WEBVTT 00:00.730 --> 00:06.490 Hello again! In this video, we are going to look at synthesised member functions. So, as we have already 00:06.490 --> 00:07.060 touched on, 00:07.510 --> 00:13.150 if you do not define certain special member functions, the compiler will synthesize them for you. 00:14.140 --> 00:18.670 So this is the constructor, the copy constructor and the assignment operator. 00:19.720 --> 00:24.910 There are also some new special member functions in modern C++ related to move operations. 00:25.390 --> 00:27.490 And we are going to look at those later on in the course. 00:28.450 --> 00:30.250 And finally, the destructor. 00:34.850 --> 00:42.740 There are rules for when the compiler will synthesize these. In traditional C++, older C++ before 2011, 00:43.070 --> 00:49.220 the compiler will always fill in the missing special functions. If we do not define a constructor or a 00:49.220 --> 00:54.470 copy constructor - because the copy constructor is regarded as a special kind of constructor - then the 00:54.470 --> 00:57.050 compiler will synthesize a default constructor. 00:57.530 --> 01:02.120 This takes no arguments, and it calls the default constructor for each data member. 01:02.840 --> 01:07.340 So if the data member is built in, that will be default initialized, which means undefined. 01:09.250 --> 01:14.380 If there is no copy constructor, the compiler will synthesize one, and this will just call the copy 01:14.380 --> 01:18.940 constructor for each data member. If there is no assignment operator, 01:18.940 --> 01:24.730 similarly, the compiler will synthesize one which calls the assignment operator for each data member 01:24.730 --> 01:25.330 of the class. 01:26.440 --> 01:32.530 And finally, if there is no destructor, the compiler will synthesize one. And that does nothing because 01:32.530 --> 01:35.290 the destructors for the members are automatically called. 01:41.060 --> 01:44.570 In modern C++, the rules are much more complex. 01:44.900 --> 01:47.690 There is five special member functions instead of three. 01:48.230 --> 01:54.610 And the rules about whether some special functions are synthesized depends on whether some of the other 01:54.620 --> 01:56.180 special member functions are defined. 01:57.140 --> 01:58.640 So we are going to leave that for now. 01:58.940 --> 02:02.240 We will come back to this in the section where we look at move operations, 02:02.240 --> 02:03.050 move semantics. 02:06.750 --> 02:12.210 So here's an example of how the compiler will synthesize these special member functions. 02:12.960 --> 02:20.040 We have our "pet" test class with an int and a string member, but we do not define any special functions, just 02:20.040 --> 02:24.330 this print member function, which is going to display the values of the members. 02:26.310 --> 02:32.490 So there is no constructor or copy constructor, so the compiler will synthesize a default constructor, 02:33.060 --> 02:36.870 and this will just use the default initialization for the data members. 02:38.260 --> 02:42.780 There is no copy constructor, so the compiler will synthesize one, which we have seen before. 02:43.350 --> 02:46.020 It will just initialize the members from the argument. 02:47.010 --> 02:51.390 If there is no assignment operator, then again, well, we have seen that. 02:51.930 --> 02:53.970 And finally, the destructor will do nothing. 02:58.530 --> 03:03.410 So we have a couple of test objects which we default construct, then we have one which we copy 03:03.430 --> 03:03.950 construct. 03:04.710 --> 03:09.030 Then we do an assignment. And we print out the data after each step. 03:12.920 --> 03:16.040 So again, we have these weird undefined values. 03:16.460 --> 03:20.840 The string's default constructor just creates an empty string, so that is not too bad. 03:20.920 --> 03:23.150 But this is really weird. 03:27.330 --> 03:31.920 Let's try and make this look a bit better. So let's add a constructor, which takes some arguments. 03:41.690 --> 03:44.510 So we are going to initialize the members from these arguments. 03:46.980 --> 03:48.740 Then we do not need to do anything else. 03:50.390 --> 03:54.560 And then we can give these some sensible initial values. 04:05.310 --> 04:10.290 So we initialize them with a string literal. We could just use a character string, but we might as well get used 04:10.350 --> 04:15.600 to using the newer versions. And let's see what we get now. 04:16.800 --> 04:17.460 So that's better. 04:18.600 --> 04:23.310 (So that is not actually.. The, er...) The output is slightly wrong. That is not the default constructor anymore. 04:23.580 --> 04:26.220 That is now calling the constructor, which takes these arguments. 04:27.330 --> 04:29.960 So "i" is 5 and "str" is "Hello". 04:30.900 --> 04:33.030 And then test2 has 7 and "Goodbye". 04:34.750 --> 04:41.530 And test3 is the same as test. And then we overwrite test with test2, so we get 7 and 04:41.530 --> 04:41.860 "Goodbye". 04:43.750 --> 04:50.050 This ability of the compiler to synthesize special member functions is quite useful. If we have a class 04:50.050 --> 04:56.470 and we do not need the constructor, destructor, et cetera, to do anything special, then we can just 04:56.470 --> 05:01.870 leave them. And the compiler will automatically provide ones which do the right thing. 05:02.320 --> 05:08.290 On the other hand, if we do need to implement them and we do not implement them, the compiler will 05:08.290 --> 05:10.030 not give us an error or a warning. 05:10.360 --> 05:13.630 Instead, it will replace them with ones which do the wrong thing. 05:14.740 --> 05:19.960 For example, if we have members which are built-in types, they will be default initialized and we 05:19.960 --> 05:20.980 will get the wrong values. 05:21.880 --> 05:28.960 If we have a member which is a pointer to allocated memory or a resource handle, then the compiler's 05:29.140 --> 05:34.540 copy constructor and assignment operator will perform what is known as a "shallow" copy. 05:34.960 --> 05:38.620 So this means they just overwrite the data in the target object. 05:42.130 --> 05:48.310 In this case, we need to provide the functions ourselves to make sure that the correct operations are 05:48.310 --> 05:48.850 performed. 05:49.090 --> 05:54.850 If we need the built-in types to have correct default values, we need to provide a default constructor which will 05:54.850 --> 06:03.580 initialize them. If we need to perform a so-called "deep copy", which will allocates new memory, obtained 06:03.580 --> 06:09.490 a new resource for the target object and then initialize it with the same data as the source object. 06:10.030 --> 06:13.570 Then we need to implement the copy constructor and the assignment operator. 06:14.710 --> 06:17.800 We are going to look at shallow and deep copying in more depth. 06:19.000 --> 06:21.730 (Get it? Deep copy? Depth? Never mind!) 06:22.450 --> 06:25.590 We are going to look at that in more detail in the next video. 06:25.660 --> 06:27.430 But until then, keep coding!