WEBVTT 00:00.180 --> 00:03.870 Hello again! In this video, we are going to look at the noexcept keyword. 00:04.800 --> 00:06.750 This came in, in C++11. 00:07.110 --> 00:10.420 It replaces the older throw() with an empty exception list. 00:11.190 --> 00:16.680 We can declare a function as noexcept and that means that the function does not throw exceptions. 00:17.160 --> 00:18.570 Or at least it is not supposed to. 00:19.320 --> 00:23.820 So this is the new version of writing throw() with an empty exception list. 00:24.870 --> 00:28.380 If an exception is thrown, then the program terminates immediately. 00:30.750 --> 00:36.030 Noexcept is a promise to callers of this function that it is not going to throw any exceptions. 00:36.660 --> 00:40.500 So a noexcept function provides the "no throw" guarantee. 00:41.130 --> 00:46.350 And that is helpful to us if we are writing exception-safe codes and we want to call that function. 00:47.340 --> 00:49.710 noexcept also has performance advantages. 00:50.130 --> 00:53.040 It helps the compiler generate better-optimized code. 00:53.820 --> 00:58.410 It gives the compiler more information about what the code might or might not do. 00:59.100 --> 01:02.460 So the compiler might be able to make some extra assumptions, which save time. 01:03.390 --> 01:07.620 And it also means that the compiler does not neede to generate code for stack unwinding for that 01:07.620 --> 01:08.100 function. 01:10.310 --> 01:16.640 In modern C++, we have optimized versions of some operators, which are not necessarily exception-safe. 01:17.570 --> 01:19.850 We will talk about this when we cover move semantics. 01:20.930 --> 01:26.690 The standard library will only use these operators if they are declared noexcept. Otherwise it is going 01:26.690 --> 01:31.550 to be cautious and call the older versions, which are not optimized but safer. 01:32.090 --> 01:38.030 So if you do not declare things as noexcept, you may lose out on these optimized operators. 01:40.370 --> 01:41.540 So when should we use 01:41.630 --> 01:42.260 noexcept? 01:42.680 --> 01:47.810 And the answer is, wherever possible. If we are certain that a function is not going to throw any 01:47.810 --> 01:54.230 exceptions, or not call functions, which might have exceptions. It is also useful if we could get 01:54.230 --> 01:56.810 an exception, but there is nothing useful we could do. 01:57.710 --> 02:01.190 And the safest thing to do is to shut down the program, before anything else goes wrong. 02:01.490 --> 02:04.160 So that is another situation where noexcept is useful. 02:05.300 --> 02:12.950 So, some examples where we could use noexcept are: overloading the standard swap() function, and for class destructors. 02:16.510 --> 02:21.930 We cannot overload on being noexcept: a function can only be one or the other. 02:21.960 --> 02:23.320 You cannot have two different versions. 02:24.190 --> 02:28.930 noexcept is part of the function's type, but it is not part of the function's signature. 02:29.320 --> 02:32.230 So that is similar reasoning to the return type. 02:32.410 --> 02:35.080 You cannot overdo it on return value in C++, either. 02:39.800 --> 02:44.210 With class hierarchies, the property of being noexcept is inherited. 02:44.780 --> 02:51.470 So if you have a virtual function in the base class, which is noexcept, then all the overrides in derived 02:51.470 --> 02:53.360 classes must also be noexcept. 02:54.740 --> 03:00.080 If you have inheriting constructors, these will be noexcept if the basic class constructor is no 03:00.080 --> 03:00.530 accept. 03:01.280 --> 03:06.680 And if we let the compiler synthesize special member functions for us, these will automatically be 03:06.680 --> 03:10.010 noexcept, if the version in the base class is noexcept. 03:12.400 --> 03:19.030 We can also make a child member function more exception safe than the parent's, but not the other way 03:19.030 --> 03:19.260 round. 03:20.080 --> 03:26.440 If we have a virtual function in the base case, which can throw, we can write an override in the child class, which 03:26.440 --> 03:27.130 is noexcept. 03:27.640 --> 03:28.780 But you cannot go the other way. 03:29.020 --> 03:35.230 You cannot have a noexcept function in the base class and then have a child override which throws exceptions. 03:35.590 --> 03:36.590 So that is similar to const. 03:36.670 --> 03:41.290 You can make the child version more const, but you cannot make it less than the parent's. 03:42.750 --> 03:49.800 And finally, if you do not make destructors noexcept, the compiler might do that for you anyway. 03:50.220 --> 03:56.070 This can happen if all the members of your class have a noexcept destructor, or they are built-in types, 03:57.030 --> 04:02.040 and all the parent classes of your class have a noexcept destructor. In that case, 04:02.040 --> 04:05.130 the compiler will also assume that your destructor is noexcept. 04:06.210 --> 04:13.410 So if you have existing code from before C++ 11, which does not use noexcept, then you just have 04:13.410 --> 04:16.920 to recompile this, and the destructors will now be noexcept. 04:17.340 --> 04:20.610 So you get the benefit of that, without having to modify your code. 04:22.050 --> 04:26.430 If you are writing new code, though, it is better to write noexcept. It is always better to make things 04:26.430 --> 04:27.420 explicit if you can. 04:28.230 --> 04:29.670 Okay, so that is it for this video. 04:30.150 --> 04:33.300 I will see you next time, but until then, keep coding!