WEBVTT 00:00.090 --> 00:00.660 Hello again! 00:01.110 --> 00:03.720 In this video, we are going to look at value categories. 00:04.470 --> 00:10.770 These are something which came in in C++ 17, although they actually describe the situation which goes 00:10.770 --> 00:12.330 back to C++11. 00:13.290 --> 00:19.290 So it is worth spending a few minutes on these, even if you are not ready to update to C++17 just yet. 00:21.580 --> 00:23.440 The characteristics of lvalues. 00:23.800 --> 00:28.840 They represent objects which are persistent, so they survive for more than one statement. 00:29.650 --> 00:32.650 They occupy memory which is accessible to the programmer. 00:33.190 --> 00:39.280 They are either on the program stack or in the heap, and they remain valid until they go out of scope 00:39.610 --> 00:40.570 or are deleted. 00:41.380 --> 00:47.320 So this could be a local variable in a scope; global or static variables; and function arguments. 00:49.690 --> 00:53.920 rvalues, on the other hand, are stored in locations which are not accessible to the programmer, 00:54.310 --> 00:55.900 such as processor registers. 00:56.380 --> 00:57.880 So we cannot take their address. 00:58.150 --> 01:00.370 And there are two different types of rvalue. 01:00.880 --> 01:07.180 There are literals like the number 2 or the letter 'c', and there are temporary objects, which are created 01:07.180 --> 01:09.460 and destroyed in the same statement. 01:12.210 --> 01:17.340 C++17 has three so-called "value categories", to describe these possibilities. 01:18.210 --> 01:22.050 The literals are called "pure" rvalues or "prvalues". 01:22.680 --> 01:26.040 These have no name, and they cannot be referred to again. 01:29.490 --> 01:31.600 Temporary objects are called "xvalues". 01:32.070 --> 01:36.270 These represent an object of some class, and their data can be moved. 01:37.080 --> 01:43.350 The original proposal was that the 'x' stands for "expert" or "exceptional", which are both pretty terrible. 01:44.040 --> 01:46.710 Then someone came up with "expiring", and that is much better. 01:47.160 --> 01:51.510 Because these objects will expire at the end of the statement in which they are created. 01:53.250 --> 01:55.560 And then finally, we have good old-fashioned lvalues. 01:57.030 --> 02:00.210 When we learn C++, we are told that every expression has a type. 02:00.780 --> 02:04.380 Now, we have to say that every expression has a type and a value category. 02:07.840 --> 02:15.400 So let's look at a diagram. Values can be either xvalues which are temporary objects, or prvalues 02:15.400 --> 02:16.390 which are literals. 02:17.020 --> 02:18.640 And then we have lvalues, on their own. 02:21.960 --> 02:23.820 In fact, that is not quite the end of the story. 02:24.150 --> 02:26.970 lvalues and xvalues do have some things in common. 02:27.360 --> 02:32.760 They are objects, and they can have a dynamic type which is different from their static type. 02:33.270 --> 02:37.530 So sometimes, it is useful to talk about lvalues and xvalues together. 02:38.220 --> 02:43.320 And there is a term for this, which is "generalized lvalue", or glvalue for short. 02:44.370 --> 02:49.920 So the final version of our diagram, which looks like a plate of fried eggs! We have rvalues which 02:49.920 --> 02:56.130 can be prvalues or xvalues, and glvalues, which can be lvalues or xvalues. 02:57.060 --> 02:59.340 So, an rvalue can be a literal or a temporary 02:59.340 --> 03:05.670 object, and a generalized lvalue can be either a "proper" lvalue or a temporary object. 03:06.900 --> 03:07.230 Okay. 03:07.230 --> 03:08.340 So that is it for this video. 03:08.820 --> 03:09.660 I will see you next time. 03:09.900 --> 03:11.810 Until then, keep coding!