WEBVTT 00:00.210 --> 00:04.080 Hello again! In this video, we are going to look at member functions and inheritance. 00:05.520 --> 00:12.180 When we inherit from a parent class which has member functions, the child will, so-called, "inherit" 00:12.180 --> 00:15.300 all these member functions, provided they are not private. 00:15.810 --> 00:17.550 So this means that it will appear 00:17.550 --> 00:21.450 that the child class defines the same functions as the parent does. 00:22.290 --> 00:26.940 For example, if we have our Vehicle class and we give it start() member function which is public, 00:27.660 --> 00:33.900 then our derived Aeroplane class will also appear to have the same start() public member function. 00:37.120 --> 00:42.910 So in this code, if we create an Aeroplane object and then call the start() member function, this 00:42.910 --> 00:47.320 is going to call the inherited member function. And there we go. 00:51.160 --> 00:55.810 What happens if the child wants the member function to do something different? For example, with 00:55.810 --> 01:01.570 an aeroplane, you cannot just jump in and turn the key, you have to do a lot more checks and so on. 01:02.260 --> 01:07.450 In that case, the child class can implement the member function itself, so it can have its own definition, 01:07.450 --> 01:09.430 which does something which is more appropriate. 01:11.240 --> 01:16.670 So with the same code again, but with the child class defining its own member function which re-implements 01:16.670 --> 01:17.150 the parent's. 01:17.630 --> 01:22.140 So you can see, this just holds the child's member function and not the parent's one. 01:23.890 --> 01:29.020 Sometimes the parent member function does something which is useful to the child, but the child 01:29.020 --> 01:30.490 needs to do some more work. 01:31.240 --> 01:36.790 So in that case, you could have re-implement the function and call the parent's member function at the 01:37.150 --> 01:38.140 strategic moment. 01:38.650 --> 01:41.440 So for example, with our aeroplane, we could carry out our checks. 01:41.830 --> 01:46.510 Then we start the engine and then we tell the air traffic control that we are ready to take off. 01:49.370 --> 01:51.170 So it will look like this. 01:53.210 --> 01:58.760 So we do our thing, then we call the parent's member function, than we do some more work. 02:00.410 --> 02:03.440 You might notice that I have been very careful with my wording. 02:03.770 --> 02:07.940 I did not say that the child inherits public member functions. 02:08.300 --> 02:12.450 I said that the child inherits member functions which are not private. 02:12.980 --> 02:17.870 And there is a reason for that, which is that there are actually three access specifiers in C++. 02:18.530 --> 02:19.820 So let's quickly recap. 02:20.390 --> 02:25.520 So when we have a class with a public member, that means that member can be accessed from anywhere 02:25.520 --> 02:26.060 in the code. 02:26.630 --> 02:29.900 That is part of the interface that the class has to the rest of the code. 02:31.510 --> 02:35.860 With a child class, the child will have access to all the parent's public members. 02:36.490 --> 02:38.920 It does not have access to any of the private members. 02:39.640 --> 02:43.060 So in that respect, the child class is no different from any other code. 02:44.490 --> 02:50.670 Sometimes it's useful to have member functions in the parent, which can be used by child classes, 02:50.670 --> 02:52.410 but not by any other code. 02:52.920 --> 02:55.200 So that would be part of the class's implementation. 02:56.100 --> 02:59.460 And to do that, we use the protected access specifier. 02:59.790 --> 03:01.860 So we put "protected", followed by a colon. 03:02.460 --> 03:08.220 And then everything which comes after that will be protected until the next access specifier, or at the 03:08.220 --> 03:09.420 end of the class definition. 03:09.930 --> 03:13.440 So this could be, for example, inside the Vehicle class definition. 03:15.440 --> 03:20.240 And anything that goes in here will be accessible to classes which are derived from Vehicle, but 03:20.570 --> 03:21.650 not to anything else. 03:22.850 --> 03:26.390 You can make data members protected, but that is not very useful. 03:26.900 --> 03:29.270 Normally, you would only make member functions protected. 03:29.870 --> 03:35.030 So this acts as a kind of second interface, which is available within the Vehicle hierarchy, but not 03:35.360 --> 03:36.440 anywhere outside it. 03:37.600 --> 03:43.470 So this will maintain encapsulation. These implementation details will be hidden from the outside world, 03:43.780 --> 03:45.910 but they will be available inside the hierarchy. 03:48.950 --> 03:53.210 So let's try that out. So we have our vehicle class with a protected member function. 03:54.240 --> 04:00.510 We have our Aeroplane class, which has a member function which calls the protected member functions, and 04:00.510 --> 04:02.010 we create a Vehicle object. 04:04.780 --> 04:09.910 So that does actually compile. We get a warning about not using this variable, but that does not matter 04:09.910 --> 04:10.210 here. 04:13.030 --> 04:19.840 So if we uncomment this, so we are going to call this protected member function in the main() function. 04:20.110 --> 04:23.800 This is not code which is part of a member function in a derived class. 04:24.310 --> 04:25.720 So this should fail. 04:30.230 --> 04:34.610 And there we are. "Cannot access protected member declared in class Vehicle". 04:36.470 --> 04:38.090 OK, so that is it for this video. 04:38.480 --> 04:41.450 I will see you next time, but until then, keep coding!