WEBVTT 00:00.180 --> 00:07.050 Hello again! In this video, we are going to look at the standard library array. C++ already has arrays. 00:07.080 --> 00:14.400 These are built into the language. They were inherited from C, in code like that. Built-in arrays are 00:14.430 --> 00:19.740 faster than using vectors, and they are directly compatible with code written in C, but there are some 00:19.740 --> 00:22.110 serious issues in using them. 00:23.100 --> 00:27.960 The most obvious is, if you have an array on the heap, then you need to manage its memory yourself. 00:28.440 --> 00:29.700 But vector will get around 00:29.700 --> 00:32.160 that. If you have an array on the stack, 00:32.160 --> 00:33.210 there are still some issues. 00:33.930 --> 00:37.430 The biggest problem is, there is no direct way to find the number of elements. 00:38.340 --> 00:43.710 If you want to know the number of elements, then either you need to have a variable which stores, it or else 00:43.710 --> 00:44.970 you need to calculate it. 00:46.920 --> 00:52.350 The language has several places where an array has to be converted into a pointer. And the compiler will 00:52.350 --> 00:54.660 do that for you, even if you do not want it to. 00:55.740 --> 01:00.960 In particular, if you pass an array as an argument to a function, or return it from a function call, then 01:00.960 --> 01:02.490 it will be converted to a pointer. 01:03.090 --> 01:06.390 And that means you need to provide another argument with the number of elements. 01:07.290 --> 01:12.390 And also, another limitation is that you are not allowed to assign one built-in array to another. 01:12.930 --> 01:21.180 To overcome these problems, Modern C++ provides the library array. This is in the library namespace std. 01:21.870 --> 01:23.640 And it is in the array header. 01:24.600 --> 01:26.880 It is a templated type with two parameters. 01:26.880 --> 01:31.560 We need to give the type of the element and the number of elements when we create an object. 01:32.460 --> 01:36.360 So this means that the number of elements in the array is part of the type of the object. 01:36.960 --> 01:41.820 If we try to use an array which has the wrong number of elements, then the compiler will not allow 01:41.820 --> 01:48.840 that. The interface is similar to containers from the standard library, but it is still just as fast as 01:48.840 --> 01:49.860 using a built-in array. 01:51.240 --> 01:52.980 It never "decays" into a pointer. 01:53.340 --> 01:58.080 If you have a library array object, it always remains a library array object. 01:58.950 --> 02:03.420 It also has information about its size, the number of elements. And if you want that, you can just 02:03.420 --> 02:05.220 get that, by calling a member function. 02:07.010 --> 02:10.460 And finally, it supports iterators, which makes writing loops a lot simpler. 02:12.290 --> 02:15.170 A library array object can only be created on the stack. 02:15.590 --> 02:18.980 If you want one on the heap, then you can use the standard vector. 02:20.510 --> 02:25.250 The compiler will allocate enough memory to hold all the elements on the stack. That will be a contiguous 02:25.250 --> 02:26.390 block, with no gaps in it. 02:26.960 --> 02:32.330 And that means that the compiler must be able to find out the number of elements, so that must be something 02:32.330 --> 02:33.770 which can be found at compile time. 02:35.540 --> 02:40.250 One difference from other containers: if you default-construct a string or vector, you will get 02:40.250 --> 02:41.120 an empty container. 02:41.840 --> 02:47.510 If you default construct a library array, then you will get an array with the right number of elements. 02:47.780 --> 02:50.450 And that is possible, because the number of elements is part of the type. 02:51.410 --> 02:53.810 These elements are going to be default-initialized. 02:54.110 --> 02:58.910 And if the elements have built-in type, that means their initial value is going to be undefined. 03:01.510 --> 03:07.720 So we need to give the type of the elements and the number of elements, when we create an array object. 03:08.680 --> 03:14.920 We can use a list initializer just like we do for vectors. And then we can use array indexing, just like 03:14.920 --> 03:16.030 we do with the built-in array. 03:19.350 --> 03:25.150 For writing loops, arrrays support iterators, so we can write a traditional iterator loop. 03:27.740 --> 03:32.540 Or a range-for loop. if we want to write an index loop, we can do that as well. 03:33.380 --> 03:38.240 And we are also allowed to assign array, provided the elements have the same type and they have the 03:38.240 --> 03:39.500 same number of elements. 03:44.480 --> 03:50.090 The interface for the array is very similar to the vector, so we can find the number of elements and 03:50.090 --> 03:56.780 whether the array is empty. We can access elements by index, with and without bounds checking. 03:58.980 --> 04:04.680 We can find the first and last element, and we can also get a built-in array, which contains all the 04:04.680 --> 04:05.130 data. 04:06.390 --> 04:11.850 The issue with function calls is that the array is converted to a pointer, so we need to pass the number 04:11.850 --> 04:13.410 of elements as a separate argument. 04:15.390 --> 04:19.410 This means we either need to know the number of elements, or we need to calculate it. 04:20.010 --> 04:24.060 And you can do it like that. Or if you want to get really clever, there is also a template trick. 04:25.770 --> 04:30.300 And then we have to pass the number of elements as the argument. With a library 04:30.330 --> 04:34.980 We can just pass it directly, because the number of elements is part of the type. 04:42.420 --> 04:43.800 Okay, so that is it for this video. 04:44.460 --> 04:47.940 I will see you next time, but until then, keep coding!