WEBVTT 00:00.180 --> 00:07.710 Hello again! In this video, we are going to look at writing our own exception class. C++ allows you to 00:07.710 --> 00:08.190 do this. 00:08.550 --> 00:14.340 Your class can do anything in theory, but usually the best approach is to derive from one of the existing 00:14.610 --> 00:18.180 standard exception classes, in the exception hierarchy. 00:18.840 --> 00:24.240 So this means we have a consistent interface with the standard exceptions. We can inherit most of the 00:24.240 --> 00:26.550 code we need, instead of watching it again from scratch. 00:27.420 --> 00:32.880 And if your program already has handlers for the base class exception, or the class that you are inheriting 00:32.880 --> 00:36.990 from, then these will automatically handle your exception class as well. 00:37.650 --> 00:42.930 You only need to go through the program and add new handlers if you want to do something different for 00:42.930 --> 00:43.650 your exception. 00:46.710 --> 00:49.890 Deriving from the base class exception is not very useful. 00:50.310 --> 00:55.790 This only has a default constructor, which means that you are not allowed to choose your own error 00:55.800 --> 00:56.220 message. 01:01.780 --> 01:03.160 So what does our class need? 01:03.580 --> 01:09.790 It needs to have constructors which take a string, so we can set the message, and these need to take 01:09.790 --> 01:13.630 both the C++ string and the C-style string. 01:14.770 --> 01:20.470 It needs a copy constructor. And usually we can either inherit that from the parent class, or get the 01:20.470 --> 01:23.440 compiler to synthesize the defaults for us. 01:24.370 --> 01:31.600 We can also override the virtual what() member function from the parent, if we want to add extra functionality 01:31.600 --> 01:38.620 for our class. And we can also add data members, if we want to store information about this particular condition. 01:39.190 --> 01:41.890 And if you do that, it is best to use built-in types. 01:45.520 --> 01:50.650 The exception object is going to be copied into memory off the local stack, so that is why it needs 01:50.650 --> 01:53.080 the copy constructor. The program 01:53.080 --> 01:58.720 may be in an unstable state, so we need to make our object as lightweight as possible. 02:00.040 --> 02:04.630 We should only use the data members that we need, if we need any at all, in fact. 02:05.290 --> 02:10.810 So we should not use any data members, unless we need them, and we should do the minimum amount of work 02:11.080 --> 02:12.400 inside our exception. 02:12.880 --> 02:15.850 And of course, we should be careful not to throw any more exceptions. 02:20.690 --> 02:25.400 As an example, we are going to create an exception test for our student grades program. 02:25.940 --> 02:28.280 We are going to call this invalid_student_grade. 02:29.060 --> 02:34.940 And the idea is if we have a student grade which is less than zero or greater than 100 percent, then 02:34.940 --> 02:35.960 we throw this exception. 02:36.680 --> 02:41.300 This is an error condition where the value is outside the defined range for it. 02:41.870 --> 02:47.600 So the obvious thing to do here is to derive our class from the out_of_range exception class. 02:49.310 --> 02:53.180 This has constructors which take strings, so we need to implement these. 02:53.730 --> 03:01.010 So C-style and C++ strings. We also implements a default constructor, and this will just call the parent 03:01.010 --> 03:04.450 class constructor and pass a suitable error string. 03:04.460 --> 03:06.020 So, "invalid grade:" 03:06.110 --> 03:06.920 "please try again". 03:09.610 --> 03:15.220 The only data we're going to use is the error string, and this is already managed for us by the parent 03:15.220 --> 03:21.310 class, so we do not need any data members of our own. And that means we do not need to do anything special 03:21.310 --> 03:25.900 in the copy constructor, so we can just get the compiler to synthesize one, which will call the parent 03:25.900 --> 03:27.100 class copy constructor. 03:28.380 --> 03:31.590 And likewise, we do not need to define a destructor. 03:34.750 --> 03:38.950 We could also override the virtual what() member function, but we do not need to. 03:39.580 --> 03:42.550 If we were going to, we need to get the signature correct. 03:43.000 --> 03:44.920 So this is a member function, which is const. 03:45.640 --> 03:47.380 It does not change the exception object. 03:48.100 --> 03:51.160 And it is also "noexcept", which we are going to look at a bit later on. 03:51.640 --> 03:57.790 This basically means it is not allowed to throw exceptions, and this should override the parent class version. 03:58.390 --> 04:01.150 And it returns the error string as a C-style string. 04:02.710 --> 04:04.690 So here is our exception class, 04:04.690 --> 04:09.220 bad_student_grade. This inherits from the standard out_of_range exception. 04:09.760 --> 04:13.750 We need to include the standard exception header to get the definition of that. 04:14.760 --> 04:20.790 We have our default, which calls the parent class constructed with a sensible default error message. 04:21.660 --> 04:27.000 Then we have the constructors, which take a string as argument, the C and C++ strings. 04:28.230 --> 04:33.240 We can use the default for the copy and assignment operators, because we do not have any data members, 04:34.020 --> 04:37.440 and we could also override the what() virtual member function. 04:40.770 --> 04:46.950 In our student_grade_class, we have the constructor, which takes the grade as argument. If the grade 04:46.960 --> 04:48.180 is greater than 100, 04:48.210 --> 04:51.540 we create an object by calling the default constructor. 04:52.470 --> 04:57.150 So that is going to set the error message to this default. If it is less than zero, 04:57.570 --> 05:01.350 then we call the constructor with a C string as argument. 05:01.650 --> 05:03.990 So that is going to set the error to be 05:04.320 --> 05:04.950 "bad grade". 05:07.620 --> 05:09.720 And then we have a main() function to try this out. 05:10.260 --> 05:14.640 So we try to create a student_grade object. If the result is invalid, 05:15.000 --> 05:19.170 it should throw an exception which will get caught here, and that will print out the error strings. 05:20.190 --> 05:24.180 So let's try this with a grade, which is greater than 100. 05:26.010 --> 05:28.140 So that calls the default constructor. 05:28.500 --> 05:30.420 And that will give us the default error string. 05:32.130 --> 05:34.500 So there is the call to the default constructor. 05:37.130 --> 05:38.870 If we try it with a negative number, 05:41.640 --> 05:46.770 then this will call the constructor which takes a string, and that will set the error string to "bad 05:46.770 --> 05:47.130 grade". 05:48.090 --> 05:49.470 Okay, so that is it for this video. 05:49.980 --> 05:53.130 I will see you next time, but until then, keep coding!