WEBVTT 00:00.180 --> 00:03.930 Hello again! In this video, we are going to start looking at virtual functions. 00:04.710 --> 00:10.260 This video is mainly going to be a recap of the previous video, about static and dynamic types. 00:10.710 --> 00:12.330 But we are going to go a little bit further. 00:13.890 --> 00:18.720 I think it is worth spending time making sure that we all understand the concepts, because it can be a 00:18.720 --> 00:21.360 bit hard to grasp at first, if you have not seen it before. 00:22.140 --> 00:29.100 And also every - I think, every - interview that I have had on C++ involved a question on virtual functions. 00:29.460 --> 00:32.040 So it certainly is worth making sure that you understand it. 00:35.380 --> 00:40.600 Normally, when we call a member function, the compiler will decide which function gets called. 00:41.200 --> 00:48.460 So in code like this, if the draw() member function is a normal member function, then the compiler will see that 00:48.460 --> 00:54.400 "rShape" is a reference to a Shape object. And it is going to call the Shape member function, even 00:54.400 --> 00:59.380 though the actual object being referred to is a Circle. So that uses the static type. 00:59.770 --> 01:02.470 And this process is known as "static binding". 01:04.910 --> 01:07.400 There are some cases where the dynamic type is used. 01:07.790 --> 01:10.700 So in that case, the compiler will not make the decision. 01:11.000 --> 01:13.280 It just puts some extra code into the binary. 01:13.940 --> 01:20.600 And then, when the program is run, that code is used to decide which function gets called. And that 01:20.610 --> 01:25.070 will use the dynamic type of the object, and that is known as "dynamic binding". 01:27.480 --> 01:30.420 Dynamic binding can only happen if two conditions are true. 01:30.840 --> 01:37.230 We must be calling a member function through a pointer or a reference to a base class. And usually 01:37.230 --> 01:42.900 the pointer will actually be pointing to a child object, or the reference will be a reference to 01:42.900 --> 01:43.740 a child object. 01:44.460 --> 01:49.020 And also, the member function has to be declared "virtual" in the base class. 01:51.560 --> 01:56.690 To make a function virtual, we put the "virtual" key word before the declaration in the base class, like that. 01:57.800 --> 02:02.900 And then this "virtual" member function will be inherited by all the subclasses of the base. 02:03.140 --> 02:05.180 So that is the children, grandchildren and so on. 02:07.230 --> 02:13.950 If a child class re-implements this virtual function, we say that the child overrides the base 02:13.950 --> 02:14.760 class version. 02:17.210 --> 02:19.580 So we have code like this. We have a Shape 02:20.030 --> 02:23.300 as the base class, with a virtual draw() member function. 02:24.050 --> 02:28.370 We have a Circle, which is the child class, and overrides the draw() member function. 02:30.380 --> 02:36.080 And then, when we call the draw() on a reference to a Shape, which is bound to a Circle object, that 02:36.080 --> 02:40.010 will use dynamic binding, and the Circle version of draw() will be called. 02:44.670 --> 02:50.040 We were talking about having a function which can take any object from a class hierarchy. 02:50.610 --> 02:55.560 If we have to use the static type, then we need to write a separate overload for each possible class. 02:56.160 --> 03:00.390 And if we add another class to the hierarchy, then we need to write another overload. 03:01.610 --> 03:03.890 With virtual functions, this becomes much easier. 03:04.340 --> 03:11.450 We can pass a reference or a pointer to the base class. And then, in the function, if we call the virtual 03:11.450 --> 03:17.120 member function, that will use dynamic binding, and it will call the right version for the actual object 03:17.120 --> 03:17.750 that was passed. 03:18.230 --> 03:24.140 So for example, if we pass a Circle by reference - or we could also pass by address - then this draw() member 03:24.140 --> 03:28.130 function will use dynamic binding, and it will call the Circle version of draw(). 03:30.700 --> 03:32.000 So let's try this out. 03:32.080 --> 03:35.980 We have our Shape class, with the virtual draw() member function. 03:36.820 --> 03:42.790 We have the child class, which overrides the draw() member function. And then we have a function which 03:42.790 --> 03:47.170 takes the Shape object by reference, and that is going to use dynamic binding. 03:47.950 --> 03:54.130 So whenever it calls the draw() member function, which was declared virtual in the Shape class, we can 03:54.130 --> 03:59.050 give it any subclass of Shape and it will call the member function of that subclass. 04:01.060 --> 04:01.720 So there you are. 04:01.750 --> 04:08.410 We give it a Circle and it calls the Circle member function. The real advantage of this is if we add another class 04:08.770 --> 04:09.690 to the hierarchy. 04:09.700 --> 04:10.510 So let's do that. 04:12.810 --> 04:17.490 So we have the same code as before, but now we have a Triangle class, which also inherits 04:17.490 --> 04:21.300 from Shape, and overrides the draw() member function. 04:23.530 --> 04:28.450 Then we call the draw_shape() function with the Triangle being passed by reference. 04:28.840 --> 04:34.090 Again, this will use dynamic binding. And this call to the draw() member function will call the Triangle 04:34.090 --> 04:36.460 version of the virtual member function. 04:39.480 --> 04:40.230 So there you are. 04:40.470 --> 04:45.420 We can add a new class and our Shape function still works, even though we did not know about the 04:45.420 --> 04:47.520 Triangle class when we vote this draw_shape(). 04:49.500 --> 04:50.970 Okay, so that is it for this video. 04:51.390 --> 04:52.230 I will see you next time. 04:52.470 --> 04:54.330 Until then, keep coding!