WEBVTT 00:00.120 --> 00:00.660 Hello again! 00:00.990 --> 00:04.500 In this video, we are going to look at the throw exception specifier. 00:05.910 --> 00:08.310 We have mentioned the "no-throw" guarantee. 00:08.700 --> 00:12.210 This means that an operation does not throw any exceptions. 00:13.200 --> 00:19.080 When we are writing code and we are trying to make it exception safe, whether we are providing the basic 00:19.230 --> 00:21.900 exception guarantee or the strong guarantee, 00:22.500 --> 00:24.520 it is very useful to know that 00:24.540 --> 00:30.510 a function we are calling, or an operation that we are using does not throw exceptions. That makes our life easier. 00:31.440 --> 00:37.380 And we have some help from C++, because none of the functions and operators in the core language or the 00:37.380 --> 00:46.260 library throw exceptions, apart from new, which can throw bad_alloc, dynamic_cast which can throw bad_cast, and 00:46.350 --> 00:48.270 obviously throw, which can throw anything! 00:50.690 --> 00:51.620 In C++, 00:51.620 --> 00:54.980 there are two ways to say that a function does not throw exceptions. 00:55.580 --> 00:57.680 In this video, we are going to look at the older one. 00:59.060 --> 01:00.710 In C++ 98, 01:01.100 --> 01:07.340 You can take a function definition and put the "throw" keyword after the argument list, and then a pair 01:07.730 --> 01:12.410 of brackets, and then you list the exceptions that the function will throw, or could throw. 01:13.400 --> 01:20.540 So in this case, we have a function which could throw a network connection error exception or a data error. 01:23.220 --> 01:28.860 If, for some reason, the function throws some other exception, one which is not in this list, then the 01:28.860 --> 01:31.260 program will be terminated immediately, by default. 01:34.470 --> 01:39.720 We can also have an empty argument list, which means that the function does not throw any exceptions 01:39.720 --> 01:42.420 at all, or at least it is not supposed to! 01:43.440 --> 01:46.440 Again, if it does throw an exception, then the program gets terminated. 01:48.150 --> 01:52.170 The problem with this is that the list of exceptions is not checked. 01:52.830 --> 01:58.920 You can put anything in there, so long as it is a valid C++ type, and the compiler will accept it. 01:59.840 --> 02:01.140 Then your program will run. 02:02.520 --> 02:08.190 If the list of exceptions is not correct, then there is a risk that your program could be terminated 02:08.190 --> 02:08.940 unexpectedly. 02:09.450 --> 02:13.110 And this actually happened to me many years ago, when I started a new job. 02:13.680 --> 02:19.620 I was given some source code, and told to look at it. So I compiled it and ran it, to see what happens. 02:20.220 --> 02:22.410 And it immediately crashed, without any explanation. 02:23.040 --> 02:27.330 It turns out there was some configuration I needed to do, which was not documented. 02:28.230 --> 02:33.120 But the reason why it crashed is that the configuration error threw an exception, and that was in a 02:33.120 --> 02:36.570 function which had an exception list, and the exception was not in the list. 02:37.140 --> 02:39.420 So the program was terminated. 02:40.740 --> 02:44.970 So really, when you call a function which has an exception list, you are trusting that the person 02:44.970 --> 02:47.910 who wrote that exception list did the job properly. 02:49.410 --> 02:52.770 And this is a bit like writing documentation or updating comments. 02:53.430 --> 02:55.890 We all know we are supposed to do it, but it is never the right time. 02:58.420 --> 03:03.910 This throw() exception specifier was actually not very popular, and it has now been removed from the language. 03:04.540 --> 03:09.220 In C++11, it was deprecated, which means you are not supposed to use it anymore. 03:10.390 --> 03:14.770 In C++17, you are no longer able to use throw() with an argument list. 03:15.580 --> 03:19.180 And finally, in C++ 20, it was removed altogether. 03:19.510 --> 03:20.980 So it is now extinct. 03:22.760 --> 03:26.840 We now have something called "noexcept", which I am going to look at in the next video. 03:27.740 --> 03:29.840 So that is all for now, but I will see you next time. 03:30.440 --> 03:32.310 Until then, keep coding!