WEBVTT 00:00.450 --> 00:06.510 Hello again! In this video, we are going to look at constructors in modern C++. As you know, the 00:06.510 --> 00:12.240 constructor is a special member function. It gets called automatically when an object of the class is 00:12.240 --> 00:12.780 created. 00:13.590 --> 00:19.320 And we can use the constructor to initialize the data members of the class and to perform any set up 00:19.320 --> 00:22.950 that we require before we actually start using the object. 00:26.150 --> 00:30.860 If we do not do anything to initialize the data members of the class, they are default initialized. 00:31.460 --> 00:36.020 So if we have a member which is an object of a class, then the default constructor for that class is 00:36.020 --> 00:36.650 going to be called. 00:37.370 --> 00:41.960 If we have members which are of built-in type, then the initial value is undefined. 00:42.530 --> 00:45.710 The initial value will be whatever happens to be in 00:45.710 --> 00:50.510 that memory, interpreted as an int, or a bool, or whatever the type is. 00:53.360 --> 00:57.830 So who is that class. We have not provided a constructor to initialize the values. 00:58.850 --> 01:03.320 We have a print member, function, which is just going to display the values of the members. 01:03.980 --> 01:10.040 We're using the boolalpha manipulator, so we get "true" or "false" instead of 0 or 1 01:10.040 --> 01:16.850 when we print out the bool members. We create an object of the class and then we call the print 01:16.850 --> 01:18.050 member function of the object. 01:19.010 --> 01:20.540 So what do you think we will get? 01:24.150 --> 01:26.790 Well, that is probably not what you were expecting. 01:27.480 --> 01:33.630 So this memory that is being used by the temperature field obviously had some strange data in it. 01:33.630 --> 01:37.290 Maybe a string or some binary data or something. 01:39.160 --> 01:42.010 And door_open, power_on, that is just 50/50. 01:45.590 --> 01:46.910 So what can you do? 01:47.270 --> 01:51.830 The obvious answer is to write a constructor which will initialize these with sensible values. 01:53.390 --> 01:59.240 We could assume that the caller will provide them, or we could have constructors which take default arguments. 01:59.960 --> 02:07.820 Or we could have default arguments in the constructor. C++ 11 has a much nicer solution. In C++ 11, 02:07.880 --> 02:11.450 You can actually initialize data members in the class definition. 02:11.930 --> 02:18.290 So here we're saying that temperature has a default initial value of 2; door_open is false and power_on 02:18.290 --> 02:18.770 is true. 02:19.190 --> 02:27.020 So you just put the initial value in curly brackets, after the name of the member. And then the rest of 02:27.020 --> 02:28.340 the code is the same. 02:29.210 --> 02:30.500 So let's see if this works. 02:32.680 --> 02:33.200 And there we are. 02:33.220 --> 02:37.390 The temperature is true, the door is not open and the power is on. 02:38.410 --> 02:42.910 And if you have some situation where these are not the values you want, you could always write a 02:42.910 --> 02:46.750 constructor, which will initialize them with the desired values. 02:47.800 --> 02:52.750 But if you do know that you want a particular initial value, or there is some sensible default, then 02:52.750 --> 02:57.760 you should always do this. Because having undefined built-in types is always a big problem. 02:58.510 --> 03:02.930 Some compilers will set them to 0, and as we saw with Visual C++, some do not. 03:03.670 --> 03:11.230 So you can get unpredictable results. When you have a more complicated class, you can have different 03:11.230 --> 03:15.970 constructors and you want to do different things with these constructors. So you may want to set some 03:15.970 --> 03:17.440 of the members and not others. 03:18.160 --> 03:20.710 And then there's also something that you need to do for them. 03:21.370 --> 03:22.980 So, as an example of this, 03:22.990 --> 03:25.150 we have updated our refrigerator. 03:25.510 --> 03:27.130 It is now the 21st century. 03:27.250 --> 03:29.500 The refrigerator can connect to the Internet. 03:30.340 --> 03:34.480 Obviously, this is just an example of constructors, so we are not actually going to connect to the 03:34.480 --> 03:34.900 Internet. 03:35.470 --> 03:36.910 We are just going to pretend to. 03:37.870 --> 03:41.860 So we have a mock Internet class, which is a member of our refrigerator. 03:42.700 --> 03:47.110 We want to arrange things so that the refrigerator always connects to the Internet when it starts up. 03:47.740 --> 03:52.210 So from the constructor, we are going to call the connect() and the login() member functions of the 03:52.430 --> 03:53.800 Internet object. 03:56.190 --> 04:00.540 We have a default username and password, and yes, those really are my username password! 04:02.560 --> 04:10.270 (Not!) So for the default constructor, we just use the default values for these members, like we did before. 04:10.870 --> 04:15.100 And we connect to the Internet using these default credentials. 04:19.370 --> 04:23.930 We may also want to be able to set the temperature of the refrigerator when it comes on. 04:24.440 --> 04:27.260 But still the connect using the default credentials. 04:27.830 --> 04:32.330 So in that case, we have a constructor which sets the temperature and then it connects with the default 04:32.330 --> 04:32.960 credentials. 04:34.610 --> 04:39.530 On the other hand, you may be happy with the default temperature, but you want to choose a particular 04:39.770 --> 04:40.970 username and password. 04:41.420 --> 04:47.070 So in that case, you would have a constructon which will use the defaults again, but it will take the user 04:47.090 --> 04:48.650 name and password and use those. 04:50.490 --> 04:56.160 And then finally, you may want to be able to control everything so you can set the temperature, and 04:56.160 --> 04:57.390 the username, and the password. 04:58.080 --> 05:02.370 So we have four different constructors, which more or less do the same thing. 05:02.970 --> 05:07.500 So that's duplicated code, which is always a sign that you might need to think about your design. 05:09.030 --> 05:16.350 One of the famous axioms of programming is DRY, d-r-y, which stands for "Don't Repeat Yourself!". Do 05:16.350 --> 05:17.220 not repeat yourself. 05:17.640 --> 05:22.650 So if you keep writing repeated codes, then you probably need to think about factoring it. 05:25.820 --> 05:29.870 So in this case, we are connecting to the Internet and logging on in all four cases. 05:30.740 --> 05:32.570 So let's check first that this does work. 05:32.570 --> 05:34.430 The rest of the code is the same. 05:36.890 --> 05:43.250 OK, so we have four constructors where we are connecting to the Internet in the body and logging on, 05:43.700 --> 05:46.160 and we should really refactor that into a single function. 05:46.490 --> 05:47.740 So here is that function. 05:47.750 --> 05:51.200 Perhaps it should be private as it is actually part of the implementation of the class. 05:52.460 --> 05:58.970 So this will just connect to the Internet and log in with a given ID and password. And then, in the default 05:58.970 --> 05:59.480 constructor, 05:59.480 --> 06:05.000 we can call this with the default credentials. In the constructor where we set the temperature and log 06:05.000 --> 06:08.270 on with the defaults, and then we call init() with the defaults. 06:08.810 --> 06:13.520 If we use the default temperature and set the username and password, then we call it with the username 06:13.520 --> 06:14.120 and password. 06:14.990 --> 06:20.390 And finally, if you want to set everything, we set the temperature and call it with the username and 06:20.390 --> 06:22.250 password. And there we are. 06:22.280 --> 06:26.780 So that works exactly the same, but we have factored out all the duplicate code. 06:29.430 --> 06:31.410 But we have added another member function. 06:33.930 --> 06:40.590 So the advantage of this is that, if the details of initializing this Internet object change, or perhaps 06:40.590 --> 06:45.460 you have to call some different function or add another argument, then we only need to change it in this function. 06:45.480 --> 06:47.220 We do not need to change all the constructors. 06:48.510 --> 06:53.460 I have always thought it would be nice if C++ gave you some way for a constructor to call another constructor. 06:53.940 --> 06:55.800 And now you can actually do that in C++11. 06:56.550 --> 06:58.740 It is known as a delegating constructor. 06:59.370 --> 07:00.330 So let's look at that. 07:02.010 --> 07:04.190 So the first part of the code is the same. 07:04.230 --> 07:11.040 We have our Internet class and the refrigerator with a member, which is an Internet object. 07:13.600 --> 07:19.900 And then we have the same constructor which sets everything. So this will set the temperature and the 07:20.590 --> 07:22.840 ID and password for logging onto the internet. 07:23.650 --> 07:26.470 And then in the other constructors, we just delegate to this. 07:27.040 --> 07:33.640 So for example, if we use the default constructor, then we call the constructor with no defaults. 07:34.180 --> 07:39.070 We passed the default temperature, 2, as the argument, and then the ID and the password that we want. 07:39.250 --> 07:44.080 Similarly, if we are using the default login and setting the temperature, then we pass the temperature 07:44.080 --> 07:50.380 and the defaults for the credentials. And for the default constructor, we pass all the defaults. 07:54.710 --> 07:59.570 And the rest of the code is the same, and the result is the same. 08:04.320 --> 08:09.300 So the advantage of this is, if you have lots of combinations of constructors with default and non-default 08:09.300 --> 08:11.580 arguments, this makes it a lot easier. 08:12.210 --> 08:16.170 Instead of writing lots of different constructors, you can just delegate to one constructor that 08:16.170 --> 08:16.860 sets everything. 08:19.050 --> 08:20.790 OK, so that is it for this video. 08:21.300 --> 08:24.510 I'll see you next time, but until then, keep coding!