WEBVTT 00:00.150 --> 00:00.630 Hello again! 00:00.900 --> 00:03.540 In this video, we are going to look at virtual inheritance. 00:06.820 --> 00:13.330 One of the problems which can arise in multiple inheritance is if the parent classes both inherit from 00:13.330 --> 00:14.560 the same base class. 00:18.120 --> 00:22.950 As an example, we could have an Employee class which has, let's say, a name, a job description 00:22.950 --> 00:23.850 and a salary. 00:25.170 --> 00:27.270 A child class, SalesEmployee. 00:27.780 --> 00:32.400 This is an Employee who also has an expense account, and gets paid a commission. 00:33.570 --> 00:35.220 We have another child class, Manager. 00:35.640 --> 00:38.400 This is an Employee, who manages other Employees. 00:39.570 --> 00:43.020 Then we have a SalesManager class which inherits from both these. 00:43.530 --> 00:50.010 So a SalesManager is an Employee, who has an expense account, and gets paid commission, and also has other 00:50.190 --> 00:51.630 Employees working for them. 00:52.620 --> 00:58.170 So you can see that the SalesManager inherits from Employee twice. once through the SalesEmployee 00:58.170 --> 01:01.140 class, and once through the Manager class. 01:04.110 --> 01:10.170 If we create a SalesManager object, this will contain objects of both the parents, so it will contain 01:10.170 --> 01:13.410 a SalesEmployee object, and a Manager object. 01:14.400 --> 01:19.380 Each of those objects will contain an object of their own parent class, so they will each contain 01:19.380 --> 01:21.030 a separate Employee object. 01:21.690 --> 01:26.790 And this means that the SalesManager object contains two Employee objects. 01:27.540 --> 01:31.860 And this is not desirable, because the SalesManager is only one Employee. 01:32.880 --> 01:38.190 If our program results in the company paying the SalesManager two salaries, then the company is not 01:38.190 --> 01:39.680 going to be very pleased with our work! 01:42.530 --> 01:43.850 So let's look at the problem. 01:44.360 --> 01:47.180 We have "skeletons" for all these classes. 01:48.230 --> 01:52.640 We have a member function which will print out the address of the Employee object. 01:53.330 --> 01:56.810 This is not virtual, so it will only be called for the Employee object. 01:57.440 --> 02:00.830 This will be inherited by all the child functions, but not overridden. 02:02.400 --> 02:08.580 We have the SalesEmployee and the Manager class, which inherits from this Employee. And then the 02:08.580 --> 02:10.860 SalesManager inherits from both those. 02:11.550 --> 02:13.980 And I am afraid I forgot this in the last video. 02:14.310 --> 02:19.740 If you want public multiple inheritance, you have to put the public keyword before both the parents. 02:20.640 --> 02:27.660 And I forgot that. But somehow my code still worked, so apologies if that caused any problems. 02:28.260 --> 02:31.080 In the main function, we have a SalesManager object. 02:31.380 --> 02:34.830 This will contain a SalesEmployee object and the Manager object. 02:36.430 --> 02:42.190 We are actually allowed to access those inner objects through pointers, so we can take the address of the 02:42.190 --> 02:49.030 SalesManager, cast it to SalesEmployee, and that will give us a pointer to the SalesEmployee part 02:49.030 --> 02:50.410 of the SalesManager. 02:51.370 --> 02:54.940 And we can also do the same thing, to get a pointer to the Manager part of the object. 02:55.690 --> 03:00.400 And then we call the address() member function. And that will display the address of the Employee part 03:00.880 --> 03:03.610 of the SalesEmployee and the Manager. 03:06.930 --> 03:10.680 So this is the address of the Employee inside the SalesEmployee part. 03:11.340 --> 03:15.510 And this is the address of the Employee object inside to the Manager part. 03:16.050 --> 03:17.190 And you can see they are different. 03:17.580 --> 03:19.560 So we do have two different objects. 03:21.600 --> 03:24.720 The way to solve this is to use what is called "virtual inheritance". 03:25.380 --> 03:33.930 So we now say that SalesEmployee uses public virtual inheritance from Employee, and Manager uses public 03:33.930 --> 03:36.120 virtual inheritance from Employee. 03:36.990 --> 03:42.150 This makes the Employee class a "virtual base" of SalesEmployee and Manager. 03:42.720 --> 03:46.170 And this means that they can both share an Employee object. 03:47.340 --> 03:50.490 And we do not need to do anything unusual for the SalesManager. 03:51.330 --> 03:57.660 So when we create a SalesManager object, this will still contain a SalesEmployee and Manager objects. 03:57.900 --> 04:01.680 But those were both the same Employee base class object. 04:03.910 --> 04:06.540 So we now have an object hierarchy that looks like this. 04:06.550 --> 04:12.580 We have our SalesManager which inherits from SalesEmployee and Manager. And contains those objects. 04:13.030 --> 04:18.160 But the SalesEmployee object and the Manager object share the same Employee. 04:19.090 --> 04:25.060 If I change this so we now use virtual inheritance for the SalesEmployee and also for the Manager. 04:25.900 --> 04:27.670 The rest of the code stays the same. 04:28.990 --> 04:35.560 So when we now make this SalesManager object, the SalesEmployee part and the Manager part will 04:35.560 --> 04:39.580 be showing the same Employee object, and we should get the same addresses. 04:41.440 --> 04:42.010 And there we are. 04:42.460 --> 04:47.470 So we now have just one Employee object, and the SalesManager only gets paid one salary! 04:48.640 --> 04:48.970 Okay. 04:48.970 --> 04:50.050 So that is it for this video. 04:50.560 --> 04:51.400 I will see you next time. 04:51.400 --> 04:53.740 But until then, keep coding!