1 00:00:00,510 --> 00:00:07,290 So we're going to wrap up polymorphism with one more sub topic, so it's a pretty short portion of the 2 00:00:07,290 --> 00:00:13,080 course, but there is one more thing to discuss and that is abstract base classes. 3 00:00:13,800 --> 00:00:15,780 So you've seen what we've done so far. 4 00:00:15,840 --> 00:00:24,060 We basically have a shape based class and then we have triangle and rectangle and circle as the derive 5 00:00:24,060 --> 00:00:24,750 classes. 6 00:00:25,770 --> 00:00:33,900 So in the last lesson we talked about instantiating these and having them be in dynamic memory as a 7 00:00:33,900 --> 00:00:41,670 pointer, and we showed how we could kind of use some of the memory functions from the shape class and 8 00:00:41,670 --> 00:00:44,310 even call them a shape pointer. 9 00:00:44,310 --> 00:00:52,610 But they would be resolved through the methods and the data members to the correct and derived class. 10 00:00:52,620 --> 00:01:00,750 So if you notice the main CBP, you remember we say Shape Pointer as an R.A. and C, and this was respectively 11 00:01:01,350 --> 00:01:03,090 rectangle triangle and circle. 12 00:01:03,090 --> 00:01:07,140 And even though we were calling the same named methods, it was able to resolve them correctly. 13 00:01:08,340 --> 00:01:14,760 So what are we going to talk about now is the fact that shape isn't really something that we should 14 00:01:14,760 --> 00:01:21,870 be instantiating really, because it's actually not specified, a shape is kind of an ambiguous thing 15 00:01:22,050 --> 00:01:24,510 like you must have a type of shape, right? 16 00:01:25,230 --> 00:01:31,380 So if someone says, oh, draw a shape, you would be like, Well, what shape do I try to draw rectangle 17 00:01:31,380 --> 00:01:33,720 or triangle or circle something more complex? 18 00:01:34,680 --> 00:01:40,710 So for that reason, we shouldn't really have shape be something that can be instantiated in the client 19 00:01:40,710 --> 00:01:41,100 code. 20 00:01:41,280 --> 00:01:45,750 And so that leads us to talk about this notion of an abstract base class. 21 00:01:46,770 --> 00:01:48,630 So if we go into this shape based class. 22 00:01:49,810 --> 00:01:55,600 What we can do is make some small changes, and we will turn this into just an abstraction where it 23 00:01:55,600 --> 00:02:01,810 can be used as a base scaffolding for the rest of the derived classes. 24 00:02:02,110 --> 00:02:06,720 So it's not meant to be instantiated, but it's just meant to help create a structure. 25 00:02:06,730 --> 00:02:11,980 So when you're designing your program, you might actually want to start out with an abstract base class 26 00:02:12,880 --> 00:02:17,490 just to solidify your idea of, Hey, I'm going to have different shapes. 27 00:02:17,500 --> 00:02:23,710 Why don't I make a base class college shape and put some of the shared information between shapes here 28 00:02:23,710 --> 00:02:24,730 in this base class? 29 00:02:24,730 --> 00:02:27,610 But I don't want people to be able to create a shape object. 30 00:02:27,970 --> 00:02:34,710 I only want them to be able to create specific types of shaped objects like triangle, circle and rectangle. 31 00:02:36,490 --> 00:02:45,010 So we only need to do a few things to change our project here into something that uses an abstract base 32 00:02:45,010 --> 00:02:45,300 class. 33 00:02:45,310 --> 00:02:52,900 So all I'm going to do is I am going to go to these virtual functions and I'm going to put in equals 34 00:02:53,020 --> 00:02:55,960 zero right here and then commented out. 35 00:03:01,610 --> 00:03:08,240 So you notice that I'm not having any implementation here, the return zero is completely committed 36 00:03:08,240 --> 00:03:12,160 out here and the return shape is come into doubt as well. 37 00:03:12,440 --> 00:03:19,610 On this tape member function the equals zero might seem kind of weird, but it's really just a syntax 38 00:03:19,610 --> 00:03:23,870 to say, Hey, this is an abstracted member function. 39 00:03:24,380 --> 00:03:28,070 It is not something that's meant to be implemented in this class. 40 00:03:28,610 --> 00:03:36,170 It is something that actually must and I say must, because it's required now that you have an area 41 00:03:36,590 --> 00:03:43,330 and a type of function in every one of the classes that inherits from the shape class. 42 00:03:43,340 --> 00:03:52,600 So this triangle circle and rectangle must all have both area and type member functions now. 43 00:03:53,180 --> 00:03:56,330 And that is because of this syntax that we just put here. 44 00:03:57,410 --> 00:03:58,940 With the equals zero. 45 00:03:59,900 --> 00:04:05,900 So once again, that makes it so these are not really ever going to be used in the shape class and really, 46 00:04:06,170 --> 00:04:12,540 that's not going to ever be a problem because we're never going to instantiate a shape object here. 47 00:04:13,370 --> 00:04:20,990 But it's required now that every class that inherits from shape must implement any one of the functions 48 00:04:20,990 --> 00:04:24,080 here that you have in the base class that say equals zero. 49 00:04:24,530 --> 00:04:27,860 These are something called pure virtual functions. 50 00:04:28,070 --> 00:04:31,490 So this one and this one are both pure virtual functions. 51 00:04:32,180 --> 00:04:40,160 Another thing that happens by doing this is it turns this base class from a normal based class into 52 00:04:40,160 --> 00:04:44,390 an abstract base class, which abstract base class? 53 00:04:44,480 --> 00:04:47,410 This is what we're talking about in this specific video. 54 00:04:47,420 --> 00:04:52,340 And really, the main thing it means is that the fact that we are not going to instantiate it. 55 00:04:53,270 --> 00:04:59,420 So since we're not going to instantiate it, let's go back to Maine and see what happens after we've 56 00:04:59,420 --> 00:05:03,860 modified these and it's turned our shape class into an abstract base class. 57 00:05:05,150 --> 00:05:08,270 So I go back here and noticed that this is underlined in red now. 58 00:05:08,990 --> 00:05:16,310 If I hover over it, it says allocating an object of abstract class type shape, and it gives me two 59 00:05:16,310 --> 00:05:17,400 options right here. 60 00:05:17,420 --> 00:05:25,070 So look, it says unemployment pure virtual method area in shape and implement pure virtual method type 61 00:05:25,070 --> 00:05:25,580 in shape. 62 00:05:25,940 --> 00:05:32,540 And remember also, I've said this reminder a few times, but whenever I say method that is synonymous 63 00:05:32,900 --> 00:05:39,590 with memory function and you see that even though we call new member functions, sea lion even puts 64 00:05:39,950 --> 00:05:40,940 method in here. 65 00:05:41,390 --> 00:05:48,170 So it gives you the option to click both of one or both of these to get rid of that pure virtual method 66 00:05:48,170 --> 00:05:49,100 area in shape. 67 00:05:49,490 --> 00:05:50,930 So if I click on this? 68 00:05:52,210 --> 00:05:58,690 Quick unemployment, that method, it takes me here, so I could actually just if I wanted to get rid 69 00:05:58,690 --> 00:06:01,940 of that, but I don't want to, I still want to be an abstract base class. 70 00:06:02,920 --> 00:06:04,890 And you know, it's giving us this problem. 71 00:06:06,280 --> 00:06:11,800 So what I'm going to need to do is comment this out or completely get rid of it because we can no longer 72 00:06:11,800 --> 00:06:12,670 implement a shape. 73 00:06:14,260 --> 00:06:18,580 Because of the House, and you get rid of this because this was as was the actual shape, and we're 74 00:06:18,580 --> 00:06:22,970 not going to be trying to print out any of this information about the generic shape. 75 00:06:22,990 --> 00:06:24,940 We need a specific type of shape now. 76 00:06:25,660 --> 00:06:32,760 So all I have to do is comment out this line here on Line 10 and these two lines 15 in line 16, assuming 77 00:06:32,800 --> 00:06:35,140 bit the rest, we left the same. 78 00:06:36,130 --> 00:06:37,840 So I'll go ahead and run this. 79 00:06:39,980 --> 00:06:45,770 And you notice that everything still runs fine, except now we are using it how we're supposed to, 80 00:06:46,100 --> 00:06:51,130 where we're using the actual type of shape rather than just some ambiguous shape object, right? 81 00:06:51,140 --> 00:06:56,300 So we have the red rectangle, the yellow triangle and the green circle with their areas being printed 82 00:06:56,300 --> 00:06:57,650 out appropriately as well. 83 00:06:58,400 --> 00:07:00,260 So we didn't have to change a whole lot. 84 00:07:01,210 --> 00:07:08,020 This part of polymorphism is really about the design idea, so it's a really important part of object 85 00:07:08,020 --> 00:07:08,860 oriented programming. 86 00:07:09,490 --> 00:07:18,070 And the idea is that when you're coming up with your design for your object or in your programming classes 87 00:07:18,070 --> 00:07:25,750 and program overall, the structure you can be thinking about how you can come up with a base class 88 00:07:25,750 --> 00:07:29,510 that will allow you to build out a derived class. 89 00:07:29,530 --> 00:07:38,800 So think of different things that have a generic commonality like maybe you know, a car like we've 90 00:07:38,800 --> 00:07:39,960 dealt with car before, right? 91 00:07:40,480 --> 00:07:44,470 We have a car and we have certain types of car or like an airplane class. 92 00:07:44,470 --> 00:07:51,100 And then you have certain types of airplane like you're not just going to instantiate a generic airplane 93 00:07:51,700 --> 00:07:55,990 type, you're going to only instantiate certain types of airplane, right? 94 00:07:56,740 --> 00:07:58,270 Just like shape. 95 00:07:58,480 --> 00:08:04,090 You know, like we said in the beginning, if I say draw a shape, you would probably ask, well, what 96 00:08:04,090 --> 00:08:04,660 type of shape? 97 00:08:05,590 --> 00:08:06,760 Because you wouldn't know what to draw. 98 00:08:06,790 --> 00:08:10,540 You would just have to dress whatever type of shape that you know of, right? 99 00:08:11,050 --> 00:08:17,380 But shape is something to ambiguous to abstract, hence the name abstract base class. 100 00:08:18,040 --> 00:08:24,240 So that's why we use this just as a base to build off of, right? 101 00:08:24,880 --> 00:08:32,680 We have some common methods or member functions here that go along with every type of shape, right 102 00:08:32,680 --> 00:08:34,320 triangle circle and rectangle. 103 00:08:34,330 --> 00:08:39,340 They all have their own type of area that should be implemented. 104 00:08:39,340 --> 00:08:41,560 Write their own formulas for it. 105 00:08:41,950 --> 00:08:44,920 They also have a name for their type of shape, right? 106 00:08:44,920 --> 00:08:50,140 So they're going to be able to return the specific string that's associated with the name or the type 107 00:08:50,560 --> 00:08:51,190 of the shape. 108 00:08:51,640 --> 00:08:57,700 That's why these are appropriate to be pure virtual functions because we know we're going to implement 109 00:08:57,700 --> 00:09:02,890 these in every type of shape or every derived class that comes from these shape. 110 00:09:02,890 --> 00:09:03,940 Abstract base class. 111 00:09:05,450 --> 00:09:11,330 OK, so that's really all I have for this lesson, and this pretty much wraps up polymorphism, so make 112 00:09:11,330 --> 00:09:17,180 sure that you review anything that you're not completely sold on or go back to the video. 113 00:09:17,570 --> 00:09:22,490 We will probably still be using it throughout the class and in the next section, we're going to build 114 00:09:22,490 --> 00:09:24,770 off of our knowledge and go into templates. 115 00:09:25,520 --> 00:09:25,820 All right. 116 00:09:25,820 --> 00:09:27,800 So with that, I will see you in the next lesson.