WEBVTT 00:00.150 --> 00:01.860 Hello again! In this video, 00:01.890 --> 00:04.080 we are going to continue looking at unions. 00:05.190 --> 00:09.780 In the last video, we saw that we can use a tagged union to make things a bit safer. 00:10.440 --> 00:15.750 However, this still requires the programmer to check that a member is in use before they access it. 00:16.650 --> 00:21.870 And also, when the programmer brings a member into use, they have to remember to set the type. 00:22.500 --> 00:24.660 So there is still scope for human error. 00:25.590 --> 00:32.640 And besides, this is a very "C language" way of doing things. We are in C++, so why not use C++ features? 00:33.450 --> 00:37.650 And one of the things we could do, is to wrap this tagged union inside a class. 00:39.280 --> 00:42.280 So we have a class where the union is a private member. 00:42.880 --> 00:47.890 There are public member functions of the class, which are the only way to access the union. 00:48.960 --> 00:55.980 These functions will make sure that the tagged member is set and checked correctly, so the right type 00:55.980 --> 00:56.730 is always used. 00:57.810 --> 01:04.020 If we try to do something which is invalid, like accessing a member which is not in use, the member 01:04.020 --> 01:09.030 functions can perform error handling. So they could return a null pointer, for example, or throw an 01:09.030 --> 01:09.570 exception. 01:10.550 --> 01:14.330 And this means that the union can only be used correctly. 01:14.630 --> 01:16.730 It is not possible to use it in the wrong way. 01:18.800 --> 01:20.270 So let's try that out. 01:20.300 --> 01:23.030 We have the same union that we had in the previous video. 01:23.630 --> 01:29.540 This has the member with the token type, which is an enumeration of the allowed types. 01:31.700 --> 01:36.110 We create a class which has an object of this union as a member. 01:36.800 --> 01:40.730 Then we have member functions which set and get the members. 01:41.300 --> 01:45.200 I have not written all of them, but I am sure you can work out how to do it! 01:46.290 --> 01:49.830 For example, with the member function which will set the character member. 01:50.070 --> 01:55.620 This first sets the token type to character, and then it brings the character member into use. 01:57.040 --> 01:59.980 The member function which gets the double member. 02:00.310 --> 02:06.760 This will first check that the double member is in use, and it only returns the double member if it is 02:06.760 --> 02:07.210 in use. 02:07.720 --> 02:09.160 Otherwise it throws an exception. 02:09.490 --> 02:13.390 So this is the standard invalid argument, from the exception hierarchy. 02:15.000 --> 02:19.500 In the main function, we create an object of this class. We call set_character(). 02:20.070 --> 02:24.120 So this rule sets the token type to character, and then bring the character member into use. 02:25.940 --> 02:28.490 We then call the get_double() member function. 02:29.210 --> 02:32.030 This will return the double member, if it is in use. 02:32.720 --> 02:34.130 Otherwise, it will throw an exception. 02:34.490 --> 02:37.430 So we have a try-catch block which will handle the exception. 02:39.490 --> 02:40.120 And then we go. 02:40.630 --> 02:43.600 We get an exception, and the double member is not in use. 02:46.680 --> 02:52.170 In C++ 17, there is now a type in the library which will do all this for us. 02:52.560 --> 02:53.670 It is called "variant". 02:54.800 --> 02:58.760 So this will do all the things that a wrapped, tagged union will do. 02:59.360 --> 03:00.380 It is type safe. 03:01.010 --> 03:05.720 You are allowed to have different members, or "alternatives", as they are called, which are the same type. 03:06.230 --> 03:08.180 Which is not possible with a union. 03:09.170 --> 03:14.540 The variant will automatically call the constructors and destructors for the members, which 03:14.840 --> 03:15.980 the union will not do. 03:16.010 --> 03:17.900 You have to call them yourself with a union. 03:18.680 --> 03:21.230 And all in all, it is simpler and safer to use. 03:22.400 --> 03:23.960 And you do not have to write all that code! 03:26.170 --> 03:29.680 Variant is a templated type. It is defined in the header. 03:30.610 --> 03:36.310 The parameters are the types of the members, or rather, alternatives. (I must get used to saying that!) 03:37.210 --> 03:42.130 So this variant will have alternatives which are character, int and double. 03:42.550 --> 03:45.010 So that is the same as having the union that we had earlier on. 03:47.430 --> 03:50.040 To use a variant we just assign to it. 03:50.040 --> 03:53.790 We do not need to assign to the members. The compiler 03:53.790 --> 03:55.950 will deduce which alternative we need. 03:56.280 --> 04:01.350 So if we assign to it and give a character argument, then this will bring the character member into use. 04:03.760 --> 04:05.170 To access the data, 04:05.170 --> 04:09.010 we call the get() function, and this works just like it does with tuples. 04:09.850 --> 04:15.160 So we can give the type as the parameter, and then we give the variant as the arguments. 04:16.090 --> 04:22.660 So this will retrieve the character member if it is in use. And if it is not in use, it will throw an exception. 04:24.370 --> 04:29.080 We can also get by index, and that is useful if there are alternatives which have the same type. 04:29.680 --> 04:37.270 So if we put zero as the parameter, that will gets the alternative which has index zero, which is 04:37.270 --> 04:38.110 the character. 04:40.990 --> 04:43.990 If we want to check whether an alternative is in use, 04:44.350 --> 04:46.420 there is the holds_alternative() function. 04:47.260 --> 04:49.480 This takes the type as the parameter. 04:50.110 --> 04:55.420 So if we want to know if double is in use, we put that as the parameter. And this will return true 04:55.420 --> 04:57.820 if it is in use and false if it is not. 04:59.680 --> 05:00.700 So here is an example. 05:00.760 --> 05:02.730 We include the variant header. 05:03.680 --> 05:08.900 We create a variant object which has the type parameters for char, int and double. 05:09.830 --> 05:12.980 We assign to a character, so that will bring the character member into use. 05:14.640 --> 05:20.520 We called holds_alternative() to find out if the double member is in use. And if it is, then we can call get(). 05:23.310 --> 05:23.700 Okay. 05:23.700 --> 05:25.320 So the double member is not in use. 05:27.660 --> 05:27.910 Right. 05:27.990 --> 05:29.430 So why is this useful? 05:29.970 --> 05:32.700 What would you use a union or variant for? 05:33.330 --> 05:36.090 One application that we have already mentioned is parsing. 05:36.690 --> 05:40.710 If you are passing source code where a variable could have one of several types. 05:42.120 --> 05:48.510 Also, if you're parsing configuration files such as .INI files on Windows, where the parameters 05:48.510 --> 05:50.760 can only be a string, a number or a Boolean. 05:51.000 --> 05:52.890 That is quite a common way of writing them. 05:53.460 --> 05:56.310 You can also use them for implementing your own language. 05:57.030 --> 06:01.650 There are some languages which have a thing called "duck typing", which means that an object can have 06:01.650 --> 06:05.910 several types, and the type which it has depends on how it is used. 06:08.340 --> 06:12.480 You can also use them for return values, if you want to return different types. 06:12.840 --> 06:17.220 So he could, for example, have a computation which can have outcomes with different types. 06:18.940 --> 06:22.390 You can also use a union or variant for error handling. 06:22.780 --> 06:26.770 So if the operation succeeds, you can return an object which has the result. 06:27.370 --> 06:29.470 And if it fails, you can return an error code. 06:31.370 --> 06:36.710 And finally, this is another way to do polymorphism. And it is actually possible to implement virtual 06:36.710 --> 06:41.170 functions using unions and variant, if you are completely insane! 06:42.560 --> 06:44.000 Okay, so that is it for this video. 06:44.600 --> 06:45.500 I will see you next time. 06:45.500 --> 06:47.810 But until then, keep coding!