1 00:00:04,470 --> 00:00:08,280 ‫So we know that we need some components on our base -- class. 2 00:00:08,280 --> 00:00:12,570 ‫We know that we need a base mesh for the bottom part of the tank and the turret. 3 00:00:12,600 --> 00:00:15,270 ‫We also need a turret mesh for the top part. 4 00:00:15,300 --> 00:00:19,500 ‫We need a capsule to handle collision and we need a projectile spawn point. 5 00:00:19,500 --> 00:00:21,270 ‫That's going to be a scene component. 6 00:00:21,300 --> 00:00:26,460 ‫Now, once we've added these things to the base pod class, then anything that we create based on this 7 00:00:26,460 --> 00:00:28,320 ‫class will inherit those things. 8 00:00:28,320 --> 00:00:34,920 ‫So we could create a tank based on base -- and then we could go and create a tower also based on base 9 00:00:34,920 --> 00:00:35,550 ‫--. 10 00:00:35,700 --> 00:00:39,620 ‫And because they're based on base pod, they inherit these components. 11 00:00:39,630 --> 00:00:46,380 ‫The tank can have its own base mesh, its own turret mesh, its own capsule and its own projectile spawn 12 00:00:46,380 --> 00:00:46,800 ‫point. 13 00:00:46,800 --> 00:00:52,500 ‫And likewise, the tower will get its own base mesh turret mesh capsule component and projectile spawn 14 00:00:52,500 --> 00:00:53,370 ‫point as well. 15 00:00:53,370 --> 00:00:58,560 ‫And we'll see very soon how we can assign different meshes to each of these so they look different. 16 00:00:58,560 --> 00:01:00,750 ‫So we know that we need some components. 17 00:01:00,750 --> 00:01:05,490 ‫Now it's time to actually add some components to our base -- class. 18 00:01:05,910 --> 00:01:11,400 ‫Now, by default, we already have a public section, a protected section, and the second public section 19 00:01:11,400 --> 00:01:12,240 ‫down here. 20 00:01:12,570 --> 00:01:17,520 ‫There are various reasons to have multiple public sections and we'll talk about those later. 21 00:01:17,520 --> 00:01:20,880 ‫But for our components, we're going to create a private section. 22 00:01:22,070 --> 00:01:28,400 ‫As you've learned previously, it's good to have member variables be private, so that way they're only 23 00:01:28,400 --> 00:01:31,790 ‫accessible from within the class that owns them. 24 00:01:31,790 --> 00:01:33,410 ‫So we need some components. 25 00:01:33,680 --> 00:01:36,190 ‫The first one I'm going to add is the capsule. 26 00:01:36,200 --> 00:01:40,760 ‫So let's create a variable of type you capsule component. 27 00:01:42,160 --> 00:01:44,030 ‫Now this is going to be a pointer. 28 00:01:44,070 --> 00:01:48,880 ‫We're going to use pointers for the majority of our variables as long as they're not base types like 29 00:01:48,880 --> 00:01:50,340 ‫integer or float. 30 00:01:50,350 --> 00:01:55,600 ‫And as you know, that's because pointers are lightweight, they're just addresses and they allow us 31 00:01:55,600 --> 00:01:56,860 ‫to be more efficient. 32 00:01:56,950 --> 00:02:03,400 ‫So we have this U capsule component pointer and we're going to call this capsule comp short for capsule 33 00:02:03,400 --> 00:02:04,030 ‫component. 34 00:02:04,030 --> 00:02:11,140 ‫Now just above capsule component, we're going to add an unreal engine specific macro and it's you property 35 00:02:11,470 --> 00:02:14,020 ‫in all caps with its own parentheses. 36 00:02:14,020 --> 00:02:20,590 ‫You property is a way to make unreal engine aware of this variable so that it can participate in unreal 37 00:02:20,590 --> 00:02:20,980 ‫engines. 38 00:02:20,980 --> 00:02:22,240 ‫Reflection System. 39 00:02:22,690 --> 00:02:28,750 ‫The reflection system allows us to do things like expose this variable to blueprints and participate 40 00:02:28,750 --> 00:02:31,060 ‫in garbage collection and things like that. 41 00:02:31,060 --> 00:02:36,400 ‫So adding the you property is going to be something that will add to our components as we create them. 42 00:02:36,400 --> 00:02:41,980 ‫So we have this capsule component and because we're using the YOU capsule component type, we need to 43 00:02:41,980 --> 00:02:44,860 ‫let the compiler know about this type. 44 00:02:44,860 --> 00:02:48,250 ‫It doesn't know by default what a capsule component is. 45 00:02:48,250 --> 00:02:54,640 ‫Now normally to tell the compiler what a capsule component is, we would include a header file up here 46 00:02:54,640 --> 00:02:59,680 ‫at the top with our include directives if we go to the unreal engine documentation and search for you 47 00:02:59,710 --> 00:03:05,200 ‫capsule component, we can see right here where it says include the header file that we would need to 48 00:03:05,200 --> 00:03:08,950 ‫include to tell the compiler about the capsule component. 49 00:03:08,950 --> 00:03:13,000 ‫Now we could include this right here at the top and then our U. 50 00:03:13,000 --> 00:03:18,160 ‫Capsule component type would be recognized by the compiler and everything would be hunky dory. 51 00:03:18,220 --> 00:03:24,070 ‫But general, good practice when it comes to coding these things is to try to keep this header file 52 00:03:24,070 --> 00:03:25,720 ‫as small as possible. 53 00:03:25,720 --> 00:03:32,380 ‫Whenever we include a header file in this file, a program called the preprocessor takes all of the 54 00:03:32,380 --> 00:03:37,720 ‫contents of that header file, strips out some things like comments, and then literally pastes all 55 00:03:37,720 --> 00:03:40,780 ‫of the contents of that header file into this file. 56 00:03:41,050 --> 00:03:46,810 ‫The result is that this file gets bigger because it has the contents of that included file paste it 57 00:03:46,810 --> 00:03:47,530 ‫in here. 58 00:03:47,530 --> 00:03:50,950 ‫We only want to include things when we use them. 59 00:03:50,950 --> 00:03:56,740 ‫So how do we get away with not including the header file here in the base -- header? 60 00:03:56,740 --> 00:04:02,380 ‫Well, we can use something in C++ known as forward declaration. 61 00:04:02,380 --> 00:04:07,990 ‫We forward declare this u capsule component type using the class keyword like this. 62 00:04:08,470 --> 00:04:15,310 ‫Notice the compiler now shows this type as green whenever we forward declare a variable were then able 63 00:04:15,310 --> 00:04:18,010 ‫to get away with not including its header file. 64 00:04:18,010 --> 00:04:23,830 ‫It's telling the compiler that this is a type so it'll no longer give us IntelliSense errors, saying 65 00:04:23,830 --> 00:04:26,170 ‫that it doesn't recognize this as a type. 66 00:04:26,170 --> 00:04:28,150 ‫Now this is an incomplete type. 67 00:04:28,150 --> 00:04:34,150 ‫If we actually try to use this type either by accessing members like functions and variables or trying 68 00:04:34,150 --> 00:04:38,560 ‫to construct an object of this type, we would get an error because it's an incomplete type. 69 00:04:38,560 --> 00:04:44,950 ‫We would need the header file included wherever we use this type to do those things so we could forward 70 00:04:44,950 --> 00:04:48,400 ‫declare a pointer of a type to avoid needing the header file. 71 00:04:48,400 --> 00:04:53,230 ‫But it's important to note that we do need header files to inherit from particular classes. 72 00:04:53,230 --> 00:05:00,130 ‫Notice that this class inherits from a pond and the pond header file is included here at the top. 73 00:05:00,130 --> 00:05:05,470 ‫So inheritance does require the header file for the type we're inheriting from. 74 00:05:05,470 --> 00:05:09,100 ‫So let's discuss forward declaration and why we want to do it. 75 00:05:09,130 --> 00:05:16,120 ‫Let's say we have a class called Base Pon which needs to have a pointer variable of type you capsule 76 00:05:16,120 --> 00:05:17,020 ‫component. 77 00:05:17,050 --> 00:05:22,060 ‫Now you capsule component is in the capsule component header file. 78 00:05:22,060 --> 00:05:28,470 ‫Now we're actually going to use this type in based upon when we construct an object of the U. 79 00:05:28,480 --> 00:05:30,010 ‫Capsule component type. 80 00:05:30,010 --> 00:05:36,820 ‫So in, based upon all we really need is to have a pointer of type U capsule component. 81 00:05:37,000 --> 00:05:44,440 ‫Now the truth is based upon H doesn't actually need the implementation details of the capsule component 82 00:05:44,440 --> 00:05:45,160 ‫class. 83 00:05:45,160 --> 00:05:50,680 ‫In other words, it doesn't need to know how the functions are defined or even how big the capsule component 84 00:05:50,680 --> 00:05:51,820 ‫class is. 85 00:05:51,850 --> 00:05:55,750 ‫A capsule component pointer is the same size as any other pointer. 86 00:05:55,750 --> 00:05:57,250 ‫It's just an address. 87 00:05:57,250 --> 00:05:58,810 ‫Now in the base plan class. 88 00:05:58,810 --> 00:06:01,420 ‫We want to construct the capsule component. 89 00:06:01,510 --> 00:06:08,260 ‫For this reason, we need to know the size of a capsule component in order to lay out the memory necessary 90 00:06:08,260 --> 00:06:09,790 ‫for creating this object. 91 00:06:09,790 --> 00:06:16,060 ‫So while the capsule component header is not strictly needed in base Ponca It's absolutely needed in 92 00:06:16,060 --> 00:06:16,330 ‫base. 93 00:06:16,350 --> 00:06:24,970 ‫Porkpie So good programming practice is to only include headers where you actually need them so we can 94 00:06:24,970 --> 00:06:32,410 ‫get away with declaring a pointer of type U capsule component pointer in base product H and then include 95 00:06:32,410 --> 00:06:39,310 ‫capsule components where we need the implementation details because we're constructing a capsule component 96 00:06:39,310 --> 00:06:41,020 ‫and we need to know the size of a. 97 00:06:41,310 --> 00:06:43,230 ‫Capsule component in memory. 98 00:06:43,350 --> 00:06:49,920 ‫Now let's discuss the consequences of including the header file in base pond FX instead. 99 00:06:50,010 --> 00:06:55,410 ‫As we've seen, we can absolutely do this and you may be thinking, well, we're including it in base 100 00:06:55,410 --> 00:07:01,230 ‫pont CP Anyway, so what difference does it make if we include it in one file versus another? 101 00:07:01,260 --> 00:07:03,180 ‫Now here's where the problem comes in. 102 00:07:03,210 --> 00:07:07,390 ‫Header files are designed to be included by other files. 103 00:07:07,410 --> 00:07:12,810 ‫So let's say that we have Base Pond H and we've included capsule components in it. 104 00:07:12,810 --> 00:07:18,600 ‫And then further down the road we have some other class and we're going to have a base pond pointer 105 00:07:18,600 --> 00:07:23,370 ‫in some other class, just like we had a capsule component pointer in base pod. 106 00:07:23,400 --> 00:07:28,230 ‫Now, rather than going through the trouble of forward declaring the base pond pointer and some other 107 00:07:28,230 --> 00:07:33,810 ‫class, we're just going to declare it as normal and include base pawn in some other class. 108 00:07:34,680 --> 00:07:41,520 ‫So now not only does Base Pond H include the capsule component header, which results in more code being 109 00:07:41,520 --> 00:07:43,190 ‫copied into that header file. 110 00:07:43,200 --> 00:07:47,820 ‫Now some other class has base upon, which includes capsule component. 111 00:07:47,820 --> 00:07:55,470 ‫So all of the content and capsule components is also inside of some other class H and it's not even 112 00:07:55,470 --> 00:07:56,400 ‫needed there. 113 00:07:56,400 --> 00:08:01,910 ‫Some other class that H only needs to know about base upon it doesn't care about capsule component. 114 00:08:01,920 --> 00:08:07,920 ‫Now say we continue this pattern and create some other class called yet another class and yet another 115 00:08:07,920 --> 00:08:13,680 ‫class may need to have a pointer of type some other class now rather than forward declaring. 116 00:08:13,680 --> 00:08:21,180 ‫We just include some other class H in yet another class H and now inside of yet another class H. 117 00:08:21,210 --> 00:08:27,840 ‫Even though we don't even need the header for base pond or capsule component H, we got it in yet another 118 00:08:27,840 --> 00:08:32,730 ‫class because some other class includes those header files inside of it. 119 00:08:32,730 --> 00:08:40,140 ‫So we have copies of all of the contents of capsule components and based upon H just stuck here in yet 120 00:08:40,140 --> 00:08:46,620 ‫another class, bloating up that file and increasing compile times all because we didn't forward declare. 121 00:08:46,620 --> 00:08:52,440 ‫So instead the better thing to do is include header files where they're actually needed. 122 00:08:52,440 --> 00:08:59,610 ‫So if we take capsule components and only include it in based upon CPP, then later down the road when 123 00:08:59,610 --> 00:09:05,340 ‫we create some other class and actually need to use the base pod, we can include the header there and 124 00:09:05,340 --> 00:09:12,840 ‫base upon does not include capsule components, so all that extra code is not included in some other 125 00:09:12,840 --> 00:09:13,440 ‫class. 126 00:09:13,440 --> 00:09:18,810 ‫And then when we go and create yet another class which needs some other class, we can include some 127 00:09:18,810 --> 00:09:24,990 ‫other class that h in yet another class that CPP where it's actually needed and then yet another class 128 00:09:24,990 --> 00:09:30,270 ‫that H does not get filled up with the contents of other header files that it doesn't need. 129 00:09:30,270 --> 00:09:36,990 ‫So when classes have dependencies like this based upon depends on capsule component, it's best to keep 130 00:09:36,990 --> 00:09:38,670 ‫those dependencies to a minimum. 131 00:09:38,880 --> 00:09:42,570 ‫So I'm going to challenge you to create a compilation error. 132 00:09:42,570 --> 00:09:44,970 ‫We shouldn't be afraid of compilation errors. 133 00:09:44,970 --> 00:09:47,550 ‫They're just a normal part of the development process. 134 00:09:47,550 --> 00:09:54,210 ‫So we're going to generate one intentionally go back and remove the class keyword and compile without 135 00:09:54,210 --> 00:09:56,160 ‫forward declaring capsule comp. 136 00:09:56,160 --> 00:10:04,080 ‫Now when I say compile, I mean you can compile from Visual Studio code, just a regular C++ compile. 137 00:10:04,110 --> 00:10:10,500 ‫You've learned how to do that, or you can simply save your file and Visual Studio code and initiate 138 00:10:10,500 --> 00:10:11,670 ‫live coding. 139 00:10:11,670 --> 00:10:16,800 ‫Either one of those is fine and once we go through this together, I'll show you what happens with both 140 00:10:16,800 --> 00:10:17,280 ‫methods. 141 00:10:17,280 --> 00:10:22,440 ‫If you use live coding, of course you're going to be looking at the generated output in the live coding 142 00:10:22,440 --> 00:10:27,270 ‫window as opposed to the output that you would see in Visual Studio code. 143 00:10:27,270 --> 00:10:33,060 ‫Now spoiler, you're going to get a compilation error, look at the compilation error message, see 144 00:10:33,060 --> 00:10:34,260 ‫what it's telling you. 145 00:10:34,470 --> 00:10:38,070 ‫This is just a familiarize ourselves with the errors that we generate. 146 00:10:38,070 --> 00:10:43,500 ‫Then after this, go back and forward, declare capsule comp using the class keyword. 147 00:10:43,620 --> 00:10:48,450 ‫Once you've done that, compile again, you should get a successful compilation this time. 148 00:10:48,780 --> 00:10:51,060 ‫Pause the video and give this a try. 149 00:10:53,520 --> 00:10:56,350 ‫All right, so let's wreak some havoc on our code here. 150 00:10:56,370 --> 00:10:58,320 ‫I'm going to remove the class keyword. 151 00:10:58,700 --> 00:11:03,540 ‫Okay, so we've attempted to compile, and here is the output in the terminal. 152 00:11:03,570 --> 00:11:09,300 ‫The first thing we see is error and it gives us an error code and it says capsule comp is not a member 153 00:11:09,300 --> 00:11:10,830 ‫of a base pawn. 154 00:11:10,860 --> 00:11:17,040 ‫We have a note that says C declaration of a base pawn and then we see that we get other errors as a 155 00:11:17,040 --> 00:11:17,940 ‫result of this. 156 00:11:17,940 --> 00:11:23,390 ‫So oftentimes students will get confused when they attempt to compile and seem multiple errors. 157 00:11:23,400 --> 00:11:28,560 ‫So this goes to show that a single mistake can propagate a series of errors. 158 00:11:28,560 --> 00:11:31,670 ‫And that doesn't mean you have to go and fix a whole bunch of files. 159 00:11:31,680 --> 00:11:34,410 ‫The error could have a single origin here. 160 00:11:34,410 --> 00:11:36,420 ‫We know what the origin of the error is. 161 00:11:36,420 --> 00:11:37,820 ‫We need to forward declare. 162 00:11:37,830 --> 00:11:45,150 ‫Now let's see what happens if we use live coding instead of regular compilation and Visual Studio code. 163 00:11:45,150 --> 00:11:51,450 ‫So I'm going to go ahead and click the button to initiate live coding and right away in red I see build 164 00:11:51,450 --> 00:11:54,270 ‫failed and we see the same thing here. 165 00:11:54,270 --> 00:11:55,050 ‫Right here. 166 00:11:55,050 --> 00:11:59,310 ‫Capsule comp is not a member of a base pawn note. 167 00:11:59,340 --> 00:12:01,860 ‫See the declaration of a base pawn. 168 00:12:01,860 --> 00:12:03,750 ‫So we see the same thing here. 169 00:12:03,750 --> 00:12:08,490 ‫So either case we're going to see an error like this when we forget to forward declare. 170 00:12:08,490 --> 00:12:16,170 ‫So now we know what it looks like and now we're more informed and less afraid of these mysterious compilation 171 00:12:16,170 --> 00:12:17,340 ‫error messages. 172 00:12:17,460 --> 00:12:23,280 ‫So what we're going to do is fix this error by forward declaring you capsule component, and then we 173 00:12:23,280 --> 00:12:24,990 ‫can attempt to compile again. 174 00:12:24,990 --> 00:12:27,780 ‫And now we've compiled without any errors. 175 00:12:27,870 --> 00:12:32,070 ‫So now we see what it looks like when we forgot to forward declare a variable. 176 00:12:32,100 --> 00:12:35,250 ‫Keep that in mind when you get compiler errors in the future. 177 00:12:35,760 --> 00:12:41,490 ‫So in summary, we discussed forward declaration when creating a new pointer and we learned that you 178 00:12:41,490 --> 00:12:48,370 ‫should only include what you use in the CP file and we should include as little as possible in the H 179 00:12:48,450 --> 00:12:49,020 ‫file. 180 00:12:49,050 --> 00:12:55,020 ‫We know that we don't need the header file to declare a new pointer as long as we forward declare and 181 00:12:55,020 --> 00:13:00,090 ‫that we do need the header file if we're going to be constructing an object of a particular type. 182 00:13:00,090 --> 00:13:05,850 ‫And we do need the header file to access members like functions and variables and we saw that we actually 183 00:13:05,850 --> 00:13:09,780 ‫do need the header file for inheritance for inheriting from a class. 184 00:13:09,780 --> 00:13:11,460 ‫We need that header file there. 185 00:13:11,580 --> 00:13:17,160 ‫So now that we know how to forward declare a type and we have our pointer capsule comp, we're ready 186 00:13:17,160 --> 00:13:21,840 ‫to actually construct a capsule component object and we'll do that in the next video. 187 00:13:22,200 --> 00:13:23,040 ‫I'll see you soon.