WEBVTT 00:00.150 --> 00:06.630 Hello again! In this video, we are going to look at run-time type information. Run-time type information, 00:06.930 --> 00:08.400 RTTI for short. 00:09.120 --> 00:12.810 As you might guess, this gives us information about types 00:12.810 --> 00:13.650 at run-time. 00:13.890 --> 00:16.290 So that means dynamic types of objects. 00:18.090 --> 00:24.750 There are three features, if you like: the typeid, the type_info and dynamic_cast. 00:26.790 --> 00:28.080 typeid() is a function. 00:28.290 --> 00:31.590 It returns information about the dynamic type of its argument. 00:32.460 --> 00:37.080 It is defined in the typeinfo header, and it is mainly used in comparisons. 00:37.680 --> 00:44.820 So if we have a pointer to base, and we want to know if that object has the same dynamic type as a circle 00:44.820 --> 00:48.420 object, then we can use this to find out. 00:51.390 --> 00:56.370 So we have our Shape hierarchy, with the Circle and Triangle child classes. 00:57.030 --> 01:02.340 We have a Circle and Triangle object, and then we have a pointer to base which is actually pointing 01:02.340 --> 01:03.510 at a Circle object. 01:04.170 --> 01:10.410 So the static type of this is pointer to Shape, and the dynamic type is pointer to Circle. 01:11.520 --> 01:16.740 So, later on in the code, we want to know: does this base plus pointer have the same dynamic type 01:17.040 --> 01:22.080 as a Circle object? So we can call typeid and compare the results to find out. 01:25.030 --> 01:28.030 And yes, pShape does point to a Circle object. 01:30.540 --> 01:31.680 If we change the code, 01:31.770 --> 01:36.630 so this p shape points to a Triangle object, then what do you think will happen? 01:38.980 --> 01:43.060 So we go into the else branch. pShape does not point to a Circle object. 01:43.660 --> 01:45.330 So this comparison returns false, 01:45.340 --> 01:53.260 if pShape is pointing to a Triangle. type_info is a class defined in the header. Note that 01:53.260 --> 01:57.730 the type_info class has an underscore and the header does not. 01:58.960 --> 02:01.990 I am not sure why anyone thought that would be a good idea, but there we are. 02:03.130 --> 02:06.970 When we call typeid, this returns a type_info object. 02:07.480 --> 02:12.070 So type_info contains information about the dynamic type of an object. 02:14.020 --> 02:17.980 It has a member function called name(), which returns a C string. 02:18.880 --> 02:20.950 So you might think that is the name of the class. 02:21.220 --> 02:24.430 In fact, it is the name of the class as used by the compiler. 02:24.790 --> 02:31.270 So it may not necessarily be useful to programmers or end users, but it is unique for each type. 02:34.550 --> 02:41.480 So with the same classes again. This time we keep the returned information from calling typeid(). 02:45.690 --> 02:46.350 And. 02:50.620 --> 02:53.800 With Visual C++, we just get the name of the class. 02:55.510 --> 02:57.550 If I compile it with gcc. 03:00.580 --> 03:02.230 Then the result is different. 03:02.650 --> 03:04.510 So we get these strange looking things. 03:05.170 --> 03:09.420 And in fact, that is more typical of the information that you get from calling typeid(). 03:12.800 --> 03:15.500 typeinfo also has a hash_code() function. 03:16.100 --> 03:19.970 This will return a number which depends on the dynamic type of its argument. 03:20.810 --> 03:25.040 So this could be used for another way of checking if two objects have the same dynamic type. 03:26.120 --> 03:30.920 And maybe you could use it for doing something clever, like writing your own hash tables, or something 03:30.920 --> 03:31.280 like that. 03:34.490 --> 03:43.130 So again, we have the same classes. We call the typeid(), we get the type_info, and then we 03:43.130 --> 03:45.950 get the hash_code()s for the two objects. 03:48.520 --> 03:50.860 And those are the results with Visual Studio. 03:53.590 --> 03:56.920 Again with GNU C++, we get different results. 03:59.660 --> 04:01.460 And finally, dynamic_cast. 04:01.850 --> 04:07.550 So we can use this to convert a pointer to a base, into a pointer to a derived class object. 04:08.270 --> 04:10.100 And it also applies to references. 04:10.460 --> 04:14.300 So we can convert a reference to base into a reference to derived. 04:15.350 --> 04:21.860 The conversion will only work if the base class is actually pointing, or referring to, a derived class object. 04:22.880 --> 04:28.340 If the conversion fails, the returned point is null, and for references, it throws an exception. 04:28.670 --> 04:31.700 The bad_cast exception from the standard hierarchy. 04:34.930 --> 04:37.360 So we have our pointer to a base class object. 04:37.900 --> 04:40.810 We are going to cast this to a pointer to Circle. 04:42.310 --> 04:44.640 If this fails, the return value will be null. 04:44.710 --> 04:47.530 So we need to check this before we dereference it. 04:50.550 --> 04:53.490 So, in this case, the pointer was pointing to a Circle object. 04:53.820 --> 04:54.730 So that all worked. 04:55.410 --> 04:57.630 If we make it point to a Triangle object... 04:59.910 --> 05:01.920 Then you can probably guess what happens. 05:05.400 --> 05:09.330 The conversion fails, and we go into the else branch, because we got a null pointer. 05:09.750 --> 05:11.370 So it could not cast the Shape. 05:13.920 --> 05:17.400 dynamic_cast is something that you should only use very rarely. 05:18.300 --> 05:22.920 Quite often when people do use it, it is because they do not really understand how inheritance works 05:22.920 --> 05:23.760 in C++. 05:24.480 --> 05:30.750 They think that you have to do dynamic_cast before you can call a member function on the child class, and 05:30.750 --> 05:31.470 that is not correct. 05:31.890 --> 05:37.080 What you should do is write virtual functions in the base class, then override them in the child class, 05:37.590 --> 05:39.600 and then the compiler will do all the work for you. 05:41.220 --> 05:46.650 One situation where it is useful to call dynamic_cast is if you want to call a member function 05:46.920 --> 05:49.740 that is only defined in a derived class. 05:51.120 --> 05:55.680 If you try to call it through pointer to base, or reference to base, the compiler will use the static 05:55.680 --> 05:59.340 type. And it will say that that function is not defined for the base class. 05:59.850 --> 06:01.140 So this is the only way to do it. 06:03.060 --> 06:06.990 So here we have a function which is only defined for the Circle class. 06:07.410 --> 06:09.150 It is not in Shape or Triangle. 06:10.530 --> 06:16.620 This time, we are going to use a reference to the base class. When we call the dynamic_cast, 06:16.650 --> 06:18.870 this will throw an exception if the conversion fails. 06:19.230 --> 06:22.860 So we need to have a try-catch block to handle the exception. 06:28.310 --> 06:29.660 So that succeeds. 06:30.980 --> 06:33.980 pShape is a reference to a Circle object. 06:34.250 --> 06:36.260 So the dynamic_cast has succeeded. 06:36.740 --> 06:40.610 And we could call the member function on the circle object. 06:43.660 --> 06:48.460 If we change this so we now have a Triangle, then we get the exception thrown. 06:49.420 --> 06:51.940 So the dynamic_cast failed and threw an exception. 06:58.490 --> 07:04.160 If you try to call the function directly on this pointer to Circle... (Change that back) 07:07.520 --> 07:11.060 Then that does not compile. "func" is not a member of Shape. 07:11.300 --> 07:17.120 So the compiler will interpret pShape as a reference to the Shape class. And that does not define 07:17.120 --> 07:17.690 this function. 07:18.500 --> 07:19.880 Okay, so that is it for this video. 07:20.300 --> 07:21.110 I will see you next time. 07:21.350 --> 07:23.330 Until then, keep coding!