WEBVTT 0 00:00.660 --> 00:04.770 Hello again! In this video, we are going to look at printing out class member data. 1 00:06.730 --> 00:11.710 So far, we have been using a print() member function, which sends the data members to cout. 2 00:12.940 --> 00:16.960 We cannot use the left shift operator because that does not compile. 3 00:21.150 --> 00:24.030 And we get all these horrible templates error messages. 4 00:24.210 --> 00:29.250 The problem is there is no left shift operator which takes a Test object as one of its arguments, 5 00:29.250 --> 00:30.960 and there is no way to convert to it. 6 00:33.410 --> 00:39.050 So if we use the print() member function that works, but it is rather limited because we can only use it 7 00:39.050 --> 00:40.010 with cout. 8 00:43.200 --> 00:46.530 Perhaps we can make this more general, so we can use it with any output stream. 9 00:49.320 --> 00:55.470 So now we have the same class again, but this time we have added an output stream as the argument to the 10 00:55.470 --> 00:59.610 print() member function. So we can now send the data members to the output stream. 11 01:00.030 --> 01:04.440 And this will work for any type of output stream, regardless of whether it is iostream, fstream 12 01:04.440 --> 01:05.430 or stringstream. 13 01:06.530 --> 01:09.750 We need to pass the argument by reference for two reasons. 14 01:10.290 --> 01:14.550 First of all, because the function is going to modify the stream. It is going to push data onto it 15 01:15.000 --> 01:18.960 and change its state, and also because ostreams cannot be copied. 16 01:19.440 --> 01:21.090 So we cannot pass this by value. 17 01:21.810 --> 01:25.840 And then, in the main() function, we pass the stream as argument. 18 01:25.860 --> 01:27.030 So if we want to 19 01:27.540 --> 01:31.260 display data, we pass cout as the arguments. 20 01:31.950 --> 01:36.810 Or alternatively, we can open a file, and use the file stream as the argument to print(). 21 01:39.280 --> 01:45.130 So there we are, we get the output and also... We get the file output as well. 22 01:50.000 --> 01:55.160 So that does work, but it would be nice if we could use the left shift operator, so our code would 23 01:55.250 --> 02:01.280 look like all the other code that uses the built-in and the library types. To do that, we need to understand 24 02:01.280 --> 02:05.360 how the library and built-in types work with the left shift operator. 25 02:05.690 --> 02:10.670 And the answer is that the core language and the standard library provide overloads of that operator, 26 02:11.180 --> 02:13.190 which take that type as the second argument. 27 02:13.910 --> 02:19.010 For example, if we send an int to standard output, that will result in the left shift operator being 28 02:19.010 --> 02:21.740 called with cout and i as the arguments. 29 02:22.460 --> 02:26.480 And this is a binary operator, so it cannot be a member function of ostream. 30 02:30.140 --> 02:35.960 We are allowed to chain together calls to this operator. So we can send multiple objects to the same 31 02:35.960 --> 02:38.510 stream in a single statement, like that. 32 02:39.230 --> 02:42.380 And this will actually result in a pair of nested function calls. 33 02:42.920 --> 02:48.200 So first, it is going to call the operator, with cout and i as the arguments, to push i into the stream. 34 02:48.920 --> 02:55.700 And then the return value from that will be used as the stream, and j will be pushed onto that stream. 35 02:57.970 --> 03:04.330 So this operator call must return cout. Again, that must be by reference, not by value. 36 03:04.750 --> 03:10.930 So the prototype is going to operator left shift, which takes an ostream by reference and the 37 03:11.020 --> 03:14.260 data type, and returns the stream by reference. 38 03:19.340 --> 03:22.790 I have implemented this operator for our class. 39 03:23.810 --> 03:24.930 This is outside the class, 40 03:24.950 --> 03:27.860 of course. It is not a member function of the class or the ostream. 41 03:28.550 --> 03:33.200 It takes the ostream and the Test object as argument, by reference. 42 03:34.100 --> 03:39.710 It calls the print() member function of the Test class, with the ostream as the argument, and then 43 03:39.830 --> 03:41.060 returns the same stream. 44 03:41.450 --> 03:45.170 So if we call it repeatedly, it will still work. 45 03:46.650 --> 03:52.260 And then in the main() function, we can use this just like we do with any other object of built-in types 46 03:52.350 --> 03:53.160 or library types. 47 03:53.940 --> 03:56.100 And it works with files as well. 48 04:02.330 --> 04:04.130 Okay, so that is it for this video. 49 04:04.610 --> 04:05.510 I will see you next time. 50 04:05.770 --> 04:07.850 Until then, keep coding!