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OK, so we're kind of continuing on with the inheritance thing, but moving on to the next step.

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So this next step is something called polymorphism, and I'm kind of going to get into the topic by

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using an example with some very simple inheritance.

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And then we're going to kind of build upon that and I'm going to explain this topic of polymorphism.

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So let's take a look at the code base I have here already.

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So it's basically just a simple shape project, very bare bones using some inheritance or stuff you've

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seen before.

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We have a shape class here, the access, our base class.

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It has a constructor where you composite a color and an X and Y coordinate.

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It has a set color, a color area and a type.

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I'm going to go ahead and move these.

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This virtual has to do with.

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What I want to do first is just kind of explain this inheritance situation, so we have area and type.

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So the area and type are also used by the derived classes, so if we go in here, this rectangle it

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inherits from the base class shape.

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And it has its own area and its own type.

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One thing that you might have not seen syntax wise, is this initialization list constructor syntax

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here.

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I'm actually calling the shape base class right here, and I'm passing the color in the X in the way.

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And why am I doing that?

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Well, because all of the rectangle, the triangle and circle and shape, they share these attributes

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like color and X coordinate and Y coordinate.

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So that's why we're just going to use the base class to hold those values and we'll still have access

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to them if we want right because I put protected here.

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So that will make it so the drive classes have access to it.

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So let's take a look at Triangle is pretty much the same thing I call the base class constructor here

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in the Triangle Constructor and then it has a base and the height they're all having their own version

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of area, right?

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And they're all having their own version of type, which is returns a string, which is whatever type

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the shape is.

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This is base times height divided by two because it's basically a right triangle, even though I'd call

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it triangle.

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If you go to circle, it's just doing PR squared here for the area and it returns a circle because it's

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a circle, right?

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Notice that this Circle Constructor is also calling the base class constructor shape.

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And the only other thing we really have is radio, so these are all like really bare bones.

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Another thing you might notice is I'm implementing everything in the header files, so I'm not even

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using CPP implementation files.

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The only CPP file I have with that extension is actually just the client file may not keep the rest.

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I can just put in the header file.

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So this is totally possible.

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You can just put them in the header file if you would like you noticed that I have like implementations

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between the curly brackets here and the initialization list as well for the constructor and like I pointed

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out already.

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So if I go over to Main, what I'm doing is I'm explicitly instantiating each one of these classes the

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the base class and then the three drive class with rectangle triangle and circle appropriately.

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So then all I do is I use ASCII color, for example, for shape and type and area.

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I do that for every one of the shapes, the base class and the three derived class shapes.

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And notice that these are just not intimately allocated or anything.

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They're just all like on the stack.

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They're just being instantiated here in this mean function, normally without pointers or anything like

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that.

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So all stuff that you've seen before, right?

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Pretty basic, so where does the polymorphism come in?

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Well, let's go ahead and run this right now.

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So everything looks good, right?

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I'm able to print this stuff out and says that there's a shape with area zero red rectangle with area

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of line, yellow triangle area above and circle with area.

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So if I go to save, you might be wondering, Well, why did you even put area and type in shape if

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we don't really know what the area of some ambiguous shape is?

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Is this base class and some type of like, you know, what is the actual type?

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It's just not having a type, it's a shape.

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So you might want to put these things in a base class for a design purpose.

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So sometimes it's good to kind of making designing a program with this base class in mind.

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Well, one of my drive classes is going to be like, this is a blatant is a relationship, right?

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Like a rectangle is a shape, a triangle is a shape, a circle is a shape.

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So you might be wanting to kind of set this class as a scaffolding for the derived classes.

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And so that kind of leads into the discussion of this polymorphism thing, so what is polymorphism even

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meet?

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Well, polymorphism means taking like one thing, taking many forms.

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And so in this specific, intense instance, it means like a shape can have many forms.

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And even more specifically.

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A shape could have, you know, could different shapes could share the same methods, but like shape

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at any time might have different implementations of those methods.

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So that's like code white, what code way is what we're trying to do?

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And we're kind of doing accomplishing that right here, right?

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Like.

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These all have their own versions of area, and they're all being called right.

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But what if I wanted to just say, well, do I have to explicitly call these rectangle triangle circle?

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Aren't they all shapes?

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Can I just say that these are all shapes?

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Maybe I had some sort of program where I was just making like a bunch of different shapes, and I just

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wanted it to automatically resolve which type of shape it was, even though I said that the data type

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was shape.

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Maybe if I use these constructors, it will know automatically which type of shape it is and it will

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call the right area method in the right area, the right type method.

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So when I say method, I really mean the member function, right?

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No function.

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So hopefully it'll call the right area and type.

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As you can probably guess, this might, you know, this is probably not going to work as you can see

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that it's getting great out, it says functional area hides a non virtual function from class shape.

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So they're not even really being used.

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So is this functional area is never used.

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So let's see what happens when we run this code now.

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So look at the output now, it actually just calls all of these a shape.

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And then it says the area is zero, so it's pretty obvious that what it's doing is that even though

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we call these constructors, when we get down here, it's actually just only using this area in this

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type from the base class, it's not able to resolve it to know which type of shape it really is.

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So how would we make that happen?

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Well, something that we could do is.

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I'll go back here to the shape.

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You are kind of already saw this because I left it there.

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But what we can do syntax wise to kind of enforce this polymorphism is to put the word virtual in front

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of these methods.

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So if I put the keyword virtual, what it does is basically let the C++ compiler know which method to

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use.

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So when I put virtual on the base class, when rectangle inherits from shape and triangle and circle

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as well, it's going to automatically and it actually calls this bindings, going to automatically bind

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the correct method to the object based on the type at the time when it's like during runtime you.

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But if the compile time, it kind of make that connection, that binding connection.

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So.

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It's pretty cool.

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You can just add this one word.

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Let's go ahead and change Maine, though, so one thing that we have to do is what I'm going to change

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up here is I'm going to actually make these pointers.

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So I'm going to say shape star se equals new shape.

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Equals new rectangle, so why make these dynamically allocated now?

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And I'm going to have to change these two points, right?

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So kind of, you know, it was me here is painfully slow at this.

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So essentially what we're doing is we are now just making it a shape pointer.

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But what's happening is these constructors are getting called and it's actually dynamically allocating

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memory for these types.

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But we're saying that the point in time is shape, but the fact that we put that virtual keyword is

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going to make us able to actually use the correct area and Typekit member functions for the correct

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objects right that we made.

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Even though we said that the type that's being pointed to where the pointer is, it can dynamically

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allocated shape.

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It's actually going to be able to resolve it because of the virtual keyword, and it's going to know

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that each one of these are actually rectangle, triangle and circle.

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So let's go ahead and run this.

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And look at that, he was actually able to resolve them, so we're back to what we saw before, when

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we explicitly put tape, rectangle and triangle and tight circle here.

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So pretty cool, right?

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All we had to do was we kind of changed things over to pointers, but we call them shape.

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And then we went in here and we put virtual in front of these functions.

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So I'm not going to go into too much more detail in this lesson, but in the next one, we will get

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into kind of more polymorphism and really using this as a base class, that's just like a structural

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thing, like it's just a scaffolding for the rest of the classes.

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We're actually going to make it so we don't even instantiate shape.

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We don't even make a shape object.

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We're just going to design our program around a shape, and we're actually going to have these virtual

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front functions, which are going to be pure virtual functions, and it'll force us to have to implement

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those virtual functions in all these classes no matter what.

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But what that does is it just kind of enforces good design and the idea of polymorphism, you know,

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one thing taking many forms is a pivotal part of object oriented programming, and it's kind of nice.

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You may be thinking, Well, why don't you just explicitly put rectangle triangle circle?

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And the cool thing is just, you know, it makes it easier for every developer is doing the client side

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code.

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You can just call it a shape and you can make whatever one of these types of shapes that you want.

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Maybe you have some type of like loop and you're making like multiple shapes.

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You have some logic or something.

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Well, it's a lot easier to just kind of have pointers, all of the same type, and the cool thing is

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is that let's say you made a point sheep type.

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You could just basically refer to the address of any other object, know any subtype like any derived

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class.

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And it would be totally fine to assign that memory to a sheep pointer that already makes it a lot more

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flexible on the client side, right?

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So it does have its use cases, and it's a very important part of API in general.

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So not a lot change, right?

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We still use the inheritance polymorphism as kind of, you know, goes hand in hand with inheritance.

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The fact that we have a base class and the right classes is a really important part of polymorphism.

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So since we have this base class available, we were able to make these virtual functions that are then

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implemented differently across all of these rectangles.

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And even though they're implemented differently, we were able to still have a shape pointer to all

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of these and have it resolved the correct method to call it.

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OK, so with that, I will leave it in this lecture and in the next lecture, we will go over abstract

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base classes and spiritual functions for polymorphism.