WEBVTT 00:00.300 --> 00:05.970 Hello again! In this video, we are going to have an overview of the copy constructor. The copy constructor 00:05.970 --> 00:07.650 is a special member function. 00:07.920 --> 00:10.590 It is actually a specialized version of the constructor. 00:11.310 --> 00:17.430 The constructor gets called when we create a new object, and it can be passed any number and type of 00:17.430 --> 00:17.910 arguments. 00:18.480 --> 00:19.440 It can even be passed 00:19.440 --> 00:22.560 no arguments at all, in which case it is the default constructor. 00:23.700 --> 00:29.160 The copy constructor, on the other hand, can only take one argument, which is an object of the same 00:29.160 --> 00:29.550 class. 00:30.810 --> 00:37.260 So when the new object is created, it will be initialized to have the same values as the other object. 00:37.680 --> 00:40.530 This other object is passed as an argument by reference. 00:40.980 --> 00:46.740 So if we have a Test class, it will take a Test object by reference, and that should usually be a 00:46.740 --> 00:49.500 reference to const, as that is more efficient. 00:49.800 --> 00:53.610 And also, it means you cannot accidentally change the thing that you are copying from. 00:56.800 --> 01:02.710 If we initialize a new object from an existing object, the copy constructor will automatically be called. 01:03.550 --> 01:09.730 So if we have some test1 object which we create and we call the constructor with arguments x and 01:09.730 --> 01:16.120 y, for example. Then if we have a new test2 object, which is initialized from test1, that 01:16.120 --> 01:21.430 will call the copy constructor, and the data in test2 will have the same values as the data members in 01:21.430 --> 01:21.970 test1. 01:23.890 --> 01:26.230 Something that looks a bit confusing at first. 01:26.920 --> 01:31.780 If you have an equals sign in here, but you are creating a new object, that will also call the copy 01:31.810 --> 01:35.210 constructor, because that is not an assignment of an existing object. 01:35.620 --> 01:37.750 It is creating a completely new object. 01:39.490 --> 01:43.510 So test3 will be a new object whose members have the same values as test1. 01:48.200 --> 01:52.010 The main situation where the copy constructor is used is in function calls. 01:52.610 --> 01:59.390 If we pass an argument by value, the called function will have a new object created on its stack, and 01:59.390 --> 02:01.160 that will be a copy of the argument. 02:01.580 --> 02:07.000 So the copy constructor will be invoked there, to make the new object a copy of the argument. 02:08.150 --> 02:11.180 Also see when we return a local variable by value. 02:11.690 --> 02:17.630 The function has a return space where it stores the return value, and the new object will be created 02:17.630 --> 02:21.530 in that return space. Which is a copy of the variable that is being returned. 02:23.360 --> 02:27.950 However, the copy constructor does not get called every time we pass or return something by value. 02:28.340 --> 02:32.960 There are some optimizations that compilers are allowed to do, which will reduce the amount of copying. 02:33.380 --> 02:35.840 And we will look at those later on, in a different video. 02:36.740 --> 02:39.680 So the copy constructor may not always be called when you think it should be. 02:42.020 --> 02:43.880 In C++, there is always an exception! 02:47.270 --> 02:52.700 If we do not actually write a copy constructor for the class, the compiler will generate one for 02:52.700 --> 02:58.280 us. That is called a synthesized copy constructor and this will just call the copy constructor for all 02:58.280 --> 02:58.790 the members. 02:59.300 --> 03:03.200 If you have a member which is an object, it will call the copy constructor for that class. 03:03.830 --> 03:06.620 If you have a member which is a built-in type, it will copy it. 03:07.430 --> 03:11.210 And for the large majority of classes, that will be good enough. 03:12.050 --> 03:16.430 There are occasionally situations where that does not give the right behaviour, and then you need 03:16.430 --> 03:18.350 to write your own copy constructor. 03:19.370 --> 03:23.720 The usual case is when you have a class that manages a resource of some kind. So that could be 03:23.720 --> 03:27.830 allocated memory, a thread object, a database connection. 03:28.190 --> 03:33.770 If you just do a naive copy, then both objects end up owning the same resource, and they are going to 03:33.770 --> 03:34.760 conflict with each other. 03:36.290 --> 03:41.300 So you want to make sure that each object has its own version of the resource. And to do that, the 03:41.300 --> 03:47.570 new object has to acquire the resource for itself, and then initialize it from the object that it is copying. 03:50.530 --> 03:52.780 So we will see an example of that later on. 03:53.170 --> 03:59.920 But for the synthesized copy constructor, if we have a class Test which has two members, an int and 03:59.920 --> 04:04.280 a string, then the compiler will generate a copy constructor, which looks like this. 04:04.540 --> 04:06.790 You cannot actually see it in the code, but it will be there. 04:07.600 --> 04:10.000 So this will be the copy constructor for the class Test. 04:10.420 --> 04:12.850 It will take some Test object by reference to const. 04:13.540 --> 04:19.600 It will initialize the "i" member from the argument's "i" member and the string from the argument's string. 04:21.400 --> 04:25.600 One of these things we are going to do throughout this course is to build up our own string 04:25.600 --> 04:30.970 class. It will be String with a capital "S", so it does not get mixed up with the library string class. 04:31.480 --> 04:38.110 And we will build up this string class as we learn more C++ features. To start off with, this is going to be 04:38.110 --> 04:41.110 very simple, because we do not really know all that many features. 04:41.530 --> 04:47.020 We're going to use a library string as a member and then delegate everything to that. 04:49.170 --> 04:54.180 We are going to have a constructor which will take a library string and initialize our member from 04:54.180 --> 04:56.130 that. The copy constructor 04:56.160 --> 05:01.950 will take an object of the same type by reference. As opposed to the constructor, which took an 05:01.950 --> 05:03.180 object of a different type. 05:04.170 --> 05:10.300 (Different spelling. C++ is case sensitive!) And then in the copy constructor, 05:10.320 --> 05:17.370 we are going to initialize our string member from the string member of the argument. And then we have 05:17.370 --> 05:20.250 a function which is going to print out the data in our string member. 05:22.410 --> 05:26.250 In our main function, we are going to create some objects of our class. 05:26.760 --> 05:30.120 So we are going to use a string literal to initialize it. 05:30.780 --> 05:32.100 Then we call the copy constructor. 05:32.730 --> 05:35.800 Then we call it again, using the syntax with an equals sign. 05:36.300 --> 05:38.400 And then we check that we have the data that we expect. 05:39.300 --> 05:41.100 So let's try this out. 05:45.020 --> 05:45.950 So there we are. 05:45.980 --> 05:52.250 We initialized a string with a library string, "world" and we can print out "world", so that works. 05:53.420 --> 05:57.500 We have initialized our string with the same data, in two different ways. 05:58.640 --> 06:03.680 OK, so that is just the beginning of our string class, with a capital "S". It will get more interesting, 06:03.680 --> 06:04.190 I promise! 06:04.670 --> 06:06.470 But that is all for this video. 06:06.890 --> 06:07.730 I will see you next time. 06:07.730 --> 06:09.620 But meanwhile, keep coding!