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Hey there.

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Thanks for making it to this lecture.

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This lecture is going to be a little short as we're just going to talk about what polymorphism is,

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but we won't really practice any examples of polymorphism in Lua as there is no practical use of this

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concept in Lua.

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Nor is there any good example that I can give you in Lua, as its effects only exist because of inheritance,

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which we've already learned about.

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So I'm just going to give you a simple explanation of polymorphism, and this is an idea that you can

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carry along with you if you learn some object oriented language like C plus, plus or C sharp.

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Anyways, polymorphism is an object oriented programming concept that allows objects of different classes

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to be treated as if they were of the same class or in other words, have classes, inherit the same

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functions, yet act differently.

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How is this achieved?

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Well, the classes would use a common interface or a set of methods known as an abstract class containing

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abstract functions.

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An object oriented programming languages.

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There's a dedicated way of doing this, but we can't really do it in Lua.

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How it's typically done is by creating what we would call an abstract class, because it doesn't really

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stand for anything, and the only purpose of it is to contain abstract functions that can be commonly

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shared among other classes.

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In fact, an abstract class doesn't even contain any implementation for any functions.

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The only purpose of an abstract class is just to contain a blueprint or a contract for what a class

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should contain, and from that point, the classes that inherit those blueprinted functions have to

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create the functionality for it themselves.

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So here's an example of polymorphism that I got working.

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But at the end of the day it's actually just inheritance.

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Anyways, so I've created this quote unquote abstract class because we can't actually make any in Lua.

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But what this abstract class is doing is defining some functions that some shapes like a circle class

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or a rectangle class or whatever, should have if we were to make those classes.

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So these classes should have a constructor, and they should have a draw function to define the position

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of the shape inside of these there is no functionality, it just errors.

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Just in case, maybe the class that is inheriting these blueprinted or abstract functions hasn't implemented

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functionality for these functions yet, so it's just going to error and say that this function has not

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been implemented.

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And then what I've done here is I've created a circle class.

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Set the index meta method to itself so we can access these functions.

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And then I've set the meta table for the circle class to the shape class.

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That means if we were to ever index any functions inside of the circle class that did not exist, but

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they existed within the shape class, it'll look into the shape class, see that this function exists,

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and then it will error saying, hey, this class here hasn't implemented any functionality for, let's

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say, the constructor or this draw function.

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So inside of these functions you know, I've implemented some stuff.

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It doesn't just error.

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When I create a new circle we set the meta table to the circle class.

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We set the x, the y and the radius.

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And then we just return it.

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And then for the draw function, it just prints the location of the circle and it tells us the radius

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of the circle.

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So when I create this new circle here has a position on the x axis of three, on the y axis of five,

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and it has a radius of ten.

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And when I go to draw the function it should print that right on the console for us.

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So when I hit run.

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There we go.

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We have a circle at X of three, y of five, and a radius of ten.

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But let's say I didn't implement this draw function.

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What if I were to just comment out and this draw function didn't exist within the circle class?

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Well, what's going to happen is that the interpreter is going to check for this draw, um, index here.

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It's going to look inside of this object and see it has this meta table of circle.

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It's going to go in here and see if the circle class has the draw function which it doesn't because

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I commented it out.

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Then it'll see okay.

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Well this circle class does have another meta table which is shape which is inheritance by the way.

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And it'll look inside the shape for an index meta method which it has.

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So it'll check this table and see that the function exists in here.

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And because we didn't put any implementation within the circle class for the draw function, it's just

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going to error and tell us that this function has not been implemented.

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So if we go and run the game.

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There we go.

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We get an error and it says draw function has not yet been implemented.

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And we can also do the same thing for the constructor.

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If we were to comment out the constructor and run the game, it'll tell us a new function has not yet

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been implemented.

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So the only purpose of this shape class is to be an abstract class that's telling us what our shape

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classes like for circles, or for rectangles or whatever, that they should have these functions in

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them by default.

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And if they don't, it's going to error and tell them that this function has not been implemented.

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But realistically, there is no practical use of polymorphism in Lua.

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The only reason why this is working is because we're using inheritance.

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So in short, you don't need to worry about polymorphism, but it's going to be an important concept.

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You'll want to learn about it for other languages that you might want to learn in the future.

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Otherwise, that's all for this lecture and I'll see you in the next one.