WEBVTT 00:00.300 --> 00:00.870 Hello again! 00:01.050 --> 00:04.410 And in this video, we are going to look at the default and delete keywords. 00:04.740 --> 00:07.630 And yes, we are still talking about special member functions. 00:07.950 --> 00:10.470 We have not suddenly started doing memory management. 00:12.290 --> 00:18.620 We have seen that the compiler will sometimes synthesize special member functions. In modern C++, 00:18.620 --> 00:23.060 the rules are more complicated, and it can be hard to work out what the compiler is doing. 00:23.450 --> 00:29.930 So now we have the ability to force the compiler to synthesize a special menu function. And the syntax 00:29.930 --> 00:33.680 for doing that is to put "= default" after the function's parameter list. 00:36.060 --> 00:41.190 For example, if we have the default constructor and then we put equals default, that will force 00:41.190 --> 00:46.590 the compiler to synthesize the default constructor, even if there are other constructors or a 00:46.590 --> 00:47.940 copy constructor for the class. 00:49.530 --> 00:55.410 If we put copy constructor equals default, that will force the compiler to synthesize a copy constructor, 00:55.410 --> 00:58.470 which copies all the members. In modern C++, the compiler 00:58.470 --> 01:01.590 may not always synthesize the copy constructor. 01:03.300 --> 01:05.190 And similarly, with the assignment operator. 01:05.550 --> 01:10.890 So we have a synthetic assignment operator which will assign all the members. And there are others. We 01:10.890 --> 01:14.250 could put the destructor in there, but that does nothing anyway. 01:14.260 --> 01:15.960 So it is a bit pointless. 01:19.170 --> 01:25.530 If you can remember the first example in the video on copy elision, we had to provide a default 01:25.530 --> 01:28.380 constructor because the compiler would not synthesize one. 01:28.830 --> 01:33.750 That was because we had a copy constructor. So we actually wrote it out, but we could equally well 01:33.750 --> 01:35.220 have put equals default. 01:37.970 --> 01:42.980 And then the compiler will synthesize a constructor which default initializes all the members. 01:43.640 --> 01:46.100 And that works exactly as it did before. 01:47.780 --> 01:49.280 Why is this useful? 01:49.610 --> 01:54.560 Well, you can look at the class definition and see immediately which special member functions the 01:54.560 --> 01:57.020 class has implemented, or defined for it. 01:58.040 --> 02:02.570 If you just let the compiler synthesize things, then you have to be able to work out what the compiler 02:02.570 --> 02:04.960 has done, and that can take some effort. 02:07.000 --> 02:12.130 It also documents that the programmer who wrote that class did actually want the default behaviour. 02:12.490 --> 02:14.770 And it is not just the case that they forgot to implement it. 02:16.840 --> 02:18.880 It avoids writing tedious code. 02:18.880 --> 02:23.350 If you do have to implement the copying instructor because the compiler will not synthesize one, then 02:23.350 --> 02:27.760 you have to write a member function, which copy initializes or assigns all the members. 02:28.450 --> 02:30.070 With this, the compiler will do it for you. 02:30.550 --> 02:36.550 And also, if the class definition changes, if we add or remove data members, then the compiler will 02:36.550 --> 02:39.820 make sure that the member functions are always up to date. 02:43.800 --> 02:48.660 Sometimes it is useful to have objects which cannot be copied. The traditional way to do this is to 02:48.660 --> 02:52.080 make the copy constructor and the assignment operator private. 02:52.680 --> 02:58.440 So for things like cout or cin, they cannot be copied because presumably the computer only 02:58.440 --> 03:00.750 has one output and one keyboard! 03:03.180 --> 03:12.330 So here is that code. If we do a copy operation, we expect to get a compile error. "Cannot access 03:12.330 --> 03:14.340 private members declared in class Test". 03:19.980 --> 03:22.020 And the same for the assignment operator. 03:22.410 --> 03:24.450 So this works, but it is a bit of a hack, really. 03:24.840 --> 03:28.260 And also, if you are looking through the code, you might miss that these are actually private. 03:29.970 --> 03:36.090 In Modern C++, we can say that the copy constructor and the assignment operator are "= delete", so 03:36.090 --> 03:36.780 they are deleted. 03:40.520 --> 03:42.440 So again, this will give us an error. 03:45.010 --> 03:51.080 But it is a different error this time. "Attempting to reference a deleted function. Function was explicitly 03:51.080 --> 03:56.990 deleted". So you can see from looking at this, that these functions are not meant to be called. 03:57.620 --> 04:04.670 And if we try to do an assignment, then we get the same result as well. Yes, "Attempting to reference 04:04.670 --> 04:05.690 a deleted function". 04:08.300 --> 04:13.160 So what does this actually mean? When we delete a function, it is still actually defined, but it 04:13.160 --> 04:13.940 cannot be called. 04:14.420 --> 04:17.270 And if you do try to call it, the compiler will give an error. 04:17.930 --> 04:24.050 And this is different from default, which only applies to special functions. For a special member function, 04:24.050 --> 04:28.520 it' i pretty easy for the compiler to guess what should be in there. If it is some other function than 04:28.520 --> 04:29.960 the compiler has no idea at all. 04:31.100 --> 04:36.500 With delete, any function can be deleted. So any member function or even non-member functions. 04:38.320 --> 04:43.240 This can be useful sometimes if you want to prevent conversions or overloads from being caught. 04:44.200 --> 04:47.230 But the main use is for copy constructors and assignment operators. 04:47.710 --> 04:53.080 If you delete the default constructor, then you cannot default construct any objects. 04:53.800 --> 04:55.480 And sometimes that can be useful as well. 04:55.660 --> 04:59.740 If you want to force users of your class to call a constructor which takes arguments. 05:02.380 --> 05:09.970 And then finally, one limitation on using the "=default". Sometimes the compiler cannot actually 05:09.970 --> 05:11.920 synthesize the special member function. 05:12.670 --> 05:16.660 The usual reason is that the class has a member which does not support that operation. 05:17.140 --> 05:19.390 So that would be a member, which is another class. 05:19.390 --> 05:21.670 And for example, it cannot be default constructed. 05:22.480 --> 05:28.600 So if the member cannot be default constructed, then the compiler cannot synthesize your own default 05:29.050 --> 05:32.240 constructor. Something that is a bit less obvious. 05:32.770 --> 05:38.590 If the class has a member with a destructor which is deleted or inaccessible. So the destructor for 05:38.590 --> 05:42.940 that class cannot be called, and that means you cannot destroy objects of that class. 05:44.050 --> 05:49.300 And even for C++, creating objects which cannot be destroyed is probably a bit too far! 05:49.450 --> 05:50.410 So that is not allowed. 05:51.970 --> 05:58.360 So the result of all this is, if the compiler cannot synthesize the operation, then it will be synthesized 05:58.360 --> 05:59.320 as "=delete". 06:00.040 --> 06:01.810 So it will still be there, but you cannot call it. 06:03.110 --> 06:05.020 Okay, so that is it for this video. 06:05.590 --> 06:08.710 I will see you next time, but until then, keep coding!