WEBVTT 00:00:00.640 --> 00:00:05.740 Copy semantics allow us to make deep copies of objects, but in some situations, 00:00:05.750 --> 00:00:09.160 we would get a better performance from a shallow copy instead. 00:00:09.540 --> 00:00:12.080 Let's get back to genetic engineering. 00:00:12.120 --> 00:00:16.160 Here I have two new subjects, a chicken and a lizard. 00:00:17.040 --> 00:00:19.280 I want to have anzu as a subject, 00:00:19.290 --> 00:00:24.260 but I don't have its genetic material so I just declared it as an empty object. 00:00:24.740 --> 00:00:26.000 In case you don't know, 00:00:26.000 --> 00:00:29.190 anzu is some kind of a dinosaur that looked like a 00:00:29.190 --> 00:00:31.550 combination of a chicken and a lizard. 00:00:31.940 --> 00:00:34.640 So since dinosaurs don't exist anymore, 00:00:34.650 --> 00:00:37.570 I figured that I could combine these two subjects to make 00:00:37.570 --> 00:00:40.850 one because that's how biology works, right? 00:00:41.740 --> 00:00:42.790 And of course, 00:00:42.790 --> 00:00:45.760 this will not work because we didn't define a plus 00:00:45.760 --> 00:00:48.060 operator for the Subject class. 00:00:48.940 --> 00:00:50.260 Let's do that now. 00:00:52.440 --> 00:00:57.560 The subject_id of this new object will be the sum total of these two objects. 00:00:58.540 --> 00:01:01.250 Then I will create an array of four characters, 00:01:01.250 --> 00:01:05.560 which will replace the string literal we usually pass to create new subjects. 00:01:05.940 --> 00:01:08.490 The genetic code of this new subject will be the 00:01:08.490 --> 00:01:11.550 combination of sequences from both objects. 00:01:11.650 --> 00:01:14.450 The first two bases are taken from the chicken, 00:01:14.460 --> 00:01:17.060 and the next two are taken from the lizard. 00:01:17.440 --> 00:01:18.460 And finally, 00:01:18.460 --> 00:01:21.110 we can use these variables to instantiate a new 00:01:21.110 --> 00:01:23.860 subject and return it from the function. 00:01:24.640 --> 00:01:29.160 Notice that I made this reference constant, and now you know why. 00:01:30.140 --> 00:01:34.330 Because in case someone decides to add more than two subjects, 00:01:34.340 --> 00:01:36.260 this operator would still work. 00:01:36.740 --> 00:01:40.860 The subject we returned from this operator function is a prvalue, 00:01:40.860 --> 00:01:44.650 because it has no identity yet. It is just a new, 00:01:44.660 --> 00:01:45.960 unassigned object. 00:01:47.340 --> 00:01:50.410 And if we didn't make this parameter constant, 00:01:50.420 --> 00:01:54.890 then this new operator call would not be able to accept it because 00:01:54.890 --> 00:01:58.650 non‑constant references only accept L values. 00:01:59.440 --> 00:02:04.260 Okay, and now this operator is finished, so let's get back to the main function. 00:02:05.040 --> 00:02:07.100 When this expression gets evaluated, 00:02:07.110 --> 00:02:11.460 a new subject will be created and assigned to this anzu object. 00:02:11.840 --> 00:02:16.080 But remember that this expression alone will result in a temporary object, 00:02:16.080 --> 00:02:17.820 which is a PR value. 00:02:17.830 --> 00:02:19.660 When I compile this program, 00:02:19.660 --> 00:02:23.560 the copy assignment operator will make a deep copy of this object. 00:02:24.240 --> 00:02:26.070 To make sure that this actually happens, 00:02:26.080 --> 00:02:29.660 I will add a special message to the copy constructor and 00:02:29.670 --> 00:02:32.160 also to the copy assignment operator. 00:02:32.840 --> 00:02:36.350 This way we will know when one of them is used by the program. 00:02:37.140 --> 00:02:38.550 And now, let's compile it. 00:02:39.540 --> 00:02:41.200 It all works as expected. 00:02:41.200 --> 00:02:44.050 The copy assignment will make a deep copy of an object, 00:02:44.060 --> 00:02:46.950 and anzu will have a new genetic sequence. 00:02:47.840 --> 00:02:51.050 But why do we need a deep copy in this situation? 00:02:51.050 --> 00:02:54.990 After all, the result of this addition is a temporary object, 00:02:54.990 --> 00:02:58.860 which will disappear as soon as the program goes past this statement. 00:02:59.240 --> 00:03:02.220 So why don't we just steal the sample pointer from this 00:03:02.220 --> 00:03:05.020 prvalue object and give it to anzu, 00:03:05.020 --> 00:03:09.890 the dinosaur? Working with temporary prvalues in this way will 00:03:09.890 --> 00:03:12.840 improve the performance of the whole program. 00:03:12.850 --> 00:03:14.540 To define this special behavior, 00:03:14.540 --> 00:03:19.260 we would have to know when this assignment operator works with prvalues. 00:03:19.640 --> 00:03:22.960 Fortunately, we learned this in the previous lesson. 00:03:23.340 --> 00:03:26.210 To define a special behavior for temporary values, 00:03:26.220 --> 00:03:30.450 we can overload a function, which will take in an rvalue reference. 00:03:31.840 --> 00:03:35.260 In this case, we need to overload the assignment operator. 00:03:36.340 --> 00:03:39.220 The definition will look similar to the copy assignment, 00:03:39.230 --> 00:03:41.980 but instead of making a copy of this genetic code, 00:03:41.990 --> 00:03:46.450 we will just steal a pointer to the genetic code of this temporary object. 00:03:47.440 --> 00:03:50.950 In other words, this is just like a shallow copy. 00:03:51.340 --> 00:03:55.450 Of course, we need to again check if this subject is assigned to itself, 00:03:55.460 --> 00:03:58.750 and also return the object itself as a reference. 00:03:59.540 --> 00:04:00.650 Nothing new here. 00:04:01.240 --> 00:04:04.150 Before stealing the address from the other object, 00:04:04.160 --> 00:04:07.700 I also need to delete the DNA object at which this sample 00:04:07.700 --> 00:04:10.780 pointer points to because if I don't do that, 00:04:10.790 --> 00:04:12.240 we would have a memory leak. 00:04:12.940 --> 00:04:16.149 And finally, when we steal everything from this object, 00:04:16.160 --> 00:04:22.070 I will completely destroy it by setting its id to 0 and its sample to a nullptr. 00:04:22.370 --> 00:04:25.160 This is done to make sure that some other part of the program 00:04:25.160 --> 00:04:28.410 doesn't mess with this temporary object after we steal its 00:04:28.410 --> 00:04:32.460 pointer because we don't want to share the ownership or the DNA 00:04:32.460 --> 00:04:34.960 object under any circumstances. 00:04:35.340 --> 00:04:37.840 Maybe that seems unlikely in this situation, 00:04:37.840 --> 00:04:42.540 but you will see that we can also use this operator on values with identity. 00:04:42.550 --> 00:04:43.860 But more on that later. 00:04:44.640 --> 00:04:46.620 The assignment operator, like this one, 00:04:46.620 --> 00:04:50.290 which uses an rvalue reference to steal from temporary object, 00:04:50.300 --> 00:04:53.760 is also known as a move assignment operator. 00:04:54.740 --> 00:04:58.870 Creating a shallow copy of an object, which we then invalidate, 00:04:58.880 --> 00:05:03.650 can be seen as moving all of the resources from one object to another. 00:05:04.340 --> 00:05:08.750 Move assignment operator is part of the so‑called move semantics. 00:05:09.840 --> 00:05:13.330 It is a good practice to mark this operator with noexcept 00:05:13.340 --> 00:05:16.350 to let the other parts of the program know that this 00:05:16.350 --> 00:05:18.860 function is resilient to exceptions. 00:05:19.240 --> 00:05:24.200 And this is not only a good practice, I urge you to do this because if you don't, 00:05:24.210 --> 00:05:28.430 some STL containers will not be able to use this operator properly. 00:05:28.440 --> 00:05:30.060 But more on this later. 00:05:30.640 --> 00:05:33.760 Now let's compile this program to see if it works. 00:05:34.440 --> 00:05:35.480 And it does. 00:05:35.490 --> 00:05:38.640 You can see that the move assignment operator was used 00:05:38.650 --> 00:05:40.560 instead of the copy alternative. 00:05:41.240 --> 00:05:44.520 We know that copy semantics assume the existence of the copy 00:05:44.520 --> 00:05:47.190 constructor and copy assignment operator, 00:05:47.200 --> 00:05:51.950 so to implement move semantics, we also need to create a move constructor. 00:05:52.540 --> 00:05:56.740 I will move this expression up to the declaration of the object so that we can 00:05:56.750 --> 00:06:00.360 immediately initialize it by the result of this combination. 00:06:01.240 --> 00:06:03.780 This should use the constructor because this 00:06:03.780 --> 00:06:06.070 statement does an implicit conversion. 00:06:06.080 --> 00:06:09.160 It is actually making a call to the copy constructor. 00:06:10.540 --> 00:06:15.640 But let's overload this constructor to make one that works with prvalues. 00:06:15.650 --> 00:06:18.460 In other words, let's make a move constructor. 00:06:19.940 --> 00:06:23.090 Move constructor will receive an rvalue reference, 00:06:23.100 --> 00:06:27.260 and I will also mark it as a function which throws no exceptions. 00:06:27.640 --> 00:06:29.290 Just like in the assignment, 00:06:29.300 --> 00:06:33.960 we are making a shallow copy by stealing the values from all data members. 00:06:34.340 --> 00:06:35.140 But of course, 00:06:35.140 --> 00:06:38.320 we cannot forget to also destroy this temporary object 00:06:38.320 --> 00:06:41.360 by invalidating all of its resources. 00:06:42.240 --> 00:06:43.120 And that's it. 00:06:43.130 --> 00:06:44.160 Nothing special. 00:06:45.340 --> 00:06:47.210 If I compile this program now, 00:06:47.220 --> 00:06:50.950 the move constructor should be used to create the anzu object, 00:06:51.940 --> 00:06:54.410 but it isn't, and to make things worse, 00:06:54.410 --> 00:06:58.890 copy constructor was not used either because if it was, we 00:06:58.890 --> 00:07:02.610 would see its special message. Join me in the next lesson 00:07:02.610 --> 00:07:04.150 to see why this happened.