Writing Native Code

Note: Some C++ and UnrealScript knowledge is required.

Introduction

Native coding in the Unreal Engine refers to code that is written in C++ instead of UnrealScript. This C++ code can be called from a script to execute tasks that are either too slow in UnrealScript (about 20 times slower than C++) or are not possible at all due to the limited features of the language. Native coding is only officially supported by Unreal Tournament from 1999 since it is the only game that has its native header files publicly available for download. Epic never released the headers for newer versions of the engine due to licensing issues with third party code. However, after countless hours of work it was possible to modify the UT99 headers in such a way that they're usable with Republic Commando. This offers a lot of new opportunities to modders or people who just like to experiment with old games like me.

The way native coding works in Unreal is that you compile your scripts into a .u package and then when you write your C++ code you compile it as a dll with the same name as your package. When the game is loading an UnrealScript class and detects that it was declared as native, it also looks for a dll which contains the native code for this UnrealScript class.

Getting Started

Before you can write any C++ code you first need to create a UnrealScript class that you declare as native. This tutorial assumes that you already know how to compile UnrealScript code with ucc. If you don't, then the download for the Republic Commando UCC contains a brief explanation in its readme.

For the sake of this tutorial I'm going create a really simple and quite useless class that has only one native function which writes a string to the log file.

class TestNativeClass extends Actor native
                                    placeable;

var int TestInt;

native final function TestNativeFunc(string param);

function PostBeginPlay(){
	Super.PostBeginPlay();

	TestNativeFunc("Hello World!");
	Log(TestInt); //Printing out TestInt which has been modified in C++ code
}

The class is going to be implemented in C++ and thus must be declared as 'native'. I added an integer variable called 'TestInt' that we're going to modify in C++ and then print from UnrealScript just to show that both, the C++ and Unrealscript classes refer to the exact same object in memory.

Furthermore, I declared a native function called 'TestNativeFunc' that takes in a string parameter. I also declared it as final since I don't want it to be overridden by a subclass but you don't have to do this. In my 'PostBeginPlay' function I simply call the native function with "Hello World!" as the string parameter and I also write 'TestInt' which at this point has been changed in C++ code to the log file.

When compiling a native class the ucc also generates a header and a source file with the C++ class definition and some other setup code. But these won't be created by default because the folder they're supposed to be created in doesn't exist so we'll have to do that before we compile our class. In the folder that contains the UnrealScript source code you have a folder with the name of your package which in turn contains a folder called 'Classes' that holds all the .uc files for this package. Navigate to your package's folder and create two new folders called 'Inc' and 'Src' in the same folder as 'Classes'.