WEBVTT 00:00.390 --> 00:03.990 Hello again! In this video, we are going to talk about resource management. 00:04.350 --> 00:07.020 This is a very important topic in C++. 00:07.380 --> 00:11.220 We have mentioned it before, but I want to spend a bit more time talking about it. 00:15.840 --> 00:21.810 When we write programs, we often use external resources: resources from the operating system or third 00:21.810 --> 00:22.590 party libraries. 00:23.160 --> 00:30.660 So things like heap memory when we call new to allocate memory on the heap; files, database connections, 00:31.080 --> 00:35.520 windows in a GUI system, threads in a multi-threaded program and so on. 00:35.910 --> 00:37.920 And we need to manage these resources. 00:38.520 --> 00:42.250 Usually, we have to get the resource somehow before we can use it. 00:42.270 --> 00:48.000 We need to allocate memory, open a file, acquire a connection to the database and so on. 00:49.320 --> 00:52.680 Once we have finished using the resource, we need to release it or close it. 00:53.790 --> 00:59.760 If we need to make a copy of something - which often happens in C++, because of value semantics - 01:00.210 --> 01:02.460 we usually need to think about what happens then. 01:02.970 --> 01:05.580 And we also need to think about what happens if there is an error. 01:10.370 --> 01:15.830 There are some classes in the standard library which manage resources. We have already met string and vector, 01:15.830 --> 01:19.460 which manage allocated memory. fstream manages files. 01:19.940 --> 01:22.670 And there are some others, like the objects used in multi-threading. 01:23.120 --> 01:27.170 And by the way, if you're interested in learning about multi-threading, I have a course on that. 01:29.910 --> 01:36.000 These all follow the object oriented principle of encapsulation. The actual details of how to manage 01:36.000 --> 01:39.330 and interact with the resource are hidden away inside the class. 01:40.170 --> 01:44.970 If you are writing code that uses the class, it does not need to know anything about how the class 01:44.970 --> 01:46.230 works or what it is doing. 01:46.860 --> 01:51.810 You just go through the class's interface, and that usually means calling the public member functions 01:51.810 --> 01:52.470 of the class. 01:56.840 --> 02:02.510 As an example of this, the fstream class encapsulates the details of how to interact with a file. 02:03.260 --> 02:05.240 If we want to use a file in C++, 02:05.240 --> 02:07.330 we just create an fstream object. 02:07.340 --> 02:10.310 We give the name of the file as the argument to the constructor. 02:10.910 --> 02:16.310 We use the overloaded left and right shift operators, depending on whether we are doing input or output. 02:16.880 --> 02:22.220 And when we have finished, we close the file. Or in fact, we can forget about that, and the stream object 02:22.220 --> 02:24.320 will close the file when it goes out of scope. 02:26.690 --> 02:30.860 So to actually access the data in the file, we just use the fstream interface. 02:31.280 --> 02:34.550 We do not need to know anything about how to work with files directly. 02:34.940 --> 02:39.600 The peculiarities of files on Windows or Linux, or whatever system we are using. 02:44.870 --> 02:52.220 These classes in the C++ library follow a common idiom: the resource or a handle to the resource is stored 02:52.220 --> 02:54.350 as a private data member of the class. 02:55.190 --> 02:58.850 The class constructor will acquire or open the resource. 02:59.900 --> 03:05.330 The class will have public member functions which control access to the resource, and then the destructor 03:05.330 --> 03:07.400 for the class will release the resource. 03:09.170 --> 03:13.190 And there is in fact a name for this idiom, which is absolutely terrible! 03:14.600 --> 03:18.890 Resource Acquisition Is Initialization. RAII for short. 03:20.030 --> 03:27.110 What this means is that when you create a class object, that will acquire a copy of the resource and 03:27.110 --> 03:33.500 manage it. And, by implication, when the class is destroyed, it will make sure that the resource is 03:33.500 --> 03:34.400 properly released. 03:36.960 --> 03:42.020 The other important thing is that, when an object is copied or assigned to, so C++ does a lot of copying 03:42.020 --> 03:42.560 objects. 03:44.300 --> 03:48.950 When that happens, the target object, the one which is the result of the operation, will have its 03:48.950 --> 03:50.420 own version of the resource. 03:50.750 --> 03:55.070 So you will not have problems where different objects are writing to the same file, scrambling the 03:55.070 --> 03:57.140 output or writing over each other's memory. 04:01.570 --> 04:08.560 The advantages of this approach, RAII: obtaining access to the resource is straightforward and deterministic, 04:09.520 --> 04:15.160 if you can access the resource, then you have a valid object which has been successfully created. 04:16.570 --> 04:21.220 If you do not get a valid object, then you will know that the resource cannot be used by the program. 04:22.360 --> 04:24.130 Using the resource is straightforward. 04:24.130 --> 04:26.500 You just call the member functions on the object. 04:28.090 --> 04:30.400 And releasing it is straightforward and deterministic. 04:31.000 --> 04:34.090 The resource will be released when the object goes out of scope. 04:34.840 --> 04:40.660 So that means either when the program reaches the end of the scope where the object was created or if 04:40.660 --> 04:44.440 an exception is thrown. And we will talk more about exceptions later on. 04:45.070 --> 04:50.980 Another teaser! And also only one object can own a given resource at one time. 04:51.490 --> 04:55.240 So there is no possibility of different objects interfering with each other. 04:57.810 --> 04:59.280 OK, so that is it for this feature. 04:59.640 --> 05:02.460 I will see you next time, but meanwhile, keep coding!