WEBVTT 0 00:00.810 --> 00:04.260 Hello again in this video, we're going look at file streams. 1 00:06.060 --> 00:08.880 We already know about input-output streams. 2 00:09.420 --> 00:14.040 cout and cin are objects of the ostream and istream classes. 3 00:15.300 --> 00:16.890 There is also fstream. 4 00:17.460 --> 00:25.200 So ofstream is a class for writing to files and ifstream is a class for reading from files. 5 00:26.660 --> 00:31.700 Unlike consoles, where there is only one console per computer, there can be many files. 6 00:32.090 --> 00:37.430 So when we use a file stream, we need to say which file we are going to use it with. 7 00:41.170 --> 00:47.230 The easiest way to do that is to pass the name of the file as the argument to the constructor of the 8 00:47.230 --> 00:47.980 file stream. 9 00:48.670 --> 00:56.970 So if we create an ifstream object for reading from a file, and we pass text dot txt as the argument, then 10 00:57.100 --> 01:01.360 the constructor of the file stream will open that file for reading. 11 01:02.020 --> 01:06.430 So that's going to set up the communication channel between the file and our program. 12 01:08.510 --> 01:14.300 The constructor will perform an open operation, which could fail; if the file does not exist or we are 13 01:14.300 --> 01:16.010 not allowed to read from it. 14 01:16.640 --> 01:21.170 So we need to check that we have a valid file stream object before we can use it. 15 01:22.190 --> 01:28.880 We can do that very easily, by putting the name of the object as a conditional. If the file stream is 16 01:28.880 --> 01:29.240 valid, 17 01:29.270 --> 01:34.130 this will be true. And then we can use the file stream to reach from the file. 18 01:35.030 --> 01:41.750 If the file stream is invalid, if the file could not be opened, then this would be false. And then 19 01:41.750 --> 01:44.360 we can go into an else branch which will handle the error. 20 01:48.450 --> 01:54.690 Incidentally, the in the version of C++ which introduced the standard string, they forgot to provide 21 01:54.690 --> 01:57.180 a constructor, which takes a standard string. 22 01:57.450 --> 02:03.270 But that has now been fixed. So you can now pass a string object as the argument to a file stream 23 02:03.270 --> 02:03.930 constructor. 24 02:07.710 --> 02:15.160 Reading from a file is the same as reading from standard input with cin. So we can just use the right 25 02:15.360 --> 02:18.000 shift operator, the double angle brackets. 26 02:19.130 --> 02:24.680 So this will cause the stream to fetch one value at a time from the file and put it in this variable. 27 02:26.150 --> 02:30.170 A value is defined as something that is separated by white space. 28 02:30.650 --> 02:37.190 So every time the stream encounters a space or tab or new line, it will decide that is the end of a 29 02:37.490 --> 02:41.810 value. And the next one starts after the end of the white space. 30 02:45.520 --> 02:49.030 And that can be problematic with keyboards, but more so with files. 31 02:49.610 --> 02:52.690 Often the space is actually part of the data that you want. 32 02:55.190 --> 03:00.500 Also, files often have some kind of structure. They have different fields which have a meaning. 33 03:01.550 --> 03:06.200 And in that case, your code has to match the structure of the file. And getting that right can 34 03:06.200 --> 03:07.220 be quite difficult. 35 03:08.430 --> 03:13.530 And also the question of error handling. If the structure of the file does not exactly match what you 36 03:13.530 --> 03:18.240 expect, perhaps the structure has changed or maybe you made a mistake? 37 03:18.660 --> 03:20.670 Or the file structure may have been corrupted. 38 03:22.050 --> 03:28.410 Let's have a look at using the right shift operator to read data from a file. So we are opening our 39 03:28.410 --> 03:32.160 file, text dot txt for input, to read from it. 40 03:33.060 --> 03:35.520 We check that the file was opened successfully. 41 03:36.300 --> 03:41.850 Then we have a string variable, which is going to hold the data from the file and then we read from 42 03:41.860 --> 03:45.630 it. One word at a time or one data field, if you like. 43 03:47.370 --> 03:51.990 And when there is no more data, this will return false and the loop terminates. 44 03:53.430 --> 03:58.110 We just print out each field as we get it, and then we close the file. 45 03:58.110 --> 04:02.430 So the close member function will cause the associated file to be closed. 46 04:03.990 --> 04:09.030 In that case, the stream object is no longer bound to the file, and it is just an empty stream object, 47 04:09.030 --> 04:10.590 which does not represent any file. 48 04:12.880 --> 04:16.480 So before we actually run this, we need to get a file to read from. 49 04:17.470 --> 04:23.260 In most IDE's, you can just take a file and drag it into the project directory and drop it. 50 04:24.380 --> 04:25.730 If you are using a command line, 51 04:26.950 --> 04:32.650 you could create the file in the directory where the program is going to run. With Visual Studio, 52 04:32.680 --> 04:37.750 you need to add it to the project, so you do Project, Add New Item. 53 04:39.850 --> 04:42.340 And then you have to find out where the text file is. 54 04:43.330 --> 04:48.610 I sometimes wonder if Microsoft has a special department that is responsible for moving things around, 55 04:48.610 --> 04:53.740 so no-one can find them! In this version of Visual Studio, it is in Utility. 56 04:54.190 --> 04:55.720 And then there is Text File. 57 04:56.980 --> 05:00.880 So let's make that lowercase, so it matches the slide. 58 05:03.440 --> 05:05.600 And then we can paste some data into that. 59 05:06.380 --> 05:06.800 There we go. 60 05:08.510 --> 05:10.310 If we look at the Solution Explorer, 61 05:13.660 --> 05:17.440 then we see that we now have this text.txt as part of the project. 62 05:18.100 --> 05:20.980 So this will actually be in the same directory that the program runs in. 63 05:22.850 --> 05:24.710 And now we can run it. 64 05:27.790 --> 05:33.700 So there we are. It has fetched the data from the file, one word at a time, and it has printed them out 65 05:34.030 --> 05:35.410 with commas between them. 66 05:40.900 --> 05:44.830 So as we said, there can be some issues with using the right shift operator. 67 05:45.880 --> 05:49.090 The best way to read input is to use the getline function. 68 05:49.720 --> 05:55.030 So this will take the stream to read from as the first argument, and a string object as the second 69 05:55.030 --> 05:55.510 argument. 70 05:56.140 --> 06:00.490 And then it will read a line, from the file, into this string object. 71 06:01.390 --> 06:04.000 If there is a new line character, it will regard 72 06:04.000 --> 06:09.490 that as the end of the line. So it will stop. And it will actually remove that newline character from the 73 06:09.490 --> 06:09.910 data. 74 06:10.390 --> 06:17.290 So this string object will contain the entire line of the text, except for the new line character. 75 06:18.800 --> 06:24.800 getline will return true when it reads a line of text. It will return false when it gets to the end 76 06:24.800 --> 06:31.970 of the file. So you can put this inside a loop and it will keep looping as long as it reads text, 77 06:32.090 --> 06:37.400 and then you can process the text inside the loop body. When it gets to the end of the file, 78 06:37.670 --> 06:39.140 the loop will terminate. 79 06:42.880 --> 06:49.270 So the advantage of this is that you have the data in a string object and then you can process it as 80 06:49.270 --> 06:49.660 you want. 81 06:51.790 --> 06:56.770 So here is the same file again. But this time we are going to use getline to get the data from it. 82 07:00.680 --> 07:01.550 So there we are. 83 07:01.670 --> 07:03.870 It has read it in as a single line of text. 84 07:03.890 --> 07:06.380 So this time there is no commas between the different words. 85 07:10.630 --> 07:17.410 If we want to write to a file then we use a ofstream. Again, we give the name of the file that we 86 07:17.410 --> 07:20.080 want to use as the argument to the constructor. 87 07:23.100 --> 07:28.650 So this will open the communication channel between the program and the file. And this time it is going 88 07:28.650 --> 07:30.870 to send data from the program to the file. 89 07:31.760 --> 07:34.650 Again, we need to check the state of the stream. 90 07:34.950 --> 07:41.700 We need to check that the stream could successfully open the file. And then, to use the stream, we just 91 07:41.700 --> 07:44.520 use the left shift operator to send data to it. 92 07:48.360 --> 07:51.570 So, for example, we could create a vector of strings. 93 07:52.110 --> 07:55.680 We have a range for loop, which iterates over the vector. 94 07:56.250 --> 08:00.720 So each time through the loop, this will be the next string from the vector. 95 08:01.350 --> 08:05.430 And then this will cause that string to be sent to the file. 96 08:12.760 --> 08:19.150 So here is that text. The solution Explorer is covering the attribute, actually, but I am sure you can work 97 08:19.150 --> 08:20.050 out what should be in there! 98 08:20.860 --> 08:27.400 So we need to include the fstream header to get the file streams, and then string for the string object 99 08:27.400 --> 08:28.930 and vector for the vector. 100 08:31.040 --> 08:36.950 So if we run this program - and by the way, make sure that this file does not have any data that you 101 08:36.950 --> 08:43.040 want to keep. When we open a file for writing, that will overwrite any data that is already in the file. 102 08:47.880 --> 08:54.960 So if we run that, then - nothing appears to have happened! But it has actually generated the file. 103 08:55.320 --> 08:59.640 This file will be in the same directory, the same folder as the one that the program ran in. 104 09:00.690 --> 09:02.790 So in [Visual Studio], we can do Projects, 105 09:03.760 --> 09:10.720 Add Existing Item, and then we see that there is the file that we just created. In the directory where 106 09:10.720 --> 09:15.520 the program read. And then that has now been added to the project. 107 09:15.580 --> 09:21.910 So it is part of the solution. And then we can just look at that file. And we have "The", comma "quick", comma 108 09:21.910 --> 09:22.240 "brown." 109 09:25.080 --> 09:30.330 So each of the elements of this vector has been sent to the file, followed by a comma. 110 09:33.490 --> 09:40.390 And finally, the destructor of the file stream class. When the destructor is called, the file is automatically 111 09:40.390 --> 09:40.810 closed. 112 09:41.230 --> 09:44.410 So this will happen just before the actual stream object is destroyed. 113 09:45.220 --> 09:50.490 If this is an output stream, then this will make sure that any data that is hanging around in memory 114 09:50.500 --> 09:52.420 buffers will get written to the file. 115 09:54.570 --> 09:59.850 So this means that if we have a file stream object, we do not actually need to call close. 116 10:00.270 --> 10:04.620 We can just leave it and let it go out of scope and it will automatically be handled. 117 10:05.240 --> 10:09.960 However, it is a good idea to explicitly put in a call to close, as that will make it clear that 118 10:09.960 --> 10:12.540 we have actually finished with using that particular file. 119 10:14.310 --> 10:16.440 Okay, so that's it's for this video. 120 10:16.890 --> 10:20.250 I'll see you next time, but meanwhile, keep coding!