WEBVTT 00:00.120 --> 00:00.690 Hello again! 00:00.960 --> 00:03.120 In this video, we are going to look at attributes. 00:04.590 --> 00:07.070 Compiler writers have various directives. 00:07.080 --> 00:12.810 These are things you can put in C++ source code, to make the compiler behave in various ways. 00:13.680 --> 00:19.080 These have names like hash pragma, double underscore attribute and double underscore declspec. 00:19.650 --> 00:21.570 These are not part of the C++ language. 00:22.080 --> 00:23.520 They are compiler specific. 00:25.740 --> 00:32.670 So these are used for things like: making the compiler generate certain types of code, laying out objects 00:32.670 --> 00:38.040 in memory in a certain way, using conventions for calling functions. And so on. 00:39.060 --> 00:45.960 C++11 introduced attributes, to provide a standard syntax for these compiler directives, and it also 00:45.960 --> 00:50.460 defined a few attributes in the language. Which we can use in our own programs. 00:52.120 --> 00:57.370 These attributes are meant to give extra information to the compiler. And they are not supposed to be 00:57.370 --> 01:00.670 used to extend the language, or change the meaning of programs. 01:01.300 --> 01:06.580 So they are nothing like Java attributes, which you can use for things like implementing database support. 01:09.120 --> 01:15.480 The syntax for an attribute is that we use a pair of double square brackets. And then the attribute's name 01:15.480 --> 01:16.350 goes inside that. 01:16.860 --> 01:19.410 So this is how we would use the "noreturn" attribute. 01:20.910 --> 01:25.650 If we are using an attribute in a declaration, it goes before the thing that it applies to. 01:26.370 --> 01:29.670 Here we have a server() function, which is going to be no return. 01:30.120 --> 01:37.620 So we put the noreturn attribute first, and then the server function prototype. You can use namespaces 01:37.620 --> 01:38.400 with attributes. 01:38.820 --> 01:45.330 So if you're working on the GNU compiler, you can put your directives in their namespace. So they will not 01:45.330 --> 01:47.160 conflict with any other attributes. 01:48.030 --> 01:50.730 And you can also provide arguments for attributes. 01:52.780 --> 01:55.900 C++11 define the noreturn attribute. 01:56.410 --> 01:59.140 This indicates that a function does not return. 02:00.640 --> 02:05.620 Usually if you write a function which does not have a return statement, it means you made a mistake. 02:06.220 --> 02:10.600 Often, compilers will give you a warning for that, so you can go back and fix it. 02:10.990 --> 02:12.100 And that is usually helpful. 02:12.670 --> 02:15.640 But sometimes, you do not actually want to return from the function. 02:16.420 --> 02:23.080 For example, if we have a web server which just has a loop which keeps processing connections until 02:23.080 --> 02:26.980 the program is terminated, then that function is never going to return. 02:27.640 --> 02:28.900 So you do not really want the warning. 02:29.770 --> 02:32.440 So you can say that this function is "noreturn". 02:32.440 --> 02:33.880 It has the noreturn attributes. 02:34.450 --> 02:40.270 That will also tell anyone reading the declaration that this function does not return, and it may help the 02:40.270 --> 02:41.710 compiler generate better code. 02:43.720 --> 02:47.620 C++11 also has the alignas keyword, which we looked at earlier. 02:47.980 --> 02:54.340 This is not actually an attribute, but it is meant to replace the hash pragme pack. But it only works if the 02:54.340 --> 02:56.080 argument is greater than the word size. 02:59.290 --> 03:01.690 So here is our never ending function. 03:02.260 --> 03:07.570 We use the no return attribute. And then we call the function, in the main() routine. 03:10.570 --> 03:11.590 And there it goes. 03:11.890 --> 03:12.670 There is no warning. 03:14.590 --> 03:17.830 C++14 added the "deprecated" attribute. 03:18.730 --> 03:23.230 Deprecated basically means "This code is going to go away soon, so stop using it!" 03:24.730 --> 03:28.930 You could, for example, have a function where you have written a new version with a different interface. 03:29.650 --> 03:32.980 You have not got around yet to changing all the code which calls this. 03:33.520 --> 03:35.410 So you still need to have the old version. 03:35.920 --> 03:40.720 But you want to discourage people who are writing new code from calling the old version. 03:40.720 --> 03:42.190 You want them to use the new version. 03:42.850 --> 03:46.210 So you could put the deprecated attribute in front of it. 03:46.750 --> 03:52.150 And then anyone who calls this function will get a compiler warning, saying that this has been deprecated. 03:52.630 --> 03:57.610 Which basically means you should try and use the new version. You can provide an argument, which will 03:57.610 --> 04:02.290 be an explanation string, and the compiler will include that in its warning message. 04:04.830 --> 04:06.840 So here is our deprecated function. 04:06.840 --> 04:11.130 We have the "deprecated" attribute, with the explanation string. 04:11.700 --> 04:15.030 So, "Please use the new version!" And then we call this. 04:17.950 --> 04:19.600 And in fact, we get an error. 04:19.870 --> 04:20.890 I am not sure that is correct. 04:21.460 --> 04:26.050 The error message is just the text of the explanation string. 04:28.330 --> 04:32.680 If we try this with g++, then we do get a warning. 04:33.130 --> 04:37.600 It says that this function is deprecated. And then it gives the explanation. 04:37.990 --> 04:39.490 So I think that is much clearer. 04:42.860 --> 04:46.600 We have already mentioned the "fallthrough" attribute in C++17. 04:47.120 --> 04:52.460 So this says that we intended to fall through from this case to this case. 04:53.090 --> 04:53.960 It is not a mistake. 04:56.290 --> 04:59.500 C++17 also has a "nodiscard" attribute. 05:00.250 --> 05:06.520 So if you have this in front of a function, anyone who calls this function, and does not check the return 05:06.520 --> 05:08.770 value, will get a compiler warning. 05:09.520 --> 05:14.410 So sometimes, it does not really matter what a function returns. The printf() function, 05:14.410 --> 05:18.430 for example. If you got a warning for not checking that, that would get really annoying! 05:19.090 --> 05:22.150 But sometimes it does return something important, like an error code. 05:22.630 --> 05:25.570 So you want to encourage people to look at the return value. 05:28.300 --> 05:31.060 So here is our function, which returns something very important. 05:31.150 --> 05:33.190 It has the nodiscard attribute. 05:34.450 --> 05:37.660 And then we are going to call this function, and ignore the returned value. 05:41.310 --> 05:42.060 So there we are. 05:42.090 --> 05:47.200 We get the compiler warning. "Discarding return value of function with nodiscard attribute". 05:48.840 --> 05:50.970 Attributes do not just apply to functions. 05:50.970 --> 05:56.610 They can also be used with variables, types, members of structs, classes, enumerations, and so on. 05:57.510 --> 06:00.720 With a type name, the attribute goes before the name of the type. 06:01.230 --> 06:07.140 So if we have a struct Test, which is nodiscard, then we put "struct", then the attribute, then 06:07.140 --> 06:08.340 the name of the struct. 06:09.450 --> 06:14.490 So this will mean that any function which returns a Test object will be nodiscard. 06:16.170 --> 06:19.770 So there is our struct Test, which is nodiscard. 06:20.280 --> 06:22.260 This function returns a Test object. 06:22.860 --> 06:25.740 So this function is, itself, nodiscard. 06:26.370 --> 06:31.350 So if we call this without checking the return value, we should get the same warning again. 06:32.950 --> 06:33.520 And we do. 06:34.240 --> 06:37.510 And the last attribute we are going to look at is the "maybe_unused". 06:38.470 --> 06:40.420 So if I just quickly remove 06:40.420 --> 06:41.560 that, for a minute. 06:43.780 --> 06:49.600 So we have a local variable here, which is not used and we get a warning for that: "unreferenced local variable". 06:51.580 --> 06:55.570 And if I put this back, then the warning goes away. 06:56.290 --> 07:00.580 So this could be useful if you have a function where you declare all the variables, but you want to 07:00.580 --> 07:02.860 test the function before you have finished it. 07:03.310 --> 07:05.980 So you may have some variables which are temporarily unused. 07:06.580 --> 07:08.380 So this will get rid of annoying warnings. 07:09.250 --> 07:10.630 Okay, so that is it for this video. 07:10.990 --> 07:11.800 I will see you next time. 07:11.800 --> 07:13.840 But until then, keep coding!