WEBVTT 00:00.120 --> 00:04.620 Hello again! In this video, we are going to look at how virtual member functions are implemented. 00:05.160 --> 00:08.580 You do not actually need to understand this to be a good C++ programmer. 00:08.970 --> 00:13.650 And you can probably get by without even knowing about it, but it is always useful to have some idea 00:13.650 --> 00:14.400 of what is going on. 00:16.320 --> 00:22.590 So first, let's remind ourselves how member functions are implemented. Member functions are not stored 00:22.590 --> 00:25.980 in the object, they are implemented as global functions. 00:26.610 --> 00:31.650 The only difference is that, when we call a member function, the compiler will add an extra argument 00:31.650 --> 00:36.750 with the "this" pointer, which is a pointer to the object that the member function was called on. 00:38.780 --> 00:45.260 So if we have a class which has some non-virtual member functions, it's really not that different from 00:45.260 --> 00:46.730 having a global function. 00:49.110 --> 00:52.090 There is quite a big difference, though, for virtual member functions. 00:52.920 --> 00:58.590 First of all, when the compiler sees that a class has a virtual member function, it will create and 00:58.590 --> 01:04.680 populate a data structure for the class, which is the virtual member function table, or vtable for 01:04.680 --> 01:05.050 short. 01:05.640 --> 01:11.160 And this has information about the class's virtual member functions. When the compiler sees that we are 01:11.160 --> 01:13.140 calling a virtual member function. 01:13.560 --> 01:17.520 It will generate some extra code, which is executed at run-time. 01:18.240 --> 01:24.900 This will check the dynamic type of the object, and then it will use the vtable for that dynamic type, 01:25.230 --> 01:27.420 to call the correct version of the function. 01:29.290 --> 01:35.020 So if we have a class which has virtual member functions, then the calls will go through this intermediate 01:35.350 --> 01:36.040 vtable. 01:38.580 --> 01:45.240 This vtable has the address of all the virtual member functions of the class. The vtable is actually 01:45.240 --> 01:51.180 an array of pointers to the class's virtual member functions, and each virtual member function of the 01:51.180 --> 01:55.200 class is identified by its index into this array. 01:58.140 --> 02:03.720 When a virtual member function is called, the compiler will replace the name of the function. Not 02:03.720 --> 02:06.600 with a pointer like it would for a normal member function, 02:06.840 --> 02:11.310 but with the index into the vtable. And then the run-time code, 02:11.310 --> 02:15.780 once it has found out which vtable to use, it will look up the element in the vtable, 02:15.780 --> 02:20.550 which has that index, and then it will call the member function through that function pointer. 02:24.600 --> 02:30.350 So this is all very clever, but there is a price to be paid. When we call a virtual member function, 02:30.360 --> 02:33.000 we have to go through this vtable, which adds indirection. 02:33.630 --> 02:40.290 So, typically, calling it takes about 25 or 50 percent longer than calling a non-virtual member function. 02:41.380 --> 02:48.040 There is also some memory usage, each class that has a virtual member function will need a vtable, so 02:48.040 --> 02:54.640 that is an array in memory at run-time, and each virtual member function in the class will require a 02:54.640 --> 02:55.930 pointer in this array. 02:56.680 --> 03:02.380 So if you have a very large hierarchy, with lots of classes with virtual member functions, or if you 03:02.380 --> 03:07.480 call virtual functions in a very tight loop, you may find that it affects the performance of your 03:07.480 --> 03:07.870 program. 03:08.560 --> 03:14.050 So the advice is, only use virtual member functions when you really need the extra flexibility. 03:15.130 --> 03:16.600 Okay, so that is it for this video. 03:17.110 --> 03:20.350 I will see you next time, but until then, keep coding!