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‫Hello and welcome.

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‫In this lecture, we're going to be understanding the main steps involved in compiling C++ files in

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‫Unreal.

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‫Let's dive in and see how they work.

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‫So it's soon going to be time for us to start using code from the outside, from other files, and we

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‫need to learn how this works in C++.

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‫To do that, we need to take a little bit of a detour and understand in more detail what's going on

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‫when we compile our C++ code into binary.

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‫So here are the compilation steps that are used.

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‫We start off with your code, your C++ files, your header files, all in various text files.

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‫These get fed into a tool called the Unreal Header Tool.

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‫Now, this tool is responsible for taking all the unreal specific pieces of syntax and using them to

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‫generate more C++ code.

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‫So let's have a look at some examples of where we've got this unreal specific syntax, this new class

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‫in capital letters that is unreal specific syntax, and so is the Crypt Reader API thing here and the

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‫generated body there.

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‫And basically what's going on here is the unreal header tool will see these different pieces of syntax

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‫in your code and go, okay, I need to do my job.

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‫I need to go in there, I need to add a bunch of stuff.

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‫It's going to replace your generated body with a bunch of code that makes our lives easier when using

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‫Unreal.

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‫It's also does things like the U properties and new functions.

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‫It adds all the stuff that it needs so that it can communicate with the blueprint world.

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‫So that is the first step.

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‫It converts all of its unreal specific stuff into actual C++.

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‫Once we've got the actual C++, we can then feed it into a standard C++ compilation process, which

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‫takes three steps.

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‫The first is to deal with the preprocessing step so this can actually touch any of the C++ syntax and

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‫actually only deals with lines that start with a hash.

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‫So we've got a few of these preprocessing directives.

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‫These are dealt with by the preprocessor.

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‫The next step is what we actually call the compiler.

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‫It takes a code and it converts it into executable binary, but it doesn't quite complete the step because

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‫it only does it on individual C++, what are known as translation units, basically C++ files.

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‫So it takes a C++ file and it translates that into binary individually, each individual C++ file.

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‫And then the third step is the linker, which brings all of that together and outputs an executable

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‫that we can actually run a single executable.

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‫So let's have a look at these standard compilation steps in a little bit more detail with an example

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‫file.

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‫So imagine that we had a game.

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‫CP And it's a very, very simple game because all it does is add one and one together, but to add one

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‫and one together is having to use a function that's declared in a different file in the math.

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‫H So the math declares.

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‫AD, but it doesn't actually define what it does.

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‫So that is done over in the math.

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‫CP And you can see it's a very simple implementation indeed.

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‫Now what happens at the preprocessor step?

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‫Well, like I said, it deals with any preprocessor directives, which are things that begin with a

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‫hash.

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‫So the hash include math, which basically means just copy everything in math into here.

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‫So what it does is it takes these two files and it combines them together to create a new output file

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‫called Game I basically games include and you can see that it is included the definition from Master

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‫H and it's replaced the hash preprocessor directive.

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‫So now Game II has got a definition for the ADD operation and then we run the preprocessor on math DHCP.

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‫But because it doesn't have any preprocessor directives, basically nothing is changed there.

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‫The next step is the compilation is probably the most important and complicated step, because what

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‫it does is it takes that C++ in each individual file and converts it in to binary, but it's not complete

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‫binary because for example, my game doesn't actually know how to implement AD, it just knows the shape

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‫of that add function.

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‫So that needs to be filled in later and that's filled in by the linker.

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‫So the linker is going to take the binary created for the game obj file and the binary created for the

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‫master file and bring them together to make a game.

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‫XY which combines the binary for both and finally is a complete executable that we can actually run

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‫because it's got the definition for everything it needs.

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‫Now, why do we have such a complicated process?

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‫The idea here is that it speeds up our compilation time because if I only made a change.

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‫To game DHCP.

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‫I don't actually need to run the entire compilation process.

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‫All I have to do is run the bits involving game DHCP, so I'd run the preprocessor only on the CP,

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‫then I'd run the compiler only on the game to I and then I can reuse the math obj file from before.

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‫I don't need to recompile it because nothing has changed in that.

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‫Similarly, if I only change math dot CP, I wouldn't have to recompile the game obj file, which is

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‫why this is a more efficient process and why we're doing it in the first place.

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‫So I'd like us to do a little experiment and try and figure out why do we include move a H in and move

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‫a CP file?

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‫What's the reason for it?

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‫Why can't we just compile the C++ file without including moved?

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‫H So the experiment we're going to do is we're going to comment out the hash include line.

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‫So using a double forward slash to make a comment in front of that line, which will basically make

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‫it get stripped out by the preprocessor instead of the preprocessor replacing it with the contents of

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‫the file.

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‫We're then going to recompile and I'd like you to have a look at the error messages that come out and

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‫see what it says to see if you can try and understand what it is that we're including this move for.

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‫Pause the video and have a go.

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‫Okay.

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‫Welcome back.

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‫So let's put a double forward slash at the beginning of this line, which basically makes it null and

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‫void.

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‫And then we're going to do a control alt f 11 and we're going to have a look at the output of live coding.

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‫So look, we've got some errors straight away very, very early on.

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‫And what it's saying is that you mover is not a class or namespace and there's some other stuff down

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‫here.

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‫It's saying that primary tech component hasn't been declared that super isn't a class or namespace get

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‫owner is an unknown identifier.

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‫And basically what this is saying is you've talked about you mover here, but you've not defined it

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‫anywhere, you've not declared it anywhere.

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‫So I don't know.

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‫The C++ compiler says, I don't know what this is.

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‫So that's why we are hash, including the mover H because mover H declares the class.

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‫It says a U mover is a class and it derives from you act a component and it's got these methods, etc.

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‫It's declaring what it means to be that class.

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‫So without that declaration, the C++ compiler can't continue.

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‫And that's why we need to include that header file.

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‫Great stuff.

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‫So hopefully you now understand a little bit better the process involved in compilation of C++ and particularly

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‫compilation of C++ in Unreal.

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‫I will see you in the next lecture.