WEBVTT 00:00.120 --> 00:04.290 Hello again! In this video, we are going to start looking at exceptions in C++. 00:04.860 --> 00:09.960 We are going to start off by looking at an example of an exception which is thrown by the standard library. 00:11.610 --> 00:14.490 This does not actually happen as often as you might expect. 00:14.910 --> 00:19.950 The reason is that the standard library has to work with every environment, including ones which do 00:19.950 --> 00:21.330 not support exceptions. 00:21.960 --> 00:25.500 So when it does throws exceptions, it is usually in something where you can avoid it. 00:27.570 --> 00:31.740 As an example, if we have a vector and we use subscript access. 00:32.900 --> 00:35.510 This will not perform any range checking. 00:35.990 --> 00:40.980 The idea is that you should know if the elements is in the vector or not. 00:41.270 --> 00:45.830 So there is no point wasting time checking. If you do access an element which is out of bounds, 00:46.220 --> 00:48.950 Then there is no warning, no error condition. 00:49.310 --> 00:52.460 The program will carry on, and probably crash or behave strangely. 00:53.060 --> 00:59.240 I'm actually going to use GNU C++ for my examples in this video, because the output is more useful for 00:59.240 --> 00:59.960 my purposes. 01:06.110 --> 01:06.740 So there we are. 01:06.860 --> 01:10.820 Segmentation fault, which is Unix's way of saying the program crashed. 01:18.130 --> 01:24.550 There is also a member function called at(), which does perform range checking. This will check its arguments 01:24.550 --> 01:25.450 at run time. 01:25.780 --> 01:30.010 And if that is not a valid index, it is going to create an exception object. 01:31.390 --> 01:34.580 As you can see, we are not actually doing anything to handle this exception object. 01:34.600 --> 01:36.280 So, what do you think will happen? 01:38.540 --> 01:40.430 And the answer is, the program should terminate. 01:41.210 --> 01:42.260 So let's see what happens. 01:48.890 --> 01:50.660 OK, so the program does terminate. 01:51.050 --> 01:56.780 It has called the library's terminate() function, which does what its name says. It has thrown an instance, 01:56.780 --> 01:58.910 or an object, of "out_of_range". 01:59.390 --> 02:00.140 So what is that? 02:00.740 --> 02:03.200 And there is also some output, which we will come back to in a minute. 02:05.820 --> 02:11.280 In C++, you are actually allowed to use any type you want for an exception object, including built-in 02:11.280 --> 02:15.060 types and your own types. That got rather confusing. 02:15.060 --> 02:19.950 So in C++ 98, they provided a standard exception class for the language. 02:21.040 --> 02:24.280 This gives a consistent interface for exception objects. 02:24.880 --> 02:27.700 It is actually the base class of an inheritance hierarchy. 02:28.120 --> 02:34.090 So this is one of the very few examples of where the library uses inheritance polymorphism. 02:37.860 --> 02:43.650 This exception class has a number of subclasses, which includes the "out_of_range" that we just saw. 02:44.430 --> 02:47.310 It also has a virtual member function, called what(). 02:47.940 --> 02:53.580 And this will return a string, a C-style string, which describes the error condition. 02:55.020 --> 02:58.710 So if we go back to our output, it actually called what(). 02:59.340 --> 03:00.660 And the string is a bit.. 03:02.420 --> 03:08.340 jargonish, but you can see there is a range check of something which is two, and the size is zero. 03:08.870 --> 03:10.250 So that is why things went wrong. 03:16.730 --> 03:22.670 Let's try now to rewrite our example, so the exception is handled and the program will continue gracefully, 03:23.000 --> 03:29.090 instead of being terminated. The compiler will actually do most of the work for us, but we need to tell 03:29.090 --> 03:31.040 the compiler that we want the extra code. 03:33.430 --> 03:38.770 To do this, we take the code, which is likely to have a run-time error, and we put that inside its 03:38.770 --> 03:43.660 own block again, and we put the "try" key word in front of this block. 03:44.140 --> 03:45.310 So this will make the compiler 03:45.310 --> 03:47.410 generate the necessary code. 03:50.420 --> 03:56.990 Then for the handler, we have another block with the catch key word in front of it. After the catch 03:56.990 --> 04:01.070 key word, we put some round brackets with the type of the exception. 04:01.460 --> 04:04.850 So this is a bit like a function argument, although not exactly the same. 04:05.870 --> 04:10.280 If we put the base class in here, the exception, this will handle any subclass 04:10.280 --> 04:16.310 of that exception class. If we catch it by reference, then dynamic bining will be used. 04:16.760 --> 04:21.200 And if we call the what() member function, we will get the correct version for the subclass. 04:23.720 --> 04:24.770 So let's try that out. 04:24.980 --> 04:26.120 We have our vector again. 04:26.600 --> 04:29.210 We have our access inside of the try block. 04:30.360 --> 04:33.720 And then we have the exception handler, which follows straight after the try block. 04:34.710 --> 04:38.430 So we are going to catch the exception, and its subclasses, by reference. 04:39.330 --> 04:45.420 And in our handler, we are just going print out that we have an exception and then call the what() member 04:45.420 --> 04:45.870 function. 04:54.890 --> 04:59.780 So there we are. We get the "exception caught", and then we get the string returned by what(). 05:05.500 --> 05:07.870 If we catch it by value, what do you think will happen? 05:13.260 --> 05:16.140 If we catch it by value, then static binding is going to be used. 05:16.590 --> 05:21.330 So the result of calling what() will be to call the exception class version. 05:25.570 --> 05:31.330 And there we are. So, "exception caught". And the exception version of what() just says this is an exception. 05:32.200 --> 05:36.910 So normally, you put the catch block straight after the try block, to catch the exceptions which are 05:36.910 --> 05:37.570 thrown in there. 05:38.560 --> 05:42.370 What happens if the catch block does not handle that particular exception. 05:43.510 --> 05:46.890 In that case, the program will jump out of the enclosing scope. 05:46.900 --> 05:49.870 So if we are in a function, it is going to return to the caller. 05:52.990 --> 05:57.710 Then it is going to look in the caller for a suitable catch handler, and if there is no suitable, it 05:57.940 --> 05:59.890 is going to go to the caller's, caller, and so on. 06:00.460 --> 06:03.900 Eventually, it will get to the main function and there is nothing to do. 06:03.940 --> 06:05.200 So the program will terminate. 06:06.310 --> 06:07.810 Okay, so that is it for this video. 06:08.050 --> 06:08.890 I will see you next time. 06:08.890 --> 06:10.960 But until then, keep coding!