WEBVTT 00:01.740 --> 00:09.200 Hello again! In this video, we're going to go over classes. A class is a compound data structure. 00:12.480 --> 00:15.570 Classes can have member functions as well as data members. 00:16.230 --> 00:19.350 So this is basically how we do object oriented programming. 00:19.650 --> 00:26.280 We have data objects which know how to manipulate themselves. And we do that by calling a function. 00:27.480 --> 00:30.060 And these member functions can have any names we like. 00:30.840 --> 00:34.050 They can do anything we want and we can choose when to call them. 00:35.640 --> 00:39.270 So these member functions define how the objects of the class behave. 00:40.530 --> 00:43.650 And when we call a member function, it performs an operation on the [object]. 00:46.580 --> 00:52.280 By default, the class members are private, which means they can only be accessed by objects of that 00:52.280 --> 00:59.960 class. Although you can actually access the private member of a different object of the same class. That's 00:59.960 --> 01:01.850 needed so you can copy data. 01:03.290 --> 01:08.200 You can make a class member public, which means it can be accessed by any code in the program. 01:08.210 --> 01:12.200 So if you have a public member function, it can be called by anything that has an object. 01:21.240 --> 01:22.980 The public members are the interface of the class. 01:23.490 --> 01:26.820 So this represents the class to the rest of the program. 01:27.270 --> 01:30.570 So this defines everything that the class can do. 01:31.770 --> 01:37.010 So that's the interface of the class, the private members provide the implementation of the class, 01:37.020 --> 01:38.970 so the actual details of how it does it. 01:39.780 --> 01:44.370 So as an example, if you have a phone, then you have the screen on the phone; you can interact with 01:44.370 --> 01:46.690 the phone, you can make it do different things. 01:47.820 --> 01:54.450 But the actual details of how the phone takes a phone call or finds things on the internet or responds 01:54.450 --> 01:56.800 when you tap on it, that's all hidden away from you. 01:58.810 --> 02:03.550 And finally, there's also a struct which is the same as class, except all the members are public by 02:03.550 --> 02:10.870 default. So normally, you would use a struct when you have some related data items that you want to keep together. 02:11.260 --> 02:16.390 So normally you wouldn't use the functions for the struct unless you're writing a throw-away class and you're 02:16.390 --> 02:17.200 being a bit lazy :) 02:18.310 --> 02:22.660 So for example, if you get some data from a database, you could have a struct, and each member of 02:22.660 --> 02:25.720 the struct would represent a column in that database. 02:30.840 --> 02:37.440 This is the Test class. You're going to be seeing rather a lot of this! We're actually going to implement the functionality 02:37.680 --> 02:38.520 of a string. 02:40.140 --> 02:46.200 But as we don't know how to do that just yet, we're going to start off by using the library string and delegating 02:46.200 --> 02:46.920 everything to that. 02:48.360 --> 02:50.440 So this has private data members. 02:50.940 --> 02:55.300 So these are the actual details of how the Test class stores its data. 02:55.800 --> 02:57.740 And then there'll be some public member functions. 02:57.750 --> 03:00.180 So we could use these to get the data from the strings, 03:00.180 --> 03:05.100 for example. Or we could perform some more interesting manipulations on it. 03:11.430 --> 03:14.880 A member function is actually implemented as a global function. 03:15.390 --> 03:18.510 So it's just a global function, which has the name of the class in front of it. 03:19.050 --> 03:23.940 So if you have a function called func, it's going to be something called test double colon func 03:23.940 --> 03:26.730 in the actual program binary code. 03:28.620 --> 03:33.000 When you call the member function, you have to provide an object, and that object will actually be 03:33.000 --> 03:35.490 passed by address in a hidden argument. 03:36.270 --> 03:44.700 So if we call func with arguments one, two and three on an object test, it'll actually have the test 03:44.700 --> 03:48.480 object passed by address as the first argument to the function call. 03:48.870 --> 03:50.970 And then the arguments that you give will come after that. 03:54.790 --> 04:00.550 (Up here again!) In the function body, you can access this object through the pointer. 04:00.910 --> 04:05.440 It's called "this", so "this" is a pointer to the test object. 04:07.170 --> 04:12.600 And if you dereference that, then you can gain access to members of the object so you can get data from data 04:12.600 --> 04:14.490 members and you can call member functions. 04:15.240 --> 04:20.190 So in the body of this function, you can write this, arrow operator i , equals one. 04:20.940 --> 04:22.800 And that will dereference the this pointer. 04:23.360 --> 04:27.780 That'll get the test object and set its i member equal to one. 04:28.560 --> 04:32.250 Normally, we don't bother writing the explicit dereference. 04:32.550 --> 04:36.270 We can just do i equals one, and that'll do it implicitly. 04:36.270 --> 04:40.320 And the compiler will work out that we mean to dereference the i member of test. 04:42.730 --> 04:46.660 And the pointer "this" will be the address of the test object. 04:49.230 --> 04:51.540 OK, so that's it, just a nice short one. 04:52.350 --> 04:55.290 I'll see you next time, but meanwhile, keep coding!