WEBVTT 00:00.180 --> 00:03.900 Hello again! In this video, we are going to look at the swap() function. 00:06.620 --> 00:10.400 The C++ Library provides a standard swap() function. 00:10.910 --> 00:12.710 This is a non-member function. 00:13.040 --> 00:20.150 It takes two arguments of the same type, and it exchanges their internal data. swap() is declared as 00:20.150 --> 00:24.410 noexcept, so it provides the "no throw" exception guarantee. 00:27.180 --> 00:30.780 swap() is a generic function, so it knows nothing about the arguments. 00:31.140 --> 00:32.550 It cannot do anything clever. 00:32.910 --> 00:33.810 It just copies them. 00:34.770 --> 00:40.680 However, there are overloads for library types and these do take advantage of the internal structure 00:40.680 --> 00:41.730 of the arguments. 00:42.540 --> 00:49.030 As an example, we looked at the string overload of swap(), earlier on in the course, and this just does 00:49.080 --> 00:54.060 some pointer manipulation. And that is much faster than allocating memory and copying data. 00:57.160 --> 01:01.390 There are some algorithms which exchange object data internally. 01:01.660 --> 01:05.530 The sort() algorithm is one of those. It does that, to put the elements in order. 01:06.430 --> 01:13.300 And there are some implementations which do that by calling the nonmember swap() function. And the function 01:13.300 --> 01:15.460 which gets called depends on the element's type. 01:16.150 --> 01:19.060 If there is an overload for the element, then that gets called. 01:19.510 --> 01:23.260 Otherwise, the generic version is used, and that will copy the elements. 01:24.010 --> 01:27.880 If you are writing your own class, and copying it takes a long time, 01:28.180 --> 01:29.740 then this could be rather inefficient. 01:30.820 --> 01:36.470 So if this is likely to happen, it could be worth implementing an overload of swap for your class. 01:37.330 --> 01:43.030 So that is if the class is expensive to copy and you expect to do a lot of swap operations on it. 01:43.540 --> 01:46.510 For example, if you have a container of these objects which get sorted. 01:48.680 --> 01:49.640 So how do we do that? 01:51.940 --> 01:57.220 The algorithm will assume that swap() is very fast, and does not throw any exceptions. 01:58.360 --> 02:02.470 It is going to be a non-member function which will need access to data members. 02:02.980 --> 02:08.320 So we need to make it a friend of the class, or perhaps delegate it to a public member function. 02:09.940 --> 02:14.410 We are going to write it as an inline function, so the compiler can optimize away the function call. 02:14.950 --> 02:20.830 I know that compiler writers say that their compilers are very clever and always do this, but in practice 02:20.830 --> 02:22.030 they sometimes need a hint. 02:23.650 --> 02:25.150 We are going to make it noexcept. 02:25.960 --> 02:29.020 And then we are just going to call swap() for each data member. 02:31.480 --> 02:36.050 We are going to use our String class again. Let's quickly remind ourselves what this does. 02:36.920 --> 02:38.750 So we have two members. 02:39.560 --> 02:41.450 We have a constructor which allocates memory. 02:42.200 --> 02:45.860 We have a copy constructor which allocates memory and copies the data. 02:47.390 --> 02:48.260 Then we have an assignment 02:48.260 --> 02:56.270 operator, destructor, and then we declare our overload of swap as a friend of this class. 02:57.730 --> 03:04.960 And here is our swap() definition is declared as in line and noexcept. We take the arguments by 03:04.960 --> 03:11.050 reference, not by const reference, because we are going to modify them, and then we just call swap() for each 03:11.050 --> 03:12.100 member of the class. 03:14.300 --> 03:18.950 We have a main function, which creates two objects. 03:19.490 --> 03:24.020 We print out their details, then we call swap(). And then we print out the details again. 03:25.070 --> 03:26.240 So let's see if this works. 03:29.250 --> 03:30.090 And there we are. 03:30.720 --> 03:34.230 So we can see that we have the details before and after. 03:34.800 --> 03:38.850 And we have just swapped around the size value and the pointers. 03:42.030 --> 03:45.510 Let's see what happens if we comment out this swap overload. 03:47.250 --> 03:51.900 So let's comment that out and then... this... 03:55.440 --> 03:57.090 So what do you think will happen now? 03:59.100 --> 04:02.130 We're going to call swap(), but there is no overload for this class. 04:02.520 --> 04:06.360 So this is going to call the generic version, and that is going to copy the data. 04:07.140 --> 04:08.520 So let's see what difference that makes. 04:12.130 --> 04:16.210 Right, well, it looks as though this has made quite a bit of difference, we have all these operations 04:16.210 --> 04:17.590 here, which are re-allocating memory. 04:19.090 --> 04:22.660 And also we have different addresses, because they have been reallocated. 04:23.080 --> 04:27.910 So you can see there is quite a lot more work involved in calling the generic version, which does copying. 04:28.420 --> 04:30.610 So the overloaded version is much faster. 04:32.110 --> 04:33.520 Okay, so that is it for this video. 04:33.970 --> 04:34.780 I will see you next time. 04:34.780 --> 04:36.750 But until then, keep coding!