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OK.

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Welcome back to another lecture in this lecture, we're going to continue on talking about object oriented

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programming, and we're going to actually continue on with this car class using as an example.

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So this lecture will have two new kind of concepts or sub concepts within the realm of classes and object

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oriented programming that we're going to go over.

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And the first one that we're going to talk about is something that's not a huge topic, but I want to

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mention it, and that is something called CONT-.

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You might have seen this already and already know what concept means, but just in case I'm going to

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go ahead and go over it and also tell you how it pertains to functions.

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So you might have seen something like this as a constant, then a variable type.

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Right?

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Sorry, a data type.

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And then we have a variable name and something like this where it says contact and and it really can

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be any data type.

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And then we say, here's the variable name calling.

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I won and we say it's five.

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The word con stands for a constant.

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And what it means is that this integer I one should always remain five.

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It should be basically read only after that.

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We can print it out.

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We can see what it is, but we should not make changes to it.

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So if I was to try something like this and I say I equals seven, let me go ahead and save this and

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I'm going to compile it in here.

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Let's say G Plus plus R.S.V.P. Dash.

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Oh, card out

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and notice that it gives me an error rate here.

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So it says assignment is this error assignment of read only variable i1.

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So it's telling us that this i1 is really only it's really read only we're not allowed to modify it

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yet.

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We tried to modify it here on Line sixty eight and this can be anything like I could have this piece,

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you know, constant flow and it could be five point seven.

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And then here I just try and change it to like 7.9 or something like that.

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It still is not going to work.

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It's going to yell at me and say it's still in assignment of re, the only variable I won.

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So that's what constant means.

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And why would you use constants?

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Well, sometimes they're a little more efficient, you know, with compilation and all that stuff,

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because it's just knowing that this is going to remain the same.

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The whole time kind of makes everything a little more optimal.

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So they're useful.

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If you can make something konst and you know that it's not going to change, then it might be a good

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idea to make it constant.

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But so that's kind of a little recap, if you might have seen that before.

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If not, now you know what cost is as far as basic variables and data types and stuff like that, so

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you can make pretty much any type.

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This can be anything here, this highlighted section, and you can put a cost in front of it.

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So it also can be used with functions.

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And what I mean by that is that when you put context after a function like, for example, I can say

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and get odometer cards.

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So this is our member functions area, remember?

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So what I'm doing here is I'm putting this contest.

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And this is the prototype that means I also need to put it down here.

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So we have get odometer, and I would need to also put a concert right here after the parentheses,

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just like I did up here.

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What it means is that if we put this here, it's saying that this function is not going to modify the

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data of the object that is calling the function.

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And what is the object that's calling the function?

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Well, let's look at an example.

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We made a car object in a car to object, right?

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Let's take it.

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Let's take a look at car for an example.

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So here we have car dot, get odometer.

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This is the function get odometer being called on.

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And I say on because there's the dot, it's car dot yet odometer.

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So get odometer function is being called on car.

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So let's go back up here.

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What I was saying previously is that Konst means that the function get odometer should not modify the

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data of car in this example, and it doesn't write it's a getter or what we would call an accessory,

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right?

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It's not a mutineer or setter.

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It really just gives us the value of odometer, it's not changing anything, it's just giving us back

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whatever stored in odometer.

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So this is something that's totally fine and I can go ahead and say this and run this, and you notice

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that we don't get any error if I compile it.

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What if I was to put a cost on something else that does modify it, let's see what happens.

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So I go ahead and I put cost here and I say, Oh, that pain is not going to change anything, I promise.

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And then I'll go down here and I'll say, OK, let's go ahead and cut the cost on here.

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So it matches.

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And then I say this, I go ahead and.

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Compile it.

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Oh, look at this, we have some errors, so what do these errors say?

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Well, you notice that it's actually yelling at this, yelling at us for this code right here.

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Where is this paint color equals color?

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It says passing constant string as this argument desk discards qualifiers.

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This is a kind of a fancy, complicated way of just saying that, hey, you said you weren't going to

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change any of the data that has to do with the calling object and calling object is actually referred

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to as this like literally the word this.

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That's why it's here in the air and the arrow.

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So that calling object, we were not supposed to change its data and we said we wouldn't.

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Yet right here, compilers noticing that that is what we are doing.

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Painkiller equals color.

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So that is an issue and we cannot have that.

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So we're going to have to take this out now.

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You don't want to put a cost on anything that is changing the data because that is not a conscious for

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it's for something that doesn't change the data, right?

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So we can put it here because it just returns odometer and it's always just going to return odometer.

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And it's totally fine.

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If we were to like, modify odometer like later on, like, I could make some function here.

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I could say like and said Odometer, and I'm not going to make this cost.

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Let's go ahead and just say it.

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And it's going to call a reset or something, so we can say with this, get a whole another thing here

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that so we don't want to return it and say this will be void and then I'll go down here and we'll just

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say boy car sets odometer.

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And here was an reset.

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That's what we're going to reset it to.

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And we just say odometer equals reset.

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So I could do something like that.

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And let's just say we call get odometer.

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And then maybe here I'll send you a car set odometer.

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And I just say like eight hundred thirty seven or something like that.

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And then we go down here and we also say, car, it will get odometer.

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OK, so.

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Now I'm showing you that I have a set of downers, so we are changing the odometer value, right?

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So we said that this was constantly get odometer.

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Yeah, I'm changing it in this other function, but let's see what happens.

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So go ahead and compile it, and it gives a little bit about this, so it says it does not match any

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class.

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That is because.

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I did set paint.

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I think I put a constant road to tears.

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So.

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Oh, yeah, I this is so.

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So let's go ahead and save this.

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All right.

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So compile again, and it's fine, and why is it fine?

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It's fine because we are not saying that, said Odometer is concerned this would be a problem, right?

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But we're just saying that get odometer.

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So we're saying that this function specifically is not going to change any data like it's not going

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to change odometer or any other data, for that matter.

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This one is changing.

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And so we're not going to put const.

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So just wanted to introduce that.

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That's kind of the first thing I wanted to show you was concert in general and then cars being used

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with member functions of a class.

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So the next thing that we're going to go.