WEBVTT 00:00.870 --> 00:03.900 Hello again! In this video, we are going to look at file modes. 00:06.350 --> 00:10.550 When we open a file in C++, there are a number of options we can use. 00:10.910 --> 00:13.310 These options are known as "file modes". 00:13.700 --> 00:15.950 These are basically the same as they are in Unix. 00:15.950 --> 00:20.300 So if you knew about file modes in Unix, you probably know all this! 00:22.560 --> 00:27.000 To use a file mode, we pass it as an optional second argument when we open the file. 00:27.600 --> 00:31.680 So that is when we call the constructor for stream or the open member function. 00:35.450 --> 00:40.730 There are two defaults, which are to use text mode and truncate mode. Text mode means that the 00:40.730 --> 00:44.220 characters are stored in the file as ASCII. 00:44.240 --> 00:52.160 So for example, if we write 42 to a file, the file will store that as the ASCII code for the character 00:52.160 --> 00:54.020 four and the character two. 00:55.070 --> 01:00.920 So those will be the corresponding binary numbers, which I think are 100 and 98. 01:03.150 --> 01:04.110 But don't quote me on that! 01:06.190 --> 01:11.290 Truncate mode means that all the data that is in the file is overwritten, so that is for output files 01:11.290 --> 01:13.600 only. Obviously input files will not be truncated. 01:14.290 --> 01:17.590 So when we open a file, it is opened in truncate mode by default. 01:17.860 --> 01:23.650 All the data is erased. And when we write to the file, that will start writing at the beginning of the 01:23.650 --> 01:24.040 file. 01:28.810 --> 01:34.300 There is also an append mode. So this will preserve the data that is in the file. 01:34.720 --> 01:40.630 And when we write data to the file, this will appear at the end of the file. So it will go after the 01:40.630 --> 01:42.160 data that was already in the file. 01:43.780 --> 01:49.900 To do this, we pass fstream double colon app as the second argument to open. 01:50.680 --> 01:52.410 This is a bitmask. 01:52.420 --> 01:53.530 It is a member of fstream. 01:53.860 --> 01:59.090 Well, actually, it is a member of ios, but you can use fstream or ios here. 02:01.400 --> 02:06.380 So this will open the file in append mode, the data in the file will be preserved and the 02:07.490 --> 02:11.150 write operation will add data at the end of the file. 02:13.130 --> 02:15.080 So here is some code to try that out. 02:15.680 --> 02:19.240 We have an important file, we open it in append more. 02:20.030 --> 02:22.130 And let's see if this file really is important. 02:25.320 --> 02:27.840 "Important data - do not remove!" 02:28.260 --> 02:29.400 Yes, that sounds important! 02:30.980 --> 02:31.740 We have to be careful here. 02:33.540 --> 02:40.650 So we open this file in append mode and then we write to the file using the left shift operator. 02:41.250 --> 02:46.680 We only use the write member function for low-level work, which we do not need here. 02:47.130 --> 02:48.300 So let's run this program. 02:51.620 --> 02:57.170 So the program has run. It has written to the file, we presume, and let's see if our important data is 02:57.170 --> 02:57.740 still there. 02:59.560 --> 02:59.890 Phew! 03:02.910 --> 03:05.520 So there we are. "Important data - do not remove!" 03:05.880 --> 03:10.110 So that is the data that was there before. And then we have "some more data" that we wrote at the end. 03:10.860 --> 03:12.750 So that has worked. Good. 03:16.240 --> 03:21.970 Instead of text mode, we can use binary mode, so the data in the file will have exactly the same bit 03:21.970 --> 03:23.740 patterns as the data in memory. 03:24.220 --> 03:30.250 So if we store the number 42 in a binary file, it will be stored as a binary number, which has 03:30.250 --> 03:36.400 the equivalent to 42. The decimal 42, binary 101010. 03:38.680 --> 03:44.500 If we used text mode, that would be stored as two separate characters, one representing '4', and one 03:44.500 --> 03:45.310 representing '2'. 03:47.780 --> 03:54.290 Binary mode is more complicated than text mode. It is low level, and it is easy to get things wrong. 03:55.220 --> 03:59.690 You could also have problems when you try to take binary files from one computer to another. 04:00.170 --> 04:05.690 If the computer that the file was written on has a different architecture from the computer that's opening 04:05.690 --> 04:12.350 the file, it may not read the data correctly. Or even the same compiler with different settings, because 04:12.350 --> 04:15.200 optimisation can change the way that the data is laid out. 04:15.770 --> 04:17.810 So generally, you should avoid binary data. 04:18.620 --> 04:24.140 However, there are some cases where it is important. If you are working with data, which is naturally 04:24.140 --> 04:24.620 binary. 04:24.980 --> 04:32.180 If you're working with file formats, so things like image files, media files, videos and so on. 04:34.760 --> 04:37.880 And we will have an example of using binary files later on. 04:42.510 --> 04:50.490 There are some other file modes. Truncate will always use truncate mode. In will open the file for input. 04:50.970 --> 04:55.650 So we have ifstream, which is an input stream and ofstream, which is an output file stream. 04:56.130 --> 05:00.210 There is also an fstream which is a general file stream, and you can use it for either. 05:00.900 --> 05:06.030 So you can use in and out to say whether you want to use it for reading or writing or both. 05:06.660 --> 05:13.530 So in will open it for input, out will open it for output. And that will also put it into truncate mode even 05:13.530 --> 05:14.880 if you did not ask for it. 05:16.750 --> 05:19.080 There is also ate or at 'e'. 05:19.120 --> 05:21.430 I do not know how you are supposed to pronounce it! 05:22.090 --> 05:23.470 This is similar to append. 05:23.860 --> 05:26.350 So it'll preserve the existing data in the file. 05:26.800 --> 05:30.640 And by default, if you write data, it will appear after the end of the file. 05:31.150 --> 05:35.980 However, you can also write the output somewhere else in the file if you want to, which you cannot 05:35.980 --> 05:36.730 do with append. 05:37.120 --> 05:39.160 Append always has to go at the end of the file. 05:45.400 --> 05:51.340 If he wants to combine different modes, these are just bitmasks, so we can use bitwise 'or', the 05:51.340 --> 05:52.720 vertical bar, to combine them. 05:53.410 --> 05:59.920 So if we have an fstream and we want to open it for output in append mode, then we combine the out and 05:59.920 --> 06:00.820 append modes. 06:02.200 --> 06:06.590 So this will open "important.txt" for output, in append mode. 06:07.270 --> 06:12.140 So that is exactly equivalent to the way that we were using the file in the example. 06:15.640 --> 06:21.490 We can also combine in and out with an fstream, and that will give us an fstream, that can both read from a file 06:21.490 --> 06:22.570 and write to it. 06:23.470 --> 06:25.900 If we do that, the file is not going to be truncated. 06:26.380 --> 06:32.410 Usually when you do this, you want to read from the file before you write to the data, so it does not 06:32.410 --> 06:34.570 make sense to overwrite the data before you look at it. 06:35.950 --> 06:41.620 If you do want to truncate it, then you can combine that as well, so we are opening "both.txt" for 06:41.620 --> 06:44.170 input and output in truncate mode. 06:46.830 --> 06:49.520 And then finally, there are some restrictions on how you can use mode. 06:50.280 --> 06:53.190 You can only use out with fstream and ofstream. 06:53.200 --> 06:57.720 You cannot use it with input stream, obviously, and you cannot use in with an [output] stream. 06:58.590 --> 07:04.200 Trunc is only available in output mode and you cannot use append and trunc together. 07:05.580 --> 07:08.880 If you do use app, the file will always be opened in output mode. 07:10.670 --> 07:11.690 So there we are! 07:12.170 --> 07:13.700 That's it for this video. 07:14.270 --> 07:17.420 I'll see you next time, but meanwhile, keep coding!