WEBVTT 00:00.240 --> 00:06.060 Hello again! In this video, we are going to look at exception safety. Exception safety means that 00:06.060 --> 00:11.280 code behaves correctly when exceptions are thrown, and obviously that is something we want. 00:11.880 --> 00:15.900 So how do we do it? In C++, 00:15.910 --> 00:18.650 there are three main ways to write exception-safe code. 00:19.320 --> 00:25.830 You can provide the so-called basic exception guarantee: if an exception is thrown during an operation, 00:26.250 --> 00:27.810 no resources will be leaked. 00:28.320 --> 00:33.180 So, for example, if you open a file and an exception is thrown, the file gets closed. 00:33.720 --> 00:40.050 If you allocate memory in an operation and an exception is thrown, the memory is released. And so on. 00:42.560 --> 00:48.350 The strong exception guarantee provides all that, and it also reverts the program to its previous 00:48.350 --> 00:50.510 state, before the operation started. 00:51.200 --> 00:53.450 So it is all as if the operation had never happened. 00:53.930 --> 00:54.980 It was all just a dream :) 00:56.320 --> 00:57.670 And then we can have the no 00:57.670 --> 01:00.700 throw guarantee, which is that we do not throw any exceptions at all. 01:01.360 --> 01:06.160 And if you do not throw any exceptions, then you do not have any problems from dealing with exceptions. 01:11.430 --> 01:17.160 The basic exception guarantee provides the minimum level of exception safety: an operation 01:17.160 --> 01:19.230 will either succeed, or throw an exception. 01:19.740 --> 01:25.860 And if there is an exception, there are no resource leaks. So all operations and functions in the C++ 01:26.160 --> 01:28.650 standard library provide this basic guarantee. 01:30.780 --> 01:36.750 With the strong exception, guarantee, an operation will either succeed or have no effect. If 01:36.750 --> 01:41.610 it throws an exception, everything will go back to the same state as it was before the operation. 01:42.270 --> 01:44.790 And that is what is known as transactional semantics. 01:45.690 --> 01:50.490 If you have worked with databases, you may have come across commit and rollback. When you were doing 01:50.490 --> 01:51.060 an update, 01:51.420 --> 01:55.620 if everything succeeds, then you "commit", and the changes are saved in the database. 01:56.340 --> 02:01.050 If something goes wrong, you "roll back" your changes, and the database is left unmodified. 02:05.300 --> 02:10.820 All or references, which you obtained before the operation, will remain valid after 02:10.820 --> 02:14.000 the operation, even if the operation throws an exception. 02:15.320 --> 02:21.050 And all the operations on the standard templates library containers provides this strong exception guarantee. 02:21.350 --> 02:28.730 There are just a couple of cases within search operations where iterators can be invalidated. To provide 02:28.730 --> 02:30.860 the basic exception guarantee, 02:31.160 --> 02:38.150 any resources which are acquired during the operation must be released when an exception is thrown. 02:38.840 --> 02:40.640 So you can either do that manually. 02:41.240 --> 02:43.850 You have a try block where you acquire resources. 02:44.330 --> 02:50.060 For example, allocating memory. And in the catch block, you release the resource - you call delete. 02:50.990 --> 02:56.510 Or alternatively, you can use objects which manage the resource and automatically release it when the 02:56.510 --> 02:57.320 destructor is called. 02:57.860 --> 03:03.440 So that will be when the program jumps out of the try block and calls the destructors for all the objects 03:03.440 --> 03:03.860 in the try 03:03.860 --> 03:04.100 block. 03:04.940 --> 03:09.710 So with string and vector, that will release the memory that is being used by the object. 03:10.280 --> 03:13.010 And for fstream, the destructor will close the file. 03:13.700 --> 03:15.920 So this should make you think of a four-letter word. 03:16.440 --> 03:17.660 No, not that four-letter word! 03:18.350 --> 03:22.040 I am thinking of RAII, but we will come back to that. 03:22.730 --> 03:24.230 Okay, so that is it for this feature. 03:24.680 --> 03:25.590 I will see you next time. 03:25.590 --> 03:27.950 But until then, keep coding!