WEBVTT 00:00.120 --> 00:04.620 Hello again! In this video, we are going to start looking at the standard exception hierarchy. 00:05.680 --> 00:12.730 C++ has a standard exception class in the library, and this is the base of an inheritance hierarchy. 00:13.270 --> 00:19.450 So if we use this hierarchy for our exceptions, we will have a consistent interface for all exception 00:19.450 --> 00:19.930 objects. 00:20.410 --> 00:25.750 And also, we can take advantage of polymorphism. So we can just write one handler, which takes the 00:25.750 --> 00:28.120 exception base class by reference. 00:28.690 --> 00:35.620 We only need to write separate handlers for subclasses if we want to do something different for a specific 00:35.620 --> 00:36.280 subclass. 00:40.050 --> 00:43.350 The hierarchy looks like this. We have the exception as the base. 00:43.860 --> 00:49.290 Then we have these classes, which we are going to look at in the next video. We can recognize bad_alloc, 00:49.290 --> 00:51.450 which is for memory allocation errors. 00:52.020 --> 00:56.760 And there is also bad_cast, which we will cover when we look at run-time type information. 00:57.600 --> 00:59.700 So these are leaf classes of the exception. 01:00.180 --> 01:05.460 There are also two sub-hierarchies, runtime_error and so-called logic_error. 01:06.030 --> 01:08.730 And we can see that out_of_range is a logic_error. 01:13.510 --> 01:18.850 The idea of these classifications is that a logic error is something that the programmer could have 01:18.850 --> 01:24.370 prevented, whereas a runtime error is something that the programmer cannot do anything about. 01:25.240 --> 01:28.050 Typically, logic errors involve preconditions. 01:28.060 --> 01:33.190 For example, if a function must never be called with argument 0, then the programmer can check 01:33.190 --> 01:34.690 for that before calling the function. 01:35.620 --> 01:40.960 On the other hand, if some program on the computer goes crazy and uses up all the memory, then there is 01:40.960 --> 01:42.490 nothing you can do about that. 01:45.050 --> 01:48.440 The exception base class defines five public member functions. 01:48.770 --> 01:50.720 So that is the interface of the hierarchy. 01:51.260 --> 01:55.970 We have the usual special member functions: constructor, copy constructor, assignment operator. 01:56.690 --> 02:02.360 There is a virtual member function called what(), and that means that the destructor has to be virtual 02:02.360 --> 02:02.780 as well. 02:03.200 --> 02:08.810 And I hope you remember why that is: it is to make sure that all the destructors are called when a child 02:09.140 --> 02:10.820 is destroyed through a reference to the base. 02:12.410 --> 02:17.480 This what() function will return a C string, and it cannot throw exceptions. 02:17.840 --> 02:19.310 And we certainly do not want it to! 02:22.700 --> 02:28.310 The exception class and its leaf children, bad_alloc and bad_cast, have default constructors. 02:29.390 --> 02:31.790 The others do not have a default constructor. 02:32.180 --> 02:37.940 Instead, they have a constructor which takes a single string argument, and that can be either a C string 02:38.300 --> 02:40.190 or a C++ library string. 02:41.290 --> 02:47.770 When the what() member function is called, this string is returned, so it is important that this constructor 02:47.770 --> 02:50.530 arguments gives information about the error condition. 02:54.330 --> 02:59.610 We saw that we can do bounds checking with vectors, by calling the at() member function, and we can actually 02:59.610 --> 03:00.720 implement that ourselves. 03:01.590 --> 03:07.560 Obviously, we cannot change the library vector, but we can write a non-member function which takes a vector 03:07.560 --> 03:08.550 and does the same thing. 03:09.420 --> 03:12.240 So this needs to take the vector and also the index. 03:12.960 --> 03:15.630 The return type will be the same as the element type. 03:16.260 --> 03:18.030 I am just using a vector of int here, but 03:18.270 --> 03:21.540 you can be generic and write a template function if you like. 03:23.010 --> 03:25.940 We need to check that the index is valid. 03:27.450 --> 03:31.410 If the index is not valid, then we need to throw the out_of_range exception. 03:31.830 --> 03:37.770 So we create an out_of_range object by calling the constructor, and we pass a string as argument. 03:38.550 --> 03:44.580 So we have a local variable, which contains some information about the error condition, and we pass that 03:44.580 --> 03:47.730 as argument. And then we throw the exception. 03:48.480 --> 03:52.350 So the program will call the destructor for this local string variable. 03:52.980 --> 03:54.540 Then it will jump out of the if block. 03:54.900 --> 03:56.940 It will look for a handler in the enclosing scope. 03:57.660 --> 04:01.230 There is nothing in this at() function, so it is going to go to the caller. 04:02.530 --> 04:08.290 In main(), we are calling this inside a try block, which has a handler for any child of exception. 04:08.800 --> 04:14.350 So this exception will be caught and handled in main(). And that is the exceptional program flow. 04:17.190 --> 04:22.860 If the index is valid, then we can just carry on and use the subscript to get the element value 04:22.860 --> 04:23.700 and return it. 04:25.310 --> 04:31.130 So let's try this. We have a vector with three elements, so we should be able to access the element 04:31.130 --> 04:36.110 with index 2 and get the value of that element, which is 3. 04:36.710 --> 04:37.280 And there we are. 04:41.550 --> 04:45.970 If we try this with an empty vector, then what should happen? 04:46.950 --> 04:51.240 This condition is going to be false, and we are going to throw an exception, which should be caused by 04:51.240 --> 04:52.380 this handler in main(). 04:53.100 --> 04:54.240 So let's see if that works. 04:56.540 --> 04:59.210 And yes, it does. So exception is caught in main(). 04:59.690 --> 05:01.400 And that is the what() string. 05:01.790 --> 05:08.480 So when we constructed the object, the argument that we gave is the message that we get when we call 05:08.480 --> 05:08.780 what(). 05:11.110 --> 05:15.430 And finally, C++11 added a few new classes to the hierarchy. 05:16.150 --> 05:19.660 So bad_alloc now has a child, which is bad_array_new_length. 05:20.320 --> 05:27.100 So if you are calling new to allocate an array and the number of elements is invalid, then this will 05:27.100 --> 05:27.540 be thrown. 05:28.570 --> 05:34.420 There are a few new exceptions for C++11 features we have not yet covered: weak pointer, standard 05:34.420 --> 05:36.460 function and standard future. 05:37.680 --> 05:40.140 And I am sorry, but this does not involve time travel. 05:40.560 --> 05:41.620 It is about multi-threading. 05:43.110 --> 05:50.070 There's also a new sub-hierarchy, system_error, underneath runtime_error, and this has one child, ios_base_failure. 05:50.070 --> 05:53.610 So we can now get streams to throw exceptions. 05:54.630 --> 05:56.070 Okay, so that is it for this video. 05:56.460 --> 05:57.350 I will see you next time. 05:57.600 --> 05:59.580 Until then, keep coding!