫a second, if I am calling shoot on a pointer of type gun pointer, I need to check that this isn't, 105 00:07:10,120 --> 00:07:14,350 ‫for example, a pistol and that this shoot method has been overridden. 106 00:07:14,350 --> 00:07:19,390 ‫So if we go ahead and hit run now, you can see we get bang and then pill. 107 00:07:19,420 --> 00:07:24,070 ‫So it is working as we'd expect with Dynamic Dispatch. 108 00:07:24,070 --> 00:07:30,370 ‫And that is the term for this thing where we basically just mean check what kind of type this is really 109 00:07:30,370 --> 00:07:33,430 ‫being held in the pointer and call the relevant method. 110 00:07:33,940 --> 00:07:40,450 ‫Now there is an extra little bit of syntax that you will often see as use and that is the override keyword. 111 00:07:40,450 --> 00:07:45,520 ‫And you put that after a method that you are trying to override as virtual. 112 00:07:45,790 --> 00:07:53,680 ‫And basically the override keyword isn't strictly necessary, nor is putting virtual again against something 113 00:07:53,680 --> 00:07:56,200 ‫on the child class of a gun. 114 00:07:56,200 --> 00:07:59,350 ‫So you don't have to put virtual, but it doesn't hurt to do it. 115 00:07:59,560 --> 00:08:06,550 ‫But override is particularly helpful because it allows us to just make sure that we've got our naming 116 00:08:06,550 --> 00:08:07,030 ‫correct. 117 00:08:07,030 --> 00:08:10,690 ‫So if I hit run, everything runs as before when I have override. 118 00:08:10,690 --> 00:08:17,290 ‫But say I had mistakenly got the name of this method wrong so I don't do the override. 119 00:08:17,290 --> 00:08:19,490 ‫And I had called it a no shoot. 120 00:08:19,870 --> 00:08:22,750 ‫For example, I had copied the name wrong. 121 00:08:22,750 --> 00:08:24,520 ‫I typed it in instead of copying it. 122 00:08:24,520 --> 00:08:29,230 ‫If I hit run now, then you see, we've got the same error again because we're trying to call to shoot. 123 00:08:29,230 --> 00:08:32,350 ‫We mistakenly overrode it incorrectly. 124 00:08:32,350 --> 00:08:33,820 ‫It's going bang, bang. 125 00:08:33,910 --> 00:08:37,840 ‫So the way I could avoid this is by saying, Well, I'm trying to override something. 126 00:08:37,840 --> 00:08:44,170 ‫So I put my override keyword in here and if I try to compile it now, you can see that it's saying shooter 127 00:08:44,170 --> 00:08:50,260 ‫is marked as override, but there is no method, no virtual method for me to override, in which case 128 00:08:50,260 --> 00:08:52,360 ‫I can go, Oh yeah, I've mistyped that. 129 00:08:52,360 --> 00:08:57,340 ‫So I'm going to go ahead and correct my code and just make it back to shoot. 130 00:08:57,340 --> 00:08:59,320 ‫And now it works as I intended. 131 00:08:59,320 --> 00:09:04,150 ‫So it's just that little bit of extra safety by putting that override there and instantly this. 132 00:09:04,240 --> 00:09:09,170 ‫It also protects us against the case where the thing that we're trying to override isn't virtual. 133 00:09:09,190 --> 00:09:14,680 ‫If I try to do that, you can see that it's saying it's marked as override, but it doesn't override 134 00:09:14,680 --> 00:09:19,410 ‫because that parent class doesn't have a virtual function for shoot. 135 00:09:19,420 --> 00:09:25,570 ‫So hopefully this has given you somewhat of an overview of how C++ uses and implements virtual functions 136 00:09:25,570 --> 00:09:28,330 ‫and why we have to be so explicit about them. 137 00:09:28,600 --> 00:09:32,890 ‫Now let's think about some more use cases for virtual functions in general. 138 00:09:32,890 --> 00:09:34,750 ‫We've already seen some from the engine. 139 00:09:34,750 --> 00:09:37,630 ‫Where have you seen virtual used before? 140 00:09:38,450 --> 00:09:40,190 ‫Where can you imagine using it? 141 00:09:40,220 --> 00:09:45,110 ‫We've just seen a use case example where you might have a gun and different overrides of that that implement 142 00:09:45,110 --> 00:09:46,880 ‫shoot in different ways. 143 00:09:47,300 --> 00:09:53,630 ‫How else could you imagine using virtual functions that override the functionality in parent classes 144 00:09:53,630 --> 00:09:55,530 ‫when you thought of an answer to those two questions? 145 00:09:55,550 --> 00:10:01,290 ‫Go and follow the link to the discussions in this lecture and post your answer there. 146 00:10:01,310 --> 00:10:06,010 ‫The use cases and the places that you've used them before and see what other people have come up with. 147 00:10:06,020 --> 00:10:11,250 ‫Maybe that will help you solidify your understanding of virtual methods. 148 00:10:11,270 --> 00:10:12,280 ‫That's it for this lecture. 149 00:10:12,290 --> 00:10:15,710 ‫I'll see you in the next one where we'll be using virtual methods in anger.