WEBVTT 00:00.490 --> 00:08.060 It's oh last time we talked about pointers and in this lecture I want to talk about a concept that's 00:08.060 --> 00:11.000 very similar which is references. 00:11.270 --> 00:18.420 So a reference it is a variable that also has an address as its value. 00:18.860 --> 00:23.670 So there are some key differences between pointer and a reference. 00:23.690 --> 00:26.870 But first let's look at how to make one. 00:27.140 --> 00:44.020 So we had X was 5 in a reference and that uses the ampersand instead of the star we have four in set 00:44.170 --> 00:46.270 to X like this. 00:47.010 --> 00:54.870 OK so don't get this ampersand can use with address when it's in a declaration for a variable this means 00:54.870 --> 00:55.770 reference 00:58.100 --> 00:59.140 so. 00:59.190 --> 01:07.880 Notice also that we set the reference and we're not taking the address of X. 01:08.190 --> 01:15.200 OK so before we had we had the address of X for pointers but we don't need to do that it possible sort 01:15.200 --> 01:18.090 of do that for us with references. 01:18.430 --> 01:21.910 K so there is also something else to note. 01:22.100 --> 01:25.290 And that's not really immediately obvious. 01:25.550 --> 01:29.370 And that is you can't use something like this. 01:29.420 --> 01:34.300 So we had another reference in reference to. 01:34.600 --> 01:39.460 You can't do this in you'll see if you try to do this. 01:39.760 --> 01:48.380 OK so says the declaration a reference variable and ref 2 requires initializer. 01:48.520 --> 01:56.050 And so it needs to have an initial value at the start k and that is because a reference is really an 01:56.080 --> 01:57.410 alias. 01:57.650 --> 02:02.560 It needs something to reference as it were right. 02:02.920 --> 02:05.690 Otherwise it it doesn't make any sense. 02:06.190 --> 02:13.600 OK so you can't have a reference that doesn't reference anything that doesn't it doesn't work. 02:13.600 --> 02:19.690 So this is actually pretty similar to constants if you're a member or so constant in. 02:20.080 --> 02:25.120 You know Max you couldn't have a concept like this for example 02:28.220 --> 02:31.340 in this. 02:31.380 --> 02:31.810 Right. 02:31.920 --> 02:37.130 So default initialization of object of conc type constant. 02:37.140 --> 02:38.150 Right. 02:38.310 --> 02:41.230 So this is very similar. 02:41.420 --> 02:49.440 So and this should make sense because you can't change Max later because it's a constant. 02:49.440 --> 02:51.900 So it's the same thing for references. 02:55.550 --> 03:06.000 So another key difference is between between the references and pointers is that you don't need to dereference 03:06.000 --> 03:12.960 a reference in order to change the value pointed to by the reference. 03:12.970 --> 03:25.330 So we had this if you want to change it to say giraffe to 10. 03:25.690 --> 03:33.020 And if you want to see out the value x is 03:35.060 --> 03:37.860 x and it's 03:40.640 --> 03:41.930 X is 10 right. 03:41.960 --> 03:45.010 So the reference actually changed x. 03:45.200 --> 03:50.380 So we noticed you didn't use the dereference operator star operator. 03:50.380 --> 03:52.840 You remember from last time. 03:52.910 --> 03:57.890 That's because again C++ sort of does that for you already. 03:57.890 --> 04:02.220 So there's no need to use it in order to change the value. 04:04.550 --> 04:11.930 So I really think of references as the original variable self. 04:12.050 --> 04:15.510 So that's really my mental model for them. 04:15.590 --> 04:19.860 I can do it any operation that I can do with the original. 04:19.920 --> 04:30.740 So might be asking then when should I use a reference over a pointer and just use a pointer when you 04:30.740 --> 04:33.890 want it to point to either nothing. 04:33.890 --> 04:42.620 I remember that null pointer or a real value and you should use reference when you don't always want 04:42.620 --> 04:45.160 to point to a real value. 04:45.550 --> 04:50.690 OK so there's this subtle but it's it's very important. 04:50.690 --> 04:54.260 So this will become much more clear as we start using them.