WEBVTT 00:00.120 --> 00:03.720 Hello again! In this video, we are going to look at maps and insertion. 00:04.650 --> 00:10.800 The reason for this is that there are some new features in C++ 17 for working with maps, and we need 00:10.800 --> 00:12.810 a bit more background before we can go onto those. 00:14.630 --> 00:18.890 So just to recap, we know two ways of inserting an element into a map. 00:19.580 --> 00:27.500 These are the subscript operator and the insert() member function. Looking first at the subscript operator. 00:27.800 --> 00:30.950 this provides so-called "insert or assign" functionality. 00:31.550 --> 00:38.060 So if there is no element in the map which has the key we are using, this will insert a new element. If 00:38.060 --> 00:40.100 there already is an element with that key, 00:40.340 --> 00:42.410 it will assign to the value of that element. 00:46.200 --> 00:51.540 So if we have some code like this: we create a map object, and then later on, we have a statement 00:51.540 --> 00:52.050 like this. 00:52.920 --> 00:59.160 There are two possible outcomes depending on what happens in here. Either there is no "Graham" element, 00:59.160 --> 01:04.530 in which case we can insert one, or there already is a "Graham" element, in which case we overwrite it. 01:05.640 --> 01:10.260 So sometimes we might want to know whether we have actually created a new element or not. 01:10.770 --> 01:13.200 And unfortunately, there is no way to know. 01:18.350 --> 01:24.080 Looking at the actual way that the subscript operator inserts an element, when it does insert one. 01:24.470 --> 01:25.880 This is done in two stages. 01:26.330 --> 01:28.730 So first, it will create the new element. 01:29.570 --> 01:35.300 The key is going to be the argument to the subscript, and the value is going to be default-initialized. 01:36.440 --> 01:42.440 So this means that the value type has to be either a built in type or something that has a default constructor. 01:43.100 --> 01:45.470 And that can be an irritating restriction sometimes. 01:46.790 --> 01:51.170 Also, if an exception gets thrown, probably by the assignment. 01:52.210 --> 01:54.230 We have not actually covered exceptions yet, 01:54.250 --> 01:59.950 I know. But, basically, what happens is that this operation will end before it has finished doing all its 01:59.950 --> 02:00.280 work. 02:00.760 --> 02:04.210 And in that case, we end up with a partially populated element. 02:04.570 --> 02:05.440 So it has a key, but 02:05.440 --> 02:07.510 the value is default-initialized. 02:09.300 --> 02:14.970 Also, if we forget to do the assignment part, then we end up with a partially populated element as 02:14.970 --> 02:15.270 well. 02:15.690 --> 02:21.180 So this will have the key "Graham" and an undefined value, or possibly default-initialized. 02:23.450 --> 02:29.210 And the return value from the operation is a reference to the value member of the element, so that tells 02:29.210 --> 02:30.560 us nothing about what happened. 02:33.770 --> 02:39.110 The insert member will only insert a new element - at least it will only directly insert one. If you want 02:39.140 --> 02:41.060 to do anything else, you have to write a bit more code. 02:42.440 --> 02:44.640 If the map already has an element with the same key, 02:44.670 --> 02:45.410 nothing happens. 02:46.250 --> 02:51.410 And the return value from insert() will actually tell us whether we actually inserted an element or not. 02:52.760 --> 02:57.020 There is no requirement that the type of the value has a default constructor. 02:57.740 --> 03:03.140 And if an exception is thrown, then the insertion has no effect. So the operation does get rolled 03:03.140 --> 03:03.440 back. 03:05.240 --> 03:10.790 So the insert() member function has some advantages over subscripts, but it does not perform "insert 03:10.790 --> 03:11.420 or assign". 03:11.780 --> 03:13.790 If we want that, we have to write code like this. 03:14.360 --> 03:19.760 So we do the insert. Then we get the return value. Then we check the second member. 03:20.600 --> 03:23.330 And if that is false, then we need to do the assignment. 03:24.140 --> 03:29.330 So it is nothing very complicated, but it is five statements for something which really is a single logical 03:29.330 --> 03:29.930 operation. 03:32.520 --> 03:35.040 So let's try this out with the map that we had before. 03:37.320 --> 03:46.870 We call insert(). And then, if the insert() does not succeed, we need to assign to the value member of this 03:47.070 --> 03:47.600 element. 03:49.380 --> 03:52.560 So there it is, we already had an element with the key "Graham". 03:52.920 --> 03:55.620 So we needed to modify the value. 03:56.950 --> 03:59.170 OK, so that is it for this video. 03:59.620 --> 04:02.680 I will see you next time, but until then, keep coding!