WEBVTT 00:00.690 --> 00:04.260 Hello again! In this video, we are going to look at the friend keyword. 00:05.760 --> 00:11.940 If we have a function which takes some object as argument, then the function can only access the public 00:11.940 --> 00:17.220 members of the class. So it can call any public member functions of the class. 00:17.910 --> 00:21.960 If that class has any public data members, it can access those as well. 00:22.470 --> 00:25.200 But anything which is private is inaccessible. 00:27.140 --> 00:33.260 When we are writing overloaded operators, we sometimes need to be able to access private data from outside 00:33.260 --> 00:33.740 the class. 00:34.790 --> 00:40.640 C++ provides a way of doing that, which I do not particularly like, but it is very convenient. 00:41.210 --> 00:44.420 And that is for the class to declare the function as a friend. 00:45.050 --> 00:51.290 For example, if we have a function which takes a Test object as argument, and the Test class has 00:51.290 --> 00:56.690 declared the print() function as a friend by putting the friend keyword in front of its signature. 00:57.470 --> 01:04.190 Then the print() function has full access to the class's members, so it can access the data members. 01:04.880 --> 01:06.410 So here is that code. 01:06.410 --> 01:11.870 I have added some members to the class with initializers. In the main function, 01:11.870 --> 01:13.490 we create an object of this class. 01:13.820 --> 01:16.400 And then we pass it to the print() function. 01:19.650 --> 01:26.670 So there we are, the print() function can access the data members. As well as making a non-member 01:26.670 --> 01:27.720 function a friend, 01:28.050 --> 01:30.450 we can also make an entire class a friend. 01:30.900 --> 01:36.120 So in this example, we have our class Test, which says that the class Example is a friend. 01:37.200 --> 01:42.840 And that means that all the member functions of Example have unlimited access to the members of Test. 01:44.220 --> 01:46.530 So here we have a class Example. 01:46.890 --> 01:49.650 So this print() is now a member function of the Example class. 01:49.650 --> 01:53.280 Example is a friend of Test. 01:54.180 --> 02:00.730 So this means that all the member functions of Example have access to test's private data. And then 02:00.740 --> 02:04.680 in the main() function, we create an object of Example as well as Test. 02:05.220 --> 02:06.690 And then we call this member function. 02:10.920 --> 02:17.880 So there we are, we can access the private data from a member function of the friend class. And some 02:17.880 --> 02:22.200 C++ programmers, including me, are not specifically fond of "friend"! 02:22.500 --> 02:24.630 The reason is that it reduces encapsulation. 02:25.260 --> 02:31.140 If you have some external function which can access any data member of the class, then in theory 02:31.140 --> 02:36.600 it can do anything to your class. So it can call member functions which are only meant for internal 02:36.600 --> 02:36.930 use. 02:37.290 --> 02:43.680 It can change the values of all your private data. And, either by design or by accident, 02:44.190 --> 02:49.170 someone can modify this friend function to do all sorts of things and creates lots of problems. 02:49.920 --> 02:55.350 So we can avoid this instead. Of having the external function as a friend, we can make its delegate 02:55.360 --> 02:56.820 to a member function of our class. 02:57.690 --> 03:01.320 So we would write a member function in our class, which does everything 03:01.320 --> 03:03.960 that the external function needs to do. 03:04.500 --> 03:07.590 And then the external function will just call this member function. 03:09.950 --> 03:14.900 So we have something like this. In our Test class. We have a member function which does everything that 03:14.900 --> 03:22.280 the print() function needs. And then the external print() function will just call this member function. 03:22.640 --> 03:23.680 And that is much safer. 03:24.620 --> 03:29.930 The external function has no access at all to the internal details of this class. 03:32.210 --> 03:34.110 So here is that code now. 03:34.130 --> 03:39.860 We have the member function, which does all the print functionality, and then we have this external 03:39.860 --> 03:42.140 function, which just calls the member function. 03:43.100 --> 03:46.280 The main() program is exactly the same as it was in the first example. 03:51.800 --> 03:57.740 So I admit, these examples are rather artificial, but we are laying a foundation. When we start actually 03:57.740 --> 03:59.120 writing overloaded operators, 03:59.480 --> 04:02.450 We will need this, and we will do something that is a bit more useful. 04:03.180 --> 04:05.180 Okay, so that is it for this video. 04:05.700 --> 04:06.580 I will see you next time. 04:06.830 --> 04:08.510 Until then, keep coding!