WEBVTT 00:00.150 --> 00:00.720 Hello again! 00:01.020 --> 00:05.340 In this video, we are going to look at special member functions in C++11. 00:06.810 --> 00:13.620 We now have these two new special functions, the move constructor and the move assignment operator. 00:14.310 --> 00:19.290 These are optimized versions of the copy constructor and the assignment operator, which move their 00:19.290 --> 00:21.300 argument instead of copying it. 00:22.470 --> 00:28.110 And they are automatically called when the object being copied or assigned from is a moveable rvalue. 00:30.790 --> 00:34.870 These have some implications on the rules for special member functions. 00:34.870 --> 00:36.220 So we are going to look at those. 00:37.270 --> 00:39.820 The compiler will not synthesize a move 00:39.820 --> 00:45.460 operator, if the class has a copy constructor, assignment operator or destructor. 00:47.110 --> 00:52.840 It will only define a move operator if none of those are defined, and also every data member of the class 00:52.840 --> 00:57.820 is either a built-in type, a user-defined type which has move operators. 00:58.450 --> 01:05.890 So something which is moveable. Or else a static data member, which is not moved, because a static data member 01:05.890 --> 01:08.800 member is shared between all the objects of a class. 01:11.340 --> 01:16.860 The move constructor which is synthesised by the compiler will call the move constructor for each 01:17.190 --> 01:21.180 member of the class, and the same for the move assignment operator. 01:25.180 --> 01:31.510 If your class defines a move operator, then booth the copy operators will be synthesized by the compiler 01:31.510 --> 01:32.380 as deleted. 01:33.130 --> 01:37.510 So that means you cannot copy or assign to objects of your class. 01:38.110 --> 01:42.250 If you want to be able to do that, then you must provide your own copy operators. 01:42.820 --> 01:44.890 Otherwise, the class will be move-only. 01:45.610 --> 01:52.180 So the easiest way to create a move only class is to implement the move operators and not the copy 01:52.180 --> 01:52.840 operators. 01:56.400 --> 02:01.890 We have talked about the rule of three for when you need to implement special member functions. 02:02.580 --> 02:08.400 And, with these two new special member functions, we now have the rule of five in C++11. 02:09.450 --> 02:15.330 If your class needs a destructor to function correctly, then probably it needs the copy operators and 02:15.330 --> 02:16.950 the move operators as well. 02:20.330 --> 02:25.850 For example, if your class allocates memory in the constructor by calling new, then it needs 02:25.850 --> 02:27.980 to have a destructor which calls delete. 02:28.520 --> 02:35.660 It needs to have a copy constructor and a copy assignment operator to perform deep copies. And move operators 02:35.660 --> 02:38.420 to make sure that the argument is correctly invalidated. 02:43.790 --> 02:45.440 So let's look at an example. 02:46.010 --> 02:49.910 I have written a very simple class, which does not actually have any data members. 02:51.080 --> 02:59.900 We implement the move operators, which are just really shells. The compiler will synthesize copy operators, 02:59.900 --> 03:02.420 but they will be declared as deleted. 03:03.230 --> 03:06.770 So this is what the code will look like, as the compiler sees it. 03:09.140 --> 03:12.590 If we called the move operators, then these will work. 03:15.250 --> 03:21.730 So we get the move constructor called and the move assignment operator called. If we try to use the copy 03:21.730 --> 03:22.330 constructor, 03:25.070 --> 03:28.490 then we get this error again: "calling deleted functions". 03:33.790 --> 03:35.800 And if I uncomment this code. 03:35.890 --> 03:38.830 So this is what the compiler actually thinks the code does. 03:40.300 --> 03:41.710 Then we get the same error again. 03:42.250 --> 03:46.000 So it is as if the copy operators were declared as deleted. 03:46.810 --> 03:51.280 So with the code as it is, you cannot copy or assign to objects of your class. 03:51.700 --> 03:55.990 If you want to do that, then you would have to implement the copy operators to do this. 03:57.100 --> 03:58.510 Okay, so that is it for this video. 03:58.900 --> 03:59.740 I will see you next time. 03:59.740 --> 04:01.930 But until then, keep coding!