WEBVTT 00:00.150 --> 00:04.380 Hello again! In this video, we are going to look at base and derived classes. 00:06.660 --> 00:12.930 We're going to create a simple hierarchy which has a vehicle class and an aeroplane class. 00:14.130 --> 00:17.760 The vehicle is the basic or the more generic one, 00:17.760 --> 00:22.800 so that is going to be the base of the hierarchy. And the aeroplane is more specific, 00:23.220 --> 00:25.200 so that is going to be the derived class. 00:28.320 --> 00:33.730 And we are going to give them a member. So the vehicle has a maximum speed, and the aeroplane has a maximum 00:33.730 --> 00:34.140 height. 00:36.830 --> 00:38.090 To define a derived 00:38.190 --> 00:44.180 class, we make the usual class definition. Then, after the name of the class, we put a colon. 00:44.840 --> 00:51.920 And then the keyword, which will almost always be "public", and then the name of the class that we deriving 00:51.920 --> 00:53.000 from - the base class. 00:53.570 --> 00:57.440 So this says this aeroplane is derived from vehicle. 00:59.420 --> 01:05.420 And then this aeroplane class. The member functions will be able to call any member functions in the 01:05.420 --> 01:08.330 vehicle class, provided they are not private. 01:08.930 --> 01:14.330 And it can also access any data members of the vehicle class. Again, if they are not private. 01:15.850 --> 01:22.000 It can also write its own version of the base class member functions. So instead of calling a member 01:22.000 --> 01:26.680 function from the base class, it can define a member function, which has the same name, and use that 01:26.680 --> 01:27.130 instead. 01:31.860 --> 01:38.820 An object of a derived class is laid out in memory as a base class object, followed by the derived 01:38.820 --> 01:39.480 class part. 01:39.810 --> 01:43.440 So we have some memory here, which is an aeroplane object. 01:44.160 --> 01:49.320 The first part of this is going to be the vehicle part of the object, and the rest of it will be the 01:49.320 --> 01:51.570 bits that are specific to an aeroplane. 01:52.560 --> 01:57.180 When we create this object, the aeroplane constructor is called, but this will call the vehicle constructor 01:57.180 --> 01:57.600 first. 01:57.990 --> 02:04.590 So this bit gets initialized before any of the aeroplane parts get initialized. And for destruction, 02:04.590 --> 02:05.520 it is the opposite way 02:05.520 --> 02:11.460 round. The aeroplane destructor is called and returns before the base class 02:11.670 --> 02:12.720 destructor is called. 02:16.470 --> 02:23.040 So we need to call the base class constructed, and we do that using the same syntax as if the base 02:23.040 --> 02:26.370 class was a member of the aeroplane class. 02:27.410 --> 02:32.030 So we just put a colon, the base class name, and any arguments in the brackets. 02:36.160 --> 02:42.340 (Just find myself some room!) if there are arguments to the constructors, then each class will in effect 02:42.340 --> 02:43.780 initialize its own members. 02:44.140 --> 02:50.980 So if we have the max speed and max height member variables again, the vehicle class will initialize 02:51.310 --> 02:52.900 the max_speed variable. 02:53.680 --> 02:58.600 And when the aeroplane constructor calls the vehicle constructor, it'll need to pass the max_speed to 02:58.600 --> 02:58.900 that. 02:59.710 --> 03:02.920 Then the aeroplane will initialize its own max_height variable. 03:05.260 --> 03:11.410 So this is our hierarchy, with the vehicle class and the max_speed member, and the aeroplane class, which 03:11.410 --> 03:14.950 is derived from the vehicle and has a max_height member. 03:16.310 --> 03:21.200 So the member functions in this can call any member functions from vehicle, and they can also access 03:21.200 --> 03:25.760 the max speed member of vehicle, as though it was a member of aeroplane. 03:28.140 --> 03:29.940 So I have added the constructors. 03:30.360 --> 03:33.330 I'm going to print out a statement, just so we can see what will happen. 03:34.080 --> 03:38.670 So there is the vehicle constructor, which is going to initialize the vehicles private member. 03:40.030 --> 03:46.630 The aeroplane constructor will call the vehicle constructor, and pass the arguments to the vehicle constructor. 03:47.170 --> 03:50.560 And then it will initialize the max_height member of the aeroplane. 03:52.880 --> 03:58.460 And then, in our main() routine, we just create a couple of objects, one for each class. 04:00.530 --> 04:05.240 So when we create the vehicle object, that calls the vehicle construction. So that is just a normal 04:05.240 --> 04:11.750 class. When we create an aeroplane object, that called the vehicle constructor first and then the aeroplane 04:11.810 --> 04:12.410 constructor. 04:14.430 --> 04:15.600 And finally, we can. 04:16.590 --> 04:21.960 And finally, we can carry on deriving. So we can derive another class from aeroplane. So we could 04:21.960 --> 04:28.080 have, for example, a fighter plane, which is a more specialized aeroplane. And we use the same syntax 04:28.080 --> 04:32.030 agains, so class fighter plane, colon, public aeroplane. 04:32.400 --> 04:35.040 So fighter plane is derived from aeroplane. 04:36.120 --> 04:39.840 It gets a bit confusing talking about base and derived classes in this context. 04:40.320 --> 04:42.330 So we often talk about parent and child. 04:45.020 --> 04:51.380 So vehicle is the parent of aeroplane. Aeroplane is the child of vehicle. Aeroplane is the parent 04:51.380 --> 04:55.400 of fighter plane, and fighter plane is the child of aeroplane. 04:56.670 --> 04:59.220 So this is what the entire hierarchy would look like. 04:59.550 --> 05:06.300 We have the vehicle class as the base, then we have the aeroplane, which is a child of the vehicle. And 05:06.300 --> 05:09.780 then we have the fighter plane, which is the child of the aeroplane class. 05:10.410 --> 05:16.320 So this fighter plane has the aeroplane as his parent, and the aeroplane has the vehicle as its parent, 05:16.320 --> 05:16.860 in turn. 05:17.880 --> 05:19.500 Okay, so that is it for this video. 05:20.010 --> 05:23.010 I will see you next time, but until then, keep coding!