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 (I think Deus Ex as well...) 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 process is not finished yet but the most important classes like Object, Actor, Pawn and some others already work.

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 a 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 one native function which writes a string to the log file.

class TestNativeClass extends Actor native
                                    placeable;

var int TestInt;

native(1024) final function TestNativeFunc(string param);

function PostBeginPlay(){
	TestNativeFunc("Hello World!");
	Log(TestInt);
}

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 use in 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. Notice the number in parentheses right after the 'native' keyword. This is the unique identifier Unreal uses to associate this UnrealScript function declaration with the actual C++ function. Some of these IDs are already used by the engine so try not to overwrite them. You can check which ones are available by entering the 'DumpNatives' console command while ingame. It will write all available IDs to the log file. However, I always start at 1024. All following numbers are definitely not in use by the game so you can really use anything up to 4095 which is the maximum amount of native functions.