WEBVTT 00:00.120 --> 00:04.860 Hello again! In this video, we are going to look at pointers, references and inheritance. 00:06.240 --> 00:12.990 Usually when we have a pointer, it has a type, and it must be the address of some variable or object, 00:12.990 --> 00:14.010 which has the same type. 00:15.090 --> 00:20.610 However, if we have a pointer to the basic class in a hierarchy, then that can be the address of 00:20.610 --> 00:23.970 an object, which is any class in that hierarchy. 00:26.160 --> 00:33.960 So as an example, we have a Shape class, and a Circle which is derived from Shape. So we can have a pointer 00:34.020 --> 00:37.270 to Shape, and that could be the address of a Circle object. 00:38.220 --> 00:40.770 But you are not allowed to go in the opposite direction. 00:41.130 --> 00:44.790 You cannot have a pointer to Circle, which is the address of a Shape object. 00:46.500 --> 00:48.540 And the reason why this works is the way 00:48.540 --> 00:50.960 that the derived class is laid out in 00:51.370 --> 00:55.110 memory. It contains an object of the base class within it. 00:55.500 --> 00:58.770 So the first part of the object in memory is actually the base class. 00:59.280 --> 01:05.100 So it does make some sense to have a pointer to Shape, which is pointing to this Shape part of the Circle 01:05.100 --> 01:05.580 object. 01:06.630 --> 01:09.180 But having a Circle pointing to a Shape makes no sense! 01:11.880 --> 01:17.460 So let's try this out. So we have a Circle object and a pointer to Shape, which is the address of 01:17.460 --> 01:18.630 the Circle object. 01:22.070 --> 01:26.210 And that compiles. If we try to go the other way, 01:27.770 --> 01:31.040 so pointer to Circle as the address of a Shape object. 01:32.450 --> 01:34.940 "Cannot convert from Shape to Circle". 01:37.740 --> 01:42.900 And by the way, it works exactly the same for references. So we can have a reference to a Shape which 01:42.900 --> 01:50.580 is bound to a Circle object, but we cannot have a reference to a Circle which is bound to a Shape object. 01:51.050 --> 01:52.260 We get the same error again. 01:54.520 --> 02:01.180 Let's now add some member functions to our classes, so we have a draw() member function which - 02:01.300 --> 02:02.890 well, how do you draw a generic Shape? 02:04.120 --> 02:07.720 Then we have the draw member function, re-implemented in the Circle. 02:07.990 --> 02:13.960 So it does something which is more appropriate for a Circle. And we have another member function which 02:13.960 --> 02:15.460 will calculate the area. 02:16.680 --> 02:23.160 Then we have our pointer again, and when we call the draw() member function, the compiler will see that 02:23.160 --> 02:30.210 pShape is a pointer to the Shape class. So it is going to call this member function. And there we are. 02:30.240 --> 02:31.710 "Drawing a generic shape". 02:32.970 --> 02:35.310 And if we try to call the area member function. 02:38.290 --> 02:41.770 Then we get a compiler error. "'area' is not a member of 'Shape'". 02:44.990 --> 02:50.300 So even though this pShape is actually representing a Circle object in memory, the compiler still 02:50.300 --> 02:51.380 thinks it is a Shape. 02:58.800 --> 03:03.780 So this is a bit of a problem, because to get the full benefits of object oriented programming, we 03:03.780 --> 03:05.990 would like to be able to use pShape 03:05.990 --> 03:11.670 to represent a pointer to a Circle, or whatever subclass of Shape that it happens to be. 03:14.490 --> 03:20.130 So, for example, we could decide that we want to have a function that takes a pointer or reference 03:20.130 --> 03:24.360 to a Shape, and calls the draw() member function of the appropriate child class. 03:24.990 --> 03:26.730 Unfortunately, we have the same problem again. 03:27.030 --> 03:31.680 The compiler will see this as a reference to a Shape, and it will call the draw() member function for the 03:31.680 --> 03:36.270 Shape class. If we want to get the right member function calls, 03:36.480 --> 03:40.170 then we have to write a function which takes a Circle, by reference. 03:41.250 --> 03:46.890 And if we add another class, then we need to add another function, which takes that class by reference. 03:46.890 --> 03:47.460 And so on. 03:50.910 --> 03:57.360 So you can see if we pass a Circle, then this will call the version which takes a Circle and calls 03:57.360 --> 03:58.350 the Circle draw() member function. 03:58.860 --> 04:04.110 If we pass a reference to a Shape, then this will call the version, which takes the Shape. 04:08.240 --> 04:13.790 Another example, with the same classes. It could be that we want to have a container of pointers to 04:14.090 --> 04:19.220 child class objects, and then iterate through them and call their draw() member functions. 04:19.640 --> 04:25.010 For example, we could have some diagram or picture, and we have a vector of all the Shape objects 04:25.370 --> 04:28.850 that are needed to draw this picture. And then we can build this up. 04:29.300 --> 04:35.270 And when we are ready, we can iterate through the container and call the draw() member function of each element 04:35.270 --> 04:35.870 in the container. 04:36.110 --> 04:37.790 And that should draw the entire picture. 04:38.120 --> 04:39.770 But what happens if you try to do it? 04:41.410 --> 04:48.940 So it calls the generic member function version again, so we can add a pointer to a Circle class when 04:48.940 --> 04:53.380 it goes into the container, but when it comes out of the container, it is now a pointer to the base class. 04:53.950 --> 04:57.130 So really, we have lost all the type information when we add it to the container. 04:58.300 --> 05:01.990 One thing we could do is to extend the Shape class, and add some kind of tag. 05:02.530 --> 05:08.590 So we know what kind of object it really represents. But that makes things rather complicated. 05:09.640 --> 05:10.780 Or we could do a cast. 05:13.970 --> 05:19.610 So when the pointer comes out of the container, we cast it back to a pointer to Circle. 05:21.820 --> 05:24.340 And that does work, we do get the Circle version of draw(). 05:25.150 --> 05:29.590 The problem is, if we add some different child class to the container. 05:30.010 --> 05:35.680 So for example, if we have a Triangle class and we push a Triangle on the vector. So we need somehow 05:35.680 --> 05:39.280 to know that we need to cast the first element to pointer to Circle. 05:39.700 --> 05:41.830 The second one to pointer to Triangle, and so on. 05:42.400 --> 05:44.440 So that is not really the way forward. 05:45.160 --> 05:49.690 Fortunately, C++ does have a solution for this, and we are going to find out what that is in the next 05:49.930 --> 05:50.710 lecture or two. 05:51.430 --> 05:55.120 But until then, I will see you next time and keep coding!