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‫Hello and welcome.

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‫In this lecture, we're going to be looking at the C++ equivalent of pure blueprint functions, which

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‫is concept functions.

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‫We're going to see what this consort keyword is, how we use it, and how it restricts what we can do

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‫in our code to make us safer.

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‫Let's dive in and see how it works.

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‫Over in blue print, we were able to make a function pure, and that meant that we could take away the

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‫need for the execution pins.

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‫Now, in C++, we can do something a little bit stronger, which is to make a function constant.

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‫Now, what does it mean to make a function const?

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‫It means that that function cannot modify the state of your class, and it is actually enforced by the

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‫rules of the compiler.

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‫So let me give you an example.

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‫If I were to take our should platform return function, this is not a function that should modify anything

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‫in the class and it would be an error really if we forgot that because it would be weird if calling

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‫this and using in an if statement actually change the location of the platform or if it updated the

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‫move distance or platform velocity.

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‫So to keep ourselves honest to that, we can make it convert.

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‫And the way we do this is by putting the contact keyword after the parentheses of the function.

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‫We often leave a space here as well.

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‫Now, in order for it to correctly match up to the function in the C++ file, we actually have to put

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‫that const keyword over there as well.

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‫Now, like I said, this now makes it an error to do anything that would modify the class.

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‫So if in should platform return, I were to try and modify a class variable such as move distance.

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‫I might try and set move distance to 200.

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‫Let's see what happens when we go over to unreal and hit the compile button.

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‫Okay.

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‫You can see that there's now an error.

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‫It does fail to build and if you look at the error, you can see that there's something being said here

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‫about not being able to modify something because it's being accessed through a constant pointer.

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‫Now don't worry about that language.

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‫It's a little bit hard to interpret, but the point is the keyword const was there and it's saying it's

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‫not able to modify something and that is crucial.

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‫Also, if you called any functions that aren't marked as being const, that's also a problem.

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‫So while it's fine to call get actor location that is a cost function doesn't change anything just gets

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‫the location.

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‫It's not okay to call set actor location that hasn't been marked as const and so it's not valid.

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‫It could be modifying the class.

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‫So any function that's not marked as const is not allowed to be called from within a cost function.

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‫And so you can see a problem emerging.

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‫We can't actually call get distance moved from within our should platform return because should platform

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‫return is const but get distance moved hasn't been marked as such.

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‫So a quick charge to you get the distance moved const make get distance move cost so in the header and

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‫see p file you're going to add that const keyword to let C++ know that it shouldn't modify the class

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‫and does it compile now pause the video and have a go.

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‫Okay.

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‫Welcome back.

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‫So we're going to go over to the header file first.

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‫And after the closing parentheses, we're going to add a space concept and then we're going to go back

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‫to the C++ file.

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‫And after a distance moved parentheses, we're going to put space concept.

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‫Now the squiggly lines under get distance moved should update in a little bit of time.

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‫But if we go over to Unreal and hit the compile button, that will be the ultimate arbiter as to whether

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‫the compiler likes what we've done or not.

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‫And this time the compilation has succeeded successfully, which means that we have correctly updated

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‫our code.

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‫And that makes sense because there is nothing being called from get distance moved that is not concept,

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‫it's not trying to change anything.

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‫And where we are calling a function in should platform return, we are only using const functions and

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‫this is a way for us to increase the safety of our code because functions won't be changing stuff without

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‫us knowing about it.

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‫Now as a little exercise to your self, you might want to go through and look at all the functions that

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‫we've been using up to now and see which one of them are marked as constant, which ones aren't.

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‫You can do this by simply hovering over a function such as get actor location and seeing whether the

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‫pop up that is included, whether after the parentheses it has contact or not.

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‫You can see with get active location hovering over it gives me const hovering over get name also gives

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‫me that this is const and it makes sense.

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‫Neither of those two should be changing the state of our class.

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‫Now if I hovered over tick you can see this one isn't marked as const and that makes sense because Tick

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‫is usually used for updating how a class is positioned in the world or any of the state of the class.

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‫And we've got our vector.

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‫If we hover over, get safe, normal, you can see that it's marked as const that's because get safe

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‫normal doesn't actually modify the vector, it just returns a value with a new vector in it.

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‫So that's it.

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‫For this lecture we've learnt about adding const to our functions to increase the safety of our code.

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‫I'll see you in the next one.