WEBVTT 00:00.210 --> 00:00.840 Hello again! 00:01.170 --> 00:05.910 In this video, we are going to look at using special member functions in C++ 11. 00:06.930 --> 00:13.890 As you may have noticed, the rules are a bit more complicated than they were before. Here is a table which 00:13.890 --> 00:16.680 was drawn up by the man who implemented move 00:16.680 --> 00:17.400 semantics. 00:18.540 --> 00:24.420 Let's have a look at that. And no, you are not expected to remember all this! In this video, 00:24.420 --> 00:28.920 I am going to try and give you some more practical guidelines on how to use special member functions. 00:31.530 --> 00:34.000 The first one is the "Rule of Zero". 00:34.620 --> 00:40.620 If your class does not need to implement a destructor, then it probably does not need to implement 00:40.620 --> 00:42.270 the copy operators either. 00:43.050 --> 00:47.460 So in this case you do not need to implement any special member functions at all. 00:48.000 --> 00:52.350 You can just leave the compiler to synthesise them for you, and they will do the right thing. 00:52.890 --> 00:55.650 And for the large majority of classes, this is quite sufficient. 00:57.550 --> 01:03.550 The compiler will also synthesize a default constructor, unless we define any constructor ourselves. 01:04.930 --> 01:10.510 If we have a constructor which takes arguments, sometimes it is useful to have a default constructor 01:10.900 --> 01:15.310 which will initialize the object with sensible values for the members. 01:18.950 --> 01:22.400 We have talked about the "Rule of Three" and the "Rule of Five". 01:23.120 --> 01:29.600 If your class implements a destructor, then it probably needs to implement the copy operators as well, 01:29.990 --> 01:33.470 because the ones that the compiler synthesizes will not do the correct thing. 01:34.640 --> 01:37.820 So they are not going to re-allocate memory or transfer the file handle. 01:39.800 --> 01:45.110 So in that case, you need to implement the destructor and also the copy constructor and copy assignment 01:45.110 --> 01:45.680 operator. 01:47.940 --> 01:52.110 And you'll probably need to implement a constructor, which will allocate the resource. 01:53.570 --> 01:57.140 In that case, the compiler will not synthesize any move operators. 01:58.100 --> 02:03.320 If copying your object involves a lot of work, and you have performance problems because of it, 02:03.860 --> 02:05.750 Then you can also add move operators. 02:06.410 --> 02:09.020 If you do not do that, then the copy operators will be used. 02:12.980 --> 02:14.840 We also talked about a move-only class. 02:15.320 --> 02:20.870 So this is in the situation where you need to declare a destructor, but you do not want to have duplicate 02:20.870 --> 02:22.910 objects managing the same resource. 02:23.750 --> 02:25.790 And in that case, you can make the class move-only. 02:26.270 --> 02:27.500 And we saw how to do that. 02:28.430 --> 02:33.890 We provided the move constructor and the move assignment operator, but not the copy operators. 02:35.360 --> 02:39.530 And in that case, the compiler will synthesize the copy operators as deleted. 02:40.340 --> 02:45.530 Or we can explicitly declare them as deleted ourselves, which is a bit clearer. 02:47.750 --> 02:50.840 And again, we may want to provide a constructor. 02:55.130 --> 02:57.370 It is possible to make an immoveable class. 02:57.380 --> 03:00.170 So this is a class which cannot be moved or copied. 03:00.920 --> 03:05.930 So objects of this class cannot be used as function arguments or returned from functions. 03:07.760 --> 03:10.280 There are some examples of this in the Concurrency Library. 03:10.970 --> 03:17.210 There are objects which will look a mutex when they are created, and then they unlock the mutex when 03:17.210 --> 03:17.900 they are destroyed. 03:18.530 --> 03:20.750 These are designed to be used within a single scope. 03:21.050 --> 03:25.850 So the idea is, when the program goes out of scope, the mutex is automatically unlocked. 03:26.780 --> 03:33.290 So the designers of the Concurrency Library have enforced this scope lifetime, by making it impossible 03:33.350 --> 03:37.070 to pass these to functions or return them from functions. 03:40.570 --> 03:42.670 To make an object immoveable, 03:42.760 --> 03:46.840 We delete the copy operators, and in that case, the compiler 03:46.840 --> 03:49.720 will not synthesize any move operators. 03:50.920 --> 03:57.100 If we try to copy or move the object, that will call the copy operators, and those are deleted, and we will 03:57.100 --> 03:58.090 get a compiler error. 04:00.480 --> 04:01.830 So here is an example. 04:02.190 --> 04:05.010 We have a class with deleted copy operators. 04:05.760 --> 04:12.690 We do not define any move operators. And in this case, the compiler is not going to synthesize any. So 04:12.690 --> 04:14.490 we can create an object of this class. 04:15.000 --> 04:17.070 But if we try to use the copy constructor, 04:19.700 --> 04:22.730 then we get the compiler error about deleted functions. 04:26.040 --> 04:32.400 And we also get that if we try to use the copy constructor. it is also possible, in theory, to make a 04:32.400 --> 04:34.410 class which can be copied, but not moved. 04:35.250 --> 04:41.010 To do that, we declare the move operators as deleted, and in that case the compiler will synthesize 04:41.370 --> 04:44.040 copy constructor and copy assignment operator. 04:46.350 --> 04:52.680 However, even though the move offices are deleted, the compiler can still decide that a move operator 04:52.680 --> 04:54.330 is the best match for a call. 04:55.080 --> 04:57.870 And then you get a compiler error when the function is called. 04:58.230 --> 05:03.300 For example, if you pass an rvalue, you may expect that this will fall back to the copy constructor, 05:03.750 --> 05:05.280 but in fact you get a compile error. 05:06.060 --> 05:09.570 So for that reason, using copy-only classes is not recommended. 05:11.660 --> 05:12.710 So let's try this out. 05:12.720 --> 05:19.250 So we have our copy operators, and we have the move operators declared as deleted. 05:20.660 --> 05:22.700 So if we call the copy constructor, 05:24.360 --> 05:32.700 we can do that. But if we try to pass an rvalue, then it does not fall back to the copy constructor. 05:33.120 --> 05:37.560 It tries to call the move operator, and then we get the compiler error again. 05:38.790 --> 05:41.340 So, copy-only objects are not very useful. 05:42.750 --> 05:44.220 Okay, so that is it for this feature. 05:44.730 --> 05:45.630 I will see you next time. 05:46.050 --> 05:47.940 But meanwhile, keep coding!