WEBVTT 00:01.020 --> 00:06.960 Hello again! In this video, we are going to have an overview of the assignment operator. The assignment 00:06.960 --> 00:08.520 operator is a member function. 00:08.520 --> 00:10.620 It is called "operator equals". 00:11.430 --> 00:18.060 The assignment operator is automatically called when we assign to existing objects of the same class. 00:18.660 --> 00:25.440 So if we have y = z, and y and z are both objects of class Test, for example. 00:27.400 --> 00:34.030 It is a member function, so it is called on the object on the left of the equals sign, and the object 00:34.030 --> 00:36.640 on the right is passed as the argument to the call. 00:37.180 --> 00:42.010 So this will actually be called as, y dot operator equals, with argument 00:42.040 --> 00:42.370 z. 00:44.560 --> 00:50.860 After the statement has been executed, the members of y, the data members, will have the same values 00:50.860 --> 00:54.790 as the data members in z. And z will be unmodified, of course. 00:56.440 --> 01:03.520 The assignment operator takes its argument z by const reference, and it returns the modified object 01:03.820 --> 01:07.540 so y equals z will return the new value of y. 01:09.750 --> 01:14.940 The reason for returning a value is that we can do multiple assignments in C++. 01:15.480 --> 01:21.210 We are allowed to "chain" operators. And we have already used that when we are outputting several variables 01:21.210 --> 01:22.170 in a single statement. 01:22.500 --> 01:24.780 We chain the left shift operators. 01:26.670 --> 01:29.070 So we can say x equals y equals z. 01:29.430 --> 01:33.300 So x and z will then have the same values as z. 01:34.710 --> 01:37.220 This is processed from right to left. 01:37.260 --> 01:39.450 So it starts by doing y equals z. 01:40.530 --> 01:43.330 So that will call y dot operator equals, with argument 01:43.330 --> 01:43.350 z. 01:44.370 --> 01:46.590 And this will return the new value of y. 01:48.060 --> 01:52.980 Then the new value of y is used as the argument to the next assignment. 01:53.460 --> 01:56.520 So X equals the new value for it. 01:57.360 --> 02:03.980 So this will actually be called as x dot operator equals, and the argument will be the result of y dot operator 02:04.140 --> 02:10.440 equals with argument z, and then the members of x and y will have the same values as z. 02:13.570 --> 02:16.510 The assignment operator actually returns by reference. 02:17.890 --> 02:22.470 That may sound a bit dangerous, but it is actually safe, because it is a reference to the object that 02:22.510 --> 02:27.670 the member function is called on. And that object will still be valid after the member function returns. 02:28.870 --> 02:33.220 You will find some sources which say it should be a const reference, but that is not correct. 02:34.210 --> 02:40.300 It is legal to modify the return value from an assignment, and there are occasions, very rare, when you want 02:40.300 --> 02:40.870 to do that. 02:42.160 --> 02:46.360 In particular, the standard library containers require that you are able to modify it. 02:47.230 --> 02:53.830 If you try to store an object in a container and the assignment operator returns a const reference instead 02:53.830 --> 02:57.190 of a modifiable reference, the code will not compile. 02:58.710 --> 03:04.530 So the correct signature for the assignment operator is operation equals, a member of the class Test. 03:05.370 --> 03:13.170 It will take a Test object by reference to const, and it will return a modifiable reference to a Test object. 03:18.740 --> 03:22.460 So how do you get a reference to the object? 03:23.210 --> 03:29.090 Well, the operator is a member function, so when it is called, it will be given an extra hidden argument, 03:29.090 --> 03:35.090 which is a pointer called "this". And "this" is a pointer to the object that the member function was called 03:35.090 --> 03:35.270 on. 03:36.110 --> 03:42.530 So if we dereference the "this" pointer, after we perform the assignments, then we will get the modified 03:42.530 --> 03:43.010 object. 03:44.570 --> 03:50.330 If we return star this, then we are returning the object. And the compiler will make sure that it is returned 03:50.330 --> 03:56.000 by reference, because the function says it's to be returned by reference. The compiler will handle that bit 03:56.000 --> 03:56.180 for us. 03:59.840 --> 04:05.000 So the rules for when you write an assignment operator are pretty similar to the rules for writing 04:05.000 --> 04:05.960 a copy constructor. 04:06.350 --> 04:07.820 Normally you do not need to bother. 04:08.350 --> 04:13.100 The compiler will synthesize one which calls the assignment operator of all the data members of the 04:13.100 --> 04:13.490 class. 04:14.210 --> 04:15.770 And usually, that is going to be good enough. 04:17.950 --> 04:22.150 There are situations in which that is not going to give the correct behaviour. 04:22.390 --> 04:25.000 And in that case, you do need to write an assignment operator. 04:25.660 --> 04:28.120 And again, it is when the class manages a resource. 04:28.600 --> 04:32.710 But the reasons are slightly different. With an assignment operator, 04:32.740 --> 04:37.300 we have two objects which are already set up and have their own resources. 04:38.170 --> 04:44.560 If we just copy the resource from one class over the resource in the other class, then the original resource 04:44.560 --> 04:45.040 is lost. 04:45.430 --> 04:51.190 For example, if we are dealing with allocated memory and we have a pointer to the memory, if we overwrite 04:51.190 --> 04:57.400 the pointer in the target object, then that pointer is now lost. The original value of the pointer. 04:57.880 --> 05:03.760 So that means we can no longer find the memory, and in particular, we cannot release the memory. So 05:03.790 --> 05:06.280 that memory is still going to be in use, but not available. 05:06.520 --> 05:07.930 So that is a memory leak. 05:08.680 --> 05:14.050 And similarly, with a file handler or a database connection, the handle or the connection is still 05:14.050 --> 05:16.150 in use, but it is no longer available. 05:18.440 --> 05:23.090 The correct way to handle this is to write an assignment operator, and the assignment operator will 05:23.150 --> 05:25.700 release the resource before doing the assignment. 05:26.240 --> 05:30.740 And then when it does the assignment, it will get its own copy of the resource. Which has the same 05:30.740 --> 05:33.620 data as the one that it is being assigned from. 05:34.280 --> 05:39.200 So in effect, this has the same effect as calling the destructor and then calling the copy constructor. 05:39.650 --> 05:44.700 Calling the destructor would release the resource, and calling the copy constructor 05:44.700 --> 05:49.910 would give the target object a new resource, but it has the same values as the one that it is being 05:49.910 --> 05:50.600 assigned from. 05:54.800 --> 05:58.100 As an example of a synthesized assignment operator. 05:58.550 --> 06:03.940 So this is the kind of code that the compiler will generate if we do not do anything. For our class 06:03.950 --> 06:04.370 Test, 06:05.060 --> 06:10.640 we have the assignment operator signature. And then it will just assign the members. 06:11.030 --> 06:14.450 So "i" will be assigned from the "i" value in the argument. 06:15.200 --> 06:18.170 The string will be assigned from the string member in the argument. 06:18.890 --> 06:25.190 And then we dereference the object that we are called on, and we return it, and the compiler will return 06:25.190 --> 06:26.210 that by reference. 06:28.300 --> 06:34.150 So let's add an assignment operator to our String class with a capital "S". We do not actually need one 06:34.150 --> 06:39.250 at the moment, but it will be good practice to get used to writing assignment operators. 06:39.520 --> 06:42.100 And we will need one when we make the class more realistic. 06:43.630 --> 06:49.720 So this is the signature. We have operator equals, which takes a const reference to an object of 06:49.720 --> 06:52.900 the class, and it returns an object by reference. 06:53.800 --> 06:55.060 Then we assign the members. 06:55.450 --> 07:03.070 So this will actually call the assignment operator of the string class and then we get the object 07:03.070 --> 07:09.400 that we were called on, and we return that. And the rest of the code is almost the same. 07:10.030 --> 07:15.430 We have a String object, but this time we are going to assign it to it, instead of copy creating it. 07:16.870 --> 07:20.350 In fact, one thing we should do in here is to add a default constructor. 07:21.010 --> 07:27.190 So this is a default constructor which takes no arguments, and it is just going to initialize the member 07:27.190 --> 07:28.870 string as an empty string. 07:32.120 --> 07:34.220 So we can actually make that an empty string... 07:54.030 --> 07:59.910 OK, so we have our string with "world", then we have a string with "bang". We assigned "world" to it, and 07:59.910 --> 08:02.250 now the string's data is "world". 08:02.790 --> 08:06.650 And then we have an empty string, and we assign "world" to that. 08:06.660 --> 08:09.240 And the data in the string is now "world". 08:10.200 --> 08:11.190 OK, so that all works. 08:11.970 --> 08:13.560 So that is it for this video. 08:14.010 --> 08:14.910 I will see you next time. 08:14.910 --> 08:17.400 But until then, keep coding!