WEBVTT 00:00.120 --> 00:00.720 Hello again! 00:01.020 --> 00:04.140 In this video, we are going to have a practical on perfect forwarding. 00:04.530 --> 00:07.500 We are going to write a slightly more realistic example. 00:09.450 --> 00:11.220 We are going to use this class. 00:11.220 --> 00:13.920 It has one member, which is a library string. 00:14.730 --> 00:20.790 It has two overloaded constructors which take a string, one for an lvalue and one for an rvalue. 00:21.690 --> 00:25.290 We're going to make a function which will create objects of this class. 00:25.710 --> 00:33.390 So this is a very simple version of make_pair(), if you like, or perhaps it should be make_single! In 00:33.390 --> 00:34.710 this make_test() function, 00:34.710 --> 00:39.180 we want to make sure that the move constructor is called whenever possible. 00:39.660 --> 00:42.600 So we are going to use perfect forwarding to achieve that. 00:43.920 --> 00:49.260 Our make_test() function is going to have a forwarding reference argument, and then it is going to call 00:49.260 --> 00:54.930 the constructor for the class. And it will use the library forward() function to make sure that the appropriate 00:55.500 --> 01:00.750 constructor is called for the argument type. And then we return this object by value. 01:01.260 --> 01:06.630 It may look as though we are introducing an extra copy operation here, but modern compilers should be 01:06.630 --> 01:07.890 able to optimize this away. 01:08.190 --> 01:11.700 And in fact, in C++ 17, they are required to elide 01:12.000 --> 01:13.380 that copy constructor call. 01:14.100 --> 01:15.990 So this is going to be very efficient, we hope. 01:18.750 --> 01:19.870 Let's look at the code. 01:19.890 --> 01:25.230 We have our class here with the string member, and the two overloaded constructors. 01:27.090 --> 01:31.050 Then we have our make_test() function which takes a forwarding reference. 01:32.010 --> 01:39.570 It calls the constructor for the class, and it uses the library forward() function to call the lvalue 01:39.570 --> 01:44.700 or the rvalue constructor as appropriate, and then we return it by value. 01:45.900 --> 01:51.950 In the main() function we call this make_test() function with an lvalue argument, and then with an 01:51.960 --> 01:52.830 rvalue argument. 01:53.730 --> 01:55.440 So let's see if this works. 01:58.320 --> 02:04.260 So, calling with an lvalue argument calls the constructor which takes lvalue, and calling with an rvalue 02:04.260 --> 02:06.450 argument calls the move constructor. 02:07.050 --> 02:07.830 So that works. 02:10.750 --> 02:16.000 I've also put in these functions similar to the ones we had before, which will tell us what type 02:16.000 --> 02:17.860 the arguments to make_test() is. 02:19.000 --> 02:20.080 So let's call those. 02:22.070 --> 02:25.250 So with the lvalue arguments, we get the modifiable version. 02:25.260 --> 02:29.690 So that is an lvalue. And with the rvalue argument, we get the move version. 02:30.050 --> 02:32.240 So x is actually an rvalue in here. 02:34.810 --> 02:36.580 So this may seem a bit pointless. 02:36.580 --> 02:41.260 Why not just call the constructors directly? And with this class you can do that. 02:41.530 --> 02:42.730 It just has one member. 02:43.420 --> 02:47.620 So there's only two possible constructors. With the pair class, 02:48.190 --> 02:53.530 there is two members, which means there is four combinations of constructors. And also the members 02:53.530 --> 02:54.940 are template types. 02:55.480 --> 02:58.900 So you'd be using templates anyway, and this approach would make more sense. 03:00.860 --> 03:05.750 C++ now has a few more functions which will make objects of library classes. 03:06.180 --> 03:09.570 We are going to meet a couple of them in the next section, with "smart" pointers. 03:10.100 --> 03:11.570 So that is why I'm going into this now. 03:12.380 --> 03:13.900 Okay, so that is it for this feature. 03:14.330 --> 03:15.170 I will see you next time. 03:15.380 --> 03:17.480 Until then, keep coding!