WEBVTT 00:00.120 --> 00:00.660 Hello again! 00:00.870 --> 00:05.850 In this video, we are going to look at inline namespaces. In C++, 00:05.850 --> 00:08.430 you can define one namespace inside another. 00:09.030 --> 00:13.440 So we have a namespace A, and inside that we have another namespace B. 00:14.160 --> 00:19.080 So these are "nested" namespaces. If we have any symbols in here, 00:19.260 --> 00:22.800 so functions, classes, variables and so on. 00:23.640 --> 00:29.430 If we want to use them outside the namespace, we have to put the namespace double colon in front. 00:30.150 --> 00:36.720 So if we are using x in this namespace, we have to put B double colon x, because it is defined in 00:36.810 --> 00:43.160 a namespace B inside this namespace. If we are outside any namespace, then we need to put another A 00:43.200 --> 00:44.490 double colon in front of that. 00:44.940 --> 00:48.210 So, A double colon B to put code on x. 00:50.900 --> 00:55.850 In C++11, we can say that the inner namespace is "inline". 00:56.330 --> 00:59.780 So namespace B is inline to namespace A. 01:00.710 --> 01:06.950 The difference is that any symbols which are defined inside the inline namespace B, will appear 01:06.950 --> 01:08.210 to be defined in A. 01:08.750 --> 01:13.220 So if we want to use anything from B in this namespace, we can just use it. 01:13.220 --> 01:15.470 We do not need to put B double colon. 01:16.490 --> 01:22.490 If we are outside the namespace, then this will appear to be defined in A. So we can just put A double 01:22.490 --> 01:23.150 colon x. 01:23.510 --> 01:26.300 And again, we do not need to put the B double colon. 01:27.740 --> 01:36.770 And, just to prove that is allowed. This is used in the C++ standard library. In C++14, there are literals 01:36.770 --> 01:37.640 for strings 01:37.640 --> 01:44.990 and chrono. If we put "s" after a C-style string, that makes it into a C++ string literal. 01:45.770 --> 01:52.490 If we put "s", "h" or "n" after a number, that makes a seconds, hours or minutes literal. These have 01:52.490 --> 01:59.390 their own namespaces, which are inside the literals namespace, which is inside the std namespace. 02:02.160 --> 02:03.150 The string literals 02:03.150 --> 02:06.720 chrono literals are inline to literals. 02:07.140 --> 02:12.930 So if we put "using namespace std::literals", that will bring in all the symbols from string_literals 02:12.930 --> 02:14.110 and chrono_literals. 02:15.210 --> 02:18.180 The literals namespace in turn, is inline to std. 02:18.630 --> 02:23.130 So if we have "using namespace std", which you should not really do unless you are writing toy 02:23.130 --> 02:26.580 programs, that will bring in everything from literals automatically. 02:29.710 --> 02:30.310 Inline 02:30.310 --> 02:35.110 namespaces are also useful if you are developing applications, if you are working with different versions 02:35.110 --> 02:35.560 of code. 02:36.640 --> 02:40.270 We have been occasionally using the Refrigerator class during this course. 02:40.630 --> 02:46.330 We have two versions, one with basic functionality, and an enhanced one which can connect to the Internet. 02:46.930 --> 02:49.960 So let's say we are writing software for refrigerators. 02:50.470 --> 02:53.290 We have a "product" name space where we write all our code. 02:54.100 --> 02:55.630 Then we have two different namespaces. 02:55.990 --> 03:01.810 The "Version1" namespace is for the basic Refrigerator, and the Version2 namespace is for the 03:01.810 --> 03:03.430 enhanced Refrigerator. 03:03.850 --> 03:06.280 And these are both inside the product namespace. 03:08.500 --> 03:09.820 So we have something like this. 03:09.850 --> 03:13.790 This is the product namespace, and we are writing all our code in it. 03:14.800 --> 03:20.260 We have the Version1 and Version2 namespaces which both have a definition of Refrigerator. 03:21.190 --> 03:26.860 If we make the Version1 namespace inline, then the Version1 definition will appear 03:26.860 --> 03:28.690 to be inside the "product" namespace. 03:29.200 --> 03:34.720 So if we just put Refrigerator without any qualification, this will pick up the basic Refrigerator 03:34.720 --> 03:35.290 definition. 03:37.340 --> 03:42.860 If instead, we have Version2 inline, then this class definition will appear to be in the product 03:42.920 --> 03:47.150 namespace, and we will pick up the enhanced Refrigerator. 03:50.240 --> 03:52.400 So you can use this for working with versions. 03:52.910 --> 03:58.160 What you do is, you put the code for the current version in an inline namespace, inside the product 03:58.160 --> 03:58.700 namespace. 03:59.330 --> 04:02.810 The code for the other versions goes in nested namespaces. 04:04.180 --> 04:09.370 If we use symbols without any scope operators, we automatically get the current version. 04:10.090 --> 04:14.800 If we want to use a different version for some reason, then we need to qualify it with the namespace 04:14.800 --> 04:18.190 for that version. And then, later on, 04:18.190 --> 04:23.290 when a new version is released, we make the namespace for that version inline, and the previous version 04:23.290 --> 04:25.480 is changed to a nested namespace. 04:27.340 --> 04:33.010 And all our code will automatically start using the new version unless it has explicit scope operators, 04:33.370 --> 04:37.210 in which case it will continue using the same version it was using before. 04:40.100 --> 04:41.360 So let's try this out. 04:41.390 --> 04:45.560 Here is the Version1 namespace, for the basic Refrigerator class. 04:46.490 --> 04:51.320 The Version2 has the enhanced Refrigerator which can connect to the internet. 04:51.740 --> 04:54.740 This is all code you have seen before, so it is not really important what it does. 04:56.150 --> 04:57.740 And then our product code. 04:58.010 --> 05:01.460 So we write all our code in the product namespace. 05:02.090 --> 05:03.920 We include the headers for both versions. 05:05.470 --> 05:08.470 At the moment, Version1 is defined as inline. 05:09.340 --> 05:16.090 So if we just use Refrigerator without any namespace qualification, this will pick up the Version1 05:16.360 --> 05:18.370 namespace. The basic Refrigerator. 05:21.480 --> 05:21.950 And there we are. 05:21.960 --> 05:23.550 That is the basic Refrigerator. 05:25.560 --> 05:26.520 If we now change this. 05:26.520 --> 05:32.850 So that Version1 becomes a normal, nested namespace and Version2 becomes inline. 05:35.700 --> 05:37.320 So this is inline in the product. 05:38.780 --> 05:39.230 (Better save that) 05:41.090 --> 05:47.060 So now if we use Refrigerator without any qualification, that is going to pick up the Version2 definition, 05:47.600 --> 05:49.250 and we should see it connecting to the Internet. 05:49.850 --> 05:50.470 And there it is. 05:51.050 --> 05:52.940 And please do not hack into my fridge! 05:53.810 --> 05:54.110 Okay. 05:54.110 --> 05:55.220 So that is it for this video. 05:55.610 --> 05:56.450 I will see you next time. 05:56.630 --> 05:58.660 Until then, keep coding!