WEBVTT 00:00.150 --> 00:07.320 Hello again! In this video, we are going to look at maps in C++17. C++17 has some new features which 00:07.320 --> 00:09.360 make working with maps a little easier. 00:10.860 --> 00:13.200 The first one is called a structured binding. 00:13.800 --> 00:16.800 This is an easy way for accessing data structures. 00:17.460 --> 00:23.790 We can write a single statement, which will declare some local variables and bind them to members of 00:23.790 --> 00:29.910 a compound data structure, and the types of these variables will be deduced for us by the compiler. 00:31.860 --> 00:35.310 This is implemented as a, kind of, more general version of auto. 00:36.450 --> 00:39.570 So in C++11, we can write statements like this. 00:40.200 --> 00:43.680 "i" will be a variable of type int, with initial value 00:43.680 --> 00:46.660 42. In 00:46.680 --> 00:47.260 C++17, 00:47.280 --> 00:51.840 You can put multiple variables in here, and you can have a compound object in here. 00:52.380 --> 00:57.480 Obviously, the number of variables here must be equal to the number of members of this object. 01:01.080 --> 01:04.560 So we can do something like this, we can have a pair object on the right hand side. 01:05.010 --> 01:11.370 This has two members, so we can use this to create and initialize two local variables. 01:12.240 --> 01:14.160 So this will define a local variable 01:14.160 --> 01:22.440 "i", of type inst with the initial value 1, and a variable "d", of type double, with initial value 01:22.440 --> 01:22.800 3.142. 01:25.530 --> 01:26.880 So let's try that out. 01:27.240 --> 01:33.810 So we have our pair, here. And then we have our structured binding, which will initialize these variables 01:33.810 --> 01:34.680 from the pair. 01:35.310 --> 01:37.800 And then we just print out the values of the variables. 01:43.970 --> 01:44.570 And there we are. 01:45.080 --> 01:47.600 "i" is 1, and "d" is 3.142. 01:50.730 --> 01:52.470 So how are these useful with maps? 01:52.770 --> 01:57.270 Well, one thing we can do is to make loops more readable. In C++11, 01:57.630 --> 01:58.320 we can do this. 01:58.320 --> 02:03.720 We can write a range-for loop, but this variable here will be an element of the map. 02:04.230 --> 02:05.370 So that is going to be a pair. 02:06.030 --> 02:09.450 So to get the data, we have to use the first member for the key, 02:09.840 --> 02:15.600 and the second member for the value. In C++17, we can write a structured binding. 02:16.020 --> 02:18.300 We can create local variables for this loop. 02:18.900 --> 02:25.860 The first variable, "name", will be the key, and the second variable, "score", will be the value. 02:26.490 --> 02:31.050 And then, in the loop body, we can use these meaningful names instead of "first" and "second". 02:33.520 --> 02:35.890 So let's try this. We have our favourite map. 02:36.730 --> 02:40.430 We are going to print it out, using the range-for loop that we have used so far. 02:41.410 --> 02:44.380 And then we're going to write another loop, with a structured binding. 02:52.010 --> 02:52.610 So there we are. 02:52.940 --> 02:58.550 So we get a list of all the elements, with the key and the value. And with the structured binding, 02:58.550 --> 02:59.900 we get the same list as well. 03:04.890 --> 03:08.730 We can also use a structured binding to check the return value from insert(). 03:09.120 --> 03:12.330 So you will remember, this returns a pair, with an iterator and a bool. 03:13.200 --> 03:19.260 We can use a structured binding to create local variables. So this represents the iterator from the returned 03:19.260 --> 03:21.300 value, and that is the bool. 03:23.390 --> 03:29.230 And then we can write code, which looks a bit more normal. If we dereference this iterator, then we get 03:29.240 --> 03:30.050 another pair. 03:30.380 --> 03:35.000 This is going to be the pair which is the element of the map. And then we can use another structured 03:35.000 --> 03:36.410 binding to get the data out of that. 03:36.950 --> 03:42.080 So the first variable, "name", is going to be the key. And the second variable, "score" is going to be the value. 03:44.690 --> 03:45.980 So let's try this. 03:46.280 --> 03:53.240 So we have our call to insert(). Then we use a structured binding to split out the iterator and the bool 03:53.240 --> 03:54.500 from the returned pair. 03:55.910 --> 04:00.590 Then we can check the bool. And if we want the data from the element, then we can just dereference 04:00.590 --> 04:03.620 the iterator and use another structured binding to get the data. 04:12.160 --> 04:18.280 So we tried to insert an element with the key "Graham". There already is one, so that failed. So we went 04:18.280 --> 04:19.750 into the "else" branch of the loop. 04:22.190 --> 04:27.200 And then we can get the details of the element which is already there, by dereferencing the iterator. 04:30.910 --> 04:36.100 In the last video, we talked about the subscript operator, which gives "insert or assign" functionality, 04:36.670 --> 04:38.290 but it has some limitations. 04:39.160 --> 04:44.170 The insert() member function does not have these limitations, but it can only insert new elements. 04:45.220 --> 04:49.690 So C++17 has a new member function, which provides the best of both worlds. 04:50.500 --> 04:55.980 This will insert a new element, if there is no element with the key that we want. 04:56.620 --> 05:01.660 If there already is an element with that key, then it will overwrite the value, so it is going to assign 05:01.720 --> 05:02.410 to the element. 05:03.160 --> 05:06.970 And this takes two arguments, which are the key and the value of the element. 05:08.410 --> 05:11.920 This will return a pair, like insert(), so we can find out what happened. 05:12.730 --> 05:15.130 The first member is an iterator to the element. 05:16.000 --> 05:19.720 If it is a new element, it will be an iterator to the new element. 05:20.530 --> 05:25.570 If it assigned to an existing element, then that will be an iterator to the old element. 05:26.890 --> 05:32.830 The bool will be true if it inserted a new element, and it will be false if it performed an assignment. 05:33.310 --> 05:35.710 So in the case where it does insert a new element, 05:36.010 --> 05:38.740 the return value is exactly the same as for insert(). 05:42.480 --> 05:44.790 So here we go again with our favourite map. 05:45.350 --> 05:47.430 (Perhaps - I am getting a bit tired of this one, actually!) 05:49.500 --> 05:53.070 So we called insert_or_assign(), instead of insert() 05:53.460 --> 05:56.490 We are going to pass a key which is already in the map. 05:56.880 --> 06:01.080 So we expect this to assign, and not create a new element. 06:01.800 --> 06:03.210 So we should go into this branch. 06:10.230 --> 06:16.490 Okay, so we had an existing element with the name "Graham". This had the score 78, and we have now assigned 06:16.500 --> 06:20.940 it to have the score 66, so we've assigned to the value of that element. 06:23.760 --> 06:29.000 And this has all the advantages of the insert() member function. We can use values which do not have 06:29.020 --> 06:34.260 a default constructor. If any exceptions are thrown, then the operation has no effect. 06:36.490 --> 06:40.000 And finally, let's go really crazy about C++17 features! 06:40.480 --> 06:44.140 So you might remember that we can have an initializer inside an if statement. 06:44.680 --> 06:50.110 So, for example, we can call a function, get its return value and then follow these different branches, 06:50.110 --> 06:51.610 depending on the returned value. 06:52.630 --> 06:56.170 When you have a structured binding, that counts as an initializer. 06:56.740 --> 07:01.570 So you are allowed to do things like this. So you can call insert_or_assign(), 07:02.850 --> 07:08.610 use a structured binding to get the bool, and then use that bool as the conditional for the if statement. 07:09.150 --> 07:14.310 So depending on whether you inserted or modified an element, you can follow different branches. 07:17.200 --> 07:18.360 So let's look at that. 07:20.450 --> 07:22.850 So here is our call to insert_or_assign(). 07:23.240 --> 07:30.080 We have now moved that inside the if statement. We are initializing this bool from the return value, and 07:30.080 --> 07:31.250 then we can branch on it. 07:39.930 --> 07:40.500 And there we are. 07:40.710 --> 07:44.100 So we get the same result as before, but with one less line of code. 07:45.270 --> 07:46.680 Okay, so that us it for this video. 07:47.010 --> 07:47.820 I will see you next time. 07:47.820 --> 07:49.890 But until then, keep coding!