WEBVTT 00:00:00.640 --> 00:00:04.070 Now that we know how to use pointers to point to objects, 00:00:04.080 --> 00:00:07.500 the rest of the module will be about using pointers inside 00:00:07.500 --> 00:00:09.760 of objects as data members. 00:00:10.140 --> 00:00:13.330 We'll begin this journey by imagining that we are genetic 00:00:13.330 --> 00:00:16.700 engineers. And just a disclaimer, I'm not very good at 00:00:16.700 --> 00:00:19.010 biology and you don't have to be either. 00:00:19.020 --> 00:00:20.760 This is just hypothetical. 00:00:21.140 --> 00:00:24.350 I created a simple DNA class, which holds the genetic 00:00:24.350 --> 00:00:27.460 code inside of this really large array. 00:00:27.840 --> 00:00:31.450 I used digit separator syntax to make this more readable. 00:00:31.460 --> 00:00:35.240 But the code array holds 60 million characters, 00:00:35.250 --> 00:00:38.360 which is equal to 60 MB of memory. 00:00:38.740 --> 00:00:42.260 The real DNA sequence would be much larger, but the reason for 00:00:42.260 --> 00:00:45.820 creating this big array is to make it impossible for us to use 00:00:45.820 --> 00:00:47.950 this DNA object on the stack. 00:00:48.340 --> 00:00:52.450 I will try to declare one just to prove that this is not possible. 00:00:53.440 --> 00:00:55.180 If I try to compile this program, 00:00:55.190 --> 00:00:59.670 I will get a segmentation fault because this is too big for stack memory. 00:00:59.680 --> 00:01:03.660 So let's instead allocate this DNA object on the heap. 00:01:04.239 --> 00:01:08.070 Instead of reading the genetic code from a file and making things more 00:01:08.070 --> 00:01:11.540 complicated, let's agree that the first four elements of the code array 00:01:11.540 --> 00:01:16.750 represent the whole sequence. And I want to initialize these four 00:01:16.750 --> 00:01:19.250 elements with a simple string literal. 00:01:20.240 --> 00:01:22.760 Of course, we need to define this constructor. 00:01:23.140 --> 00:01:27.090 We learned in one of the previous lessons that we can pass the string literal 00:01:27.090 --> 00:01:30.960 to a function by using the constant pointer to a char value. 00:01:31.340 --> 00:01:34.130 It has to be constant because the string literal is 00:01:34.140 --> 00:01:35.960 in a read only part of memory. 00:01:36.340 --> 00:01:40.570 I will simply rip out all of the characters from this literal and store 00:01:40.570 --> 00:01:43.960 them at the start of the code array in the same order. 00:01:44.340 --> 00:01:47.170 This is one of those situations where we actually use the 00:01:47.170 --> 00:01:50.310 array of characters as an array of characters. 00:01:50.320 --> 00:01:52.950 I don't want to use this code array as a string. 00:01:53.340 --> 00:01:57.370 It is literally just a sequence of characters and each one of these 00:01:57.370 --> 00:02:01.070 characters should be one of the four possible genetic bases, 00:02:01.080 --> 00:02:03.660 C, G, T, or A. 00:02:04.240 --> 00:02:08.150 I also want a way to print out this code, so I will create another 00:02:08.150 --> 00:02:11.600 function for that. We will simply loop over the first four 00:02:11.600 --> 00:02:14.310 characters and print them out to the terminal. 00:02:14.320 --> 00:02:17.090 We could have used the null character at the end of this 00:02:17.090 --> 00:02:20.370 sequence, but I still want to pretend that all of the 00:02:20.370 --> 00:02:22.260 other elements are initialized. 00:02:22.640 --> 00:02:25.250 And now this constructor should work. 00:02:25.260 --> 00:02:28.850 Let's test it by using the printGeneticCode function. 00:02:29.240 --> 00:02:33.350 Don't forget to use the arrow operator because sample is a pointer. 00:02:35.440 --> 00:02:38.300 The code is presented correctly, so it seems that 00:02:38.300 --> 00:02:40.360 we are done with the DNA class. 00:02:40.740 --> 00:02:42.880 Since we are now scientists, 00:02:42.890 --> 00:02:46.050 we also need to have subjects for the experiments. I 00:02:46.540 --> 00:02:49.150 will create a simple Subject class. 00:02:49.640 --> 00:02:54.070 Every subject has a unique integer ID and also a DNA sample 00:02:54.070 --> 00:02:56.560 which represents its genetic material. 00:02:56.940 --> 00:02:59.470 Since DNA class is really large, 00:02:59.480 --> 00:03:02.020 this kind of implementation would prevent us from 00:03:02.030 --> 00:03:04.860 instantiating subjects on the stack. 00:03:04.870 --> 00:03:08.820 So instead of having the sample inside of the subject itself, 00:03:08.830 --> 00:03:11.960 we can turn it into a pointer to a DNA object. 00:03:12.840 --> 00:03:17.130 Now, let's define the constructor. It will accept the ID of the 00:03:17.130 --> 00:03:21.060 subject and also the string literal for the DNA sample, 00:03:21.440 --> 00:03:26.120 I want to pass this string literal pointer to the DNA object, but the object 00:03:26.120 --> 00:03:29.890 doesn't exist yet, we only have an empty sample pointer. 00:03:29.900 --> 00:03:32.360 So inside of this constructor, 00:03:32.360 --> 00:03:37.220 we actually need to allocate space on the heap for this DNA object and 00:03:37.220 --> 00:03:41.560 store its location inside of the sample pointer. I will pass this 00:03:41.560 --> 00:03:46.360 pointer to a string literal down to the DNA constructor and this is 00:03:46.360 --> 00:03:49.680 where we need to start thinking about all of the side effects from 00:03:49.680 --> 00:03:52.160 using pointers as data members. 00:03:52.540 --> 00:03:56.250 Since we allocated this DNA resource inside of the constructor, 00:03:56.250 --> 00:03:59.370 we need to deallocate it inside of the destructor. 00:03:59.380 --> 00:04:05.520 So let's do this. I hope that you remember the concept of RAII 00:04:05.520 --> 00:04:08.550 from the first module because this is it. 00:04:09.340 --> 00:04:12.070 All of the resources that we allocate inside of the 00:04:12.070 --> 00:04:15.840 constructor need to be released inside of the destructor; 00:04:15.840 --> 00:04:18.550 otherwise, we are risking a memory leak. 00:04:18.940 --> 00:04:22.260 I also want to create a default constructor, which will just 00:04:22.270 --> 00:04:25.960 instantiate an empty subject with the ID of 0. 00:04:26.340 --> 00:04:30.180 The genetic placeholder code will also be a string of 0s 00:04:30.180 --> 00:04:33.030 because I want to prepare this place on the heap for 00:04:33.030 --> 00:04:35.280 potential future modifications, 00:04:35.400 --> 00:04:38.360 we will need this for some of the following lessons. 00:04:38.740 --> 00:04:39.500 Finally, 00:04:39.500 --> 00:04:43.780 let me also declare the printSubjectData function so that we can easily see 00:04:43.780 --> 00:04:46.860 all that we need to know about it on the terminal screen. 00:04:47.240 --> 00:04:52.140 I will first print out the subject's ID and then invoke the printGeneticCode 00:04:52.140 --> 00:04:55.160 function to print the genetic sequence from the sample. 00:04:56.140 --> 00:04:57.550 Now we can try this out. 00:04:58.340 --> 00:05:01.580 Let's remove all of this code from the main function and create a 00:05:01.580 --> 00:05:05.660 sheep subject with some random ID and genetic code. 00:05:06.340 --> 00:05:09.850 And let's also print the information about this subject. 00:05:10.740 --> 00:05:16.550 Okay, I'm going to compile this, and it seems that everything works as expected. 00:05:16.940 --> 00:05:19.480 Hopefully, I thought of a good enough example to 00:05:19.480 --> 00:05:22.550 make you see why RAII is important. 00:05:22.940 --> 00:05:26.750 This also applies to using pointers for any kind of resource, 00:05:26.760 --> 00:05:28.850 not just objects on the heap. 00:05:29.240 --> 00:05:32.720 Maybe you need to access a file through the pointer, and you open a 00:05:32.720 --> 00:05:36.860 connection to it in the constructor, but remember to always release 00:05:36.860 --> 00:05:39.160 resources inside of the destructor. 00:05:39.540 --> 00:05:41.790 Now that we defined this subject class, 00:05:41.790 --> 00:05:45.260 we can continue expanding it in the following lessons. 00:05:45.740 --> 00:05:47.550 And as you will see, 00:05:47.560 --> 00:05:50.380 managing resources with pointers from inside of the 00:05:50.380 --> 00:05:53.150 class is not as simple as it seems.