WEBVTT 00:00.180 --> 00:07.920 Hello again! In this video, we are going to look at library function objects. The C++ library defines 00:07.920 --> 00:09.330 some function objects. 00:09.870 --> 00:16.950 So these are generic operators which perform all the usual arithmetic, logical and relational operations. 00:17.220 --> 00:21.030 So things like plus, greater than, not equals, and so on. 00:22.320 --> 00:24.860 These are implemented as functors. 00:24.900 --> 00:29.120 So these are classes which have a function call operator, and they are templated. 00:29.430 --> 00:30.930 So these are completely generic. 00:31.680 --> 00:34.710 They will work for any type which supports the underlying operator. 00:35.340 --> 00:35.850 All these do, 00:35.850 --> 00:38.580 is they just call the corresponding operator for the object. 00:38.940 --> 00:45.540 For example, with the less-than operator, which is actually called less. This will have a function 00:45.540 --> 00:53.190 call operator, which takes two arguments of the template type. And it returns a bool, which is the result 00:53.190 --> 00:55.200 of calling the less than operator. 00:57.330 --> 01:01.130 If we want to use this operator, we need to create an object. 01:01.170 --> 01:04.890 So we need to call the constructor or do something else which will create an object. 01:05.490 --> 01:11.790 We also need to give the paramater type. Although, in C++ 17, you can actually skip that. 01:12.750 --> 01:18.300 So this will create an object of this functor, and then the sort() algorithm will call the function 01:18.300 --> 01:22.170 call operator, which will in turn call the string's less-than operator. 01:25.430 --> 01:29.120 So let's have a look at this. So we have a vector of strings. 01:30.500 --> 01:37.310 We have our call to sort() with the function object as the arguments. Actually, let's remind ourselves 01:37.310 --> 01:39.740 what happens without that. 01:40.670 --> 01:43.190 So this is just the default ordering. 01:44.480 --> 01:47.420 So these are ordered in ascending alphabetical order. 01:47.420 --> 01:51.830 And that's because the sort() will use the string less-than operator by default. 01:58.840 --> 02:04.870 So if we put this back, it is now going to use the library less, for type string, and that will also 02:04.870 --> 02:07.100 use the less-than operator of the string class. 02:07.600 --> 02:12.610 So we should expect to get exactly the same results and that is what we do. 02:15.130 --> 02:18.170 There is something slightly more interesting if we use greater. 02:19.400 --> 02:22.790 So this will now call the greater-than operator of the string class. 02:23.360 --> 02:26.300 So we expect to get these sorted in the reverse order. 02:26.660 --> 02:30.470 So with the highest first, in reverse alphabetical order. 02:33.420 --> 02:33.720 OK. 02:33.810 --> 02:34.680 And that is what we get. 02:37.790 --> 02:41.480 So there is quite a few of these. They provide all the usual operations. 02:41.870 --> 02:44.750 It is pretty obvious what the object does. 02:45.380 --> 02:48.080 It may not always be obvious what the name of the object should be. 02:48.080 --> 02:50.960 I am not sure I would have chosen the same ones - but there you go. 02:52.100 --> 02:53.690 So those are for arithmetic. 02:54.470 --> 03:01.640 Then we have the relational operators: equal to, not equal to, greater, less, greater equal, less equal. 03:03.430 --> 03:13.000 And logical operators: logical and, logical or, logical not, and some bitwise operators as well. The 03:13.000 --> 03:18.490 not operator did not get in until C++ 14. For some reason. 03:19.270 --> 03:20.980 OK, so that's it for this video. 03:21.370 --> 03:24.340 I will see you next time, but until then, keep coding!