WEBVTT 00:00.180 --> 00:04.680 Hello again! In this video, we are going to look at static type and dynamic type. 00:06.530 --> 00:12.200 In the last video, we were looking at this "pShape", which is declared as pointed to a Shape. 00:12.620 --> 00:18.950 But the actual address is the address of a Circle object and Circle is a child class of Shape. 00:19.550 --> 00:22.190 So what actually is the type of pShape? 00:22.700 --> 00:25.850 Is it a pointer to Shape, or pointer to Circle? 00:27.950 --> 00:35.510 And the correct answer is both! In C++, variables actually have two types: the static type and the dynamic 00:35.510 --> 00:35.900 type. 00:36.590 --> 00:37.970 So let's have a quick look at this. 00:40.050 --> 00:46.200 The static type is the one that is used in the declaration of the variable, so that is what the compiler 00:46.200 --> 00:46.620 sees. 00:47.310 --> 00:54.000 So we have int, pointer to int, Circle, pointer to Circle, reference to Circle and so on. 00:57.240 --> 01:01.290 In the case of our pShape, this is declared as a pointer to a Shape. 01:01.710 --> 01:09.240 So the static type is pointer to Shape. And similarly for reference, the static type is reference 01:09.240 --> 01:09.990 to Shape. 01:11.270 --> 01:16.910 And similarly, for references, the static type is reference to Shape. 01:22.460 --> 01:28.010 The dynamic type depends on the variable that is actually in memory. And usually that is going to be the 01:28.010 --> 01:29.420 same as the declaration. 01:29.780 --> 01:33.230 So if we have an int, there is going to be an int object in memory. 01:33.740 --> 01:37.940 If we have a pointer to int, that's going to be pointing to an int object, and so on. 01:40.800 --> 01:47.310 However, if we have a pointer or reference to base class, it can be different. With our pShape, 01:47.310 --> 01:49.140 the actual object is a Circle. 01:49.590 --> 01:56.970 So the dynamic type of pShape is pointer to Circle. And for a reference, that would be reference 01:56.970 --> 01:57.630 to Circle. 02:02.310 --> 02:09.540 C++ is very, very keen on static typing! The compiler uses it for type checking and deciding which 02:09.570 --> 02:10.350 function to call. 02:11.190 --> 02:13.950 So this means there is less work to be done 02:14.220 --> 02:18.810 at run time, so the program runs faster, and also it makes it easier to optimize the code. 02:20.260 --> 02:26.350 The dynamic type is only used when we have a pointer or reference to a base class. In that case, the 02:26.350 --> 02:29.020 compiler does not decide which member function is called. 02:29.770 --> 02:34.960 Instead, the program will choose the function at run time, and it will choose the correct one for the object 02:35.020 --> 02:36.010 that is in memory. 02:40.430 --> 02:44.980 To do this, we need to declare the member function as virtual in the base class. 02:47.740 --> 02:52.750 So, in the Shape class, we put "virtual", before we declare the function. 02:53.710 --> 02:59.460 And then, when we call it through the pointer to Shape, the compiler will not get to decide that this 02:59.470 --> 03:01.060 is a member function of Shape. 03:01.720 --> 03:04.960 Instead, the program will decide that this is a Circle object. 03:05.410 --> 03:07.960 So it is going to call the Circle version of draw(). 03:10.040 --> 03:12.110 So let's quickly try this out. 03:12.530 --> 03:16.090 This is almost the same program as we had before, with just one small change. 03:17.730 --> 03:21.360 So we have our vector of pointers to Shape objects. 03:22.290 --> 03:29.520 We push a pointer to Circle onto it, and then we iterate through the elements and we call the draw() member function. 03:30.870 --> 03:32.590 Before, we were using the static type. 03:32.610 --> 03:37.050 So the compiler always choose the Shape version of draw(). 03:38.070 --> 03:41.370 We have now declared the draw() member function virtual. 03:42.060 --> 03:47.220 So the program should decide that we have a Circle object, and it is going to call the Circle version of 03:47.220 --> 03:47.580 draw(). 03:49.770 --> 03:52.620 So there it is. We have actually called the Circle version. 03:53.340 --> 03:59.220 So the pointer to Shape has the static type of pointer to Shape, but the dynamic tight is pointer 03:59.220 --> 04:02.700 to a Circle. And that called the Circle member function. 04:03.930 --> 04:05.310 Okay, so that is it for this video. 04:05.670 --> 04:06.480 I will see you next time. 04:06.480 --> 04:08.480 But until then, keep coding!