WEBVTT 00:00.120 --> 00:07.140 Hello again! In this video, we are going to look at member function pointers. C++ allows us to define 00:07.140 --> 00:13.530 pointers to member functions of classes. Using them is more complicated than with normal function pointers. 00:13.530 --> 00:16.320 Pointers to non-member functions. 00:17.220 --> 00:23.490 The member function pointers use different syntax, and we may also need to provide the object that they are 00:23.490 --> 00:23.940 called on. 00:24.840 --> 00:27.210 When you call a member function, you have to provide an object. 00:27.540 --> 00:33.060 Normally the compiler will do that for you, if you are calling it directly. But if you are going through a pointer 00:33.060 --> 00:34.620 then you may need to do that yourself. 00:35.730 --> 00:41.550 Member function pointers cannot be called directly. Unlike function pointers, we have to explicitly 00:41.730 --> 00:46.200 dereference them. And that means that member function pointers are not callable objects. 00:49.650 --> 00:56.170 So let's say we have a class with a member function and we want to have a pointer to this member function. 00:57.570 --> 01:00.890 The syntax for doing this is even worse than with normal function pointers, 01:00.890 --> 01:03.150 because we have to give the name of the class as well. 01:03.780 --> 01:06.890 So "pfunc" is a pointer to a member of "Test". 01:07.440 --> 01:11.430 It takes int and const string ref, and returns void. 01:12.540 --> 01:17.130 And then, to initialize this variable, we have to give it the address of the member function. 01:17.460 --> 01:19.740 And the ampersand is compulsory here. 01:20.220 --> 01:22.830 It is not optional, like it is with function pointers. 01:26.300 --> 01:28.450 To call a member function pointer, 01:28.460 --> 01:35.660 we first need an object. Then we need to get the pointer as a member of the object, the member function 01:35.660 --> 01:36.080 pointer. 01:36.440 --> 01:37.610 And then we dereference it. 01:37.940 --> 01:43.160 So we are doing both the "dot" and the "star" operations. And there is actually a special operator for this, which 01:43.160 --> 01:43.700 does both. 01:44.480 --> 01:47.210 So that is how you call a pointer to member function, 01:47.570 --> 01:53.600 when you have an object. If you have a pointer to an object, then you need to use the arrow operator for 01:53.600 --> 01:55.640 the member access. 01:56.120 --> 01:58.040 So there is an "arrow star" operator. 02:00.150 --> 02:02.340 So that is an operator with three characters. 02:04.840 --> 02:08.310 C++11 has some features which help us when we're working with member 02:08.320 --> 02:14.470 function pointers. First of all, the auto keyword, so we can just initialize the pointer with 02:14.470 --> 02:17.800 the address, and let the compiler work out what the type is. 02:19.210 --> 02:21.220 And we can also use type aliases. 02:21.580 --> 02:27.640 So if we want to create a vector, for example, of member function pointers, we can create a type alias 02:27.640 --> 02:28.600 and then use that. 02:29.200 --> 02:30.850 So that makes the code look a lot cleaner. 02:32.860 --> 02:33.940 So let's try this out. 02:33.940 --> 02:35.920 We have our class with the member function. 02:36.970 --> 02:46.720 We define a pointer to this member function. Or there is the C++11 syntax. And then we call the 02:47.110 --> 02:48.610 member function through the pointer. 02:50.140 --> 02:50.770 And there we are. 02:53.280 --> 03:00.810 C++11 has a function called mem_fn() in the header, and this can be used to convert a member function 03:00.810 --> 03:02.910 pointer into a callable object. 03:03.750 --> 03:09.960 So we pass our member function pointer as an argument to mem_fn(), and this will return a callable object. 03:11.310 --> 03:17.370 When we invoke this callable, we need to pass an object for the function to be called on. 03:17.970 --> 03:21.600 So we need to give an object of the class as the first argument. 03:22.110 --> 03:24.630 And then we give the arguments to the member function call. 03:26.840 --> 03:29.150 So, the same member function again. 03:32.280 --> 03:34.580 We use the C++11 syntax this time. 03:34.920 --> 03:38.640 Then we called mem_fn(). We get our callable object. 03:39.000 --> 03:42.300 And then we invoke it with an object of the class as the first argument. 03:46.730 --> 03:49.520 We can also use bind() with member function pointers. 03:50.000 --> 03:51.470 So let's go back to the example 03:51.470 --> 03:56.750 in the last video. We had a vector of strings, and we were counting the number of cats in that vector. 03:57.770 --> 04:01.580 So we have a class with a member function, which will do the match() function. 04:03.810 --> 04:08.610 Then we use bind(), with the pointer to this member function as the first argument. 04:09.360 --> 04:13.560 Then we have the address of a object of this class as the second argument. 04:14.520 --> 04:18.540 Then we have a placeholder. That is going to be one of the strings from the vector. 04:19.440 --> 04:21.780 That will be the first argument to this match() call. 04:22.500 --> 04:25.050 And then we bind "cat" as the second argument to 04:25.050 --> 04:26.130 that match() call. 04:28.610 --> 04:29.720 So here is our code again. 04:29.720 --> 04:34.610 We have the same count_strings() function, with the function object as argument. 04:35.330 --> 04:37.280 We have our vector of strings, again. 04:39.640 --> 04:43.570 We have this class, the same as the one on the slide, which will do the comparison. 04:46.000 --> 04:48.070 And then we have our call to bind(). 04:48.550 --> 04:50.800 So this is going to return a callable object. 04:51.190 --> 04:58.030 It will call the match() member function on this object, with "cat" as the second argument, and a first 04:58.030 --> 05:00.160 argument which will be provided by the caller. 05:01.150 --> 05:04.120 And then we pass that as the argument to count_strings(). 05:05.110 --> 05:11.430 So count_string() will iterate through the vector, and it will call this member function, with each element 05:11.440 --> 05:13.780 of the vector as the first argument. 05:16.190 --> 05:16.760 And there we are. 05:18.740 --> 05:21.050 So when would you use member function pointers? 05:21.650 --> 05:26.120 I have to admit the only time I have ever used them, is in concurrency. In C++, 05:26.660 --> 05:31.460 with threads, you have to put the code for the thread inside a function, and then create a thread object 05:31.760 --> 05:33.860 which has the address of that function. 05:34.490 --> 05:40.160 If you're writing a class which has member functions which run in their own thread, that means you need to pass 05:40.370 --> 05:43.640 member function pointers as arguments to the thread constructor. 05:45.260 --> 05:49.430 You could also use member function pointers for making decisions at runtime. 05:50.030 --> 05:55.760 You could have several objects of the same class, and then you decide which object has the member function 05:55.760 --> 05:56.120 called. 05:56.630 --> 06:03.170 So, for that, you would use bind(). You would bind the arguments to a member function call. You would leave the 06:03.500 --> 06:08.750 object as a placeholder. And then, later on, someone will call this bound function. 06:09.050 --> 06:12.710 They will provide the object they want, and that will call the member function with the arguments. 06:14.370 --> 06:19.580 Or, alternatively, you could have a container of member function pointers, and then you decide which one 06:19.580 --> 06:21.770 of these member functions gets called. 06:22.790 --> 06:28.820 And in both cases, you can use a switch statement, or some if-else branches, to make the actual decision. 06:29.360 --> 06:33.200 So you could use that for implementing state machines, callbacks and so on. 06:33.920 --> 06:35.420 Okay, so that is it for this video. 06:35.780 --> 06:36.620 I will see you next time. 06:36.620 --> 06:38.960 But until then, keep coding!