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C++ Mutator Tutorial
Contents [hide]
C++ Mutator Tutorial
Requirements
Directory Structure
UPlugin File
Build.cs File
Header File
Plugin Module Interface
The Mutator
Building your Mutator
Testing your Mutator
C++ Mutator Tutorial

This tutorial will teach you how to make a simple mutator for UT using C++. We'll be making a UE4 plugin that defines a child class of AUTMutator.

Requirements

Engine version: 4.3
Skill level: intermediate C++ knowledge
Directory Structure

If UnrealTournament\Plugins does not already exist, create it
Create a directory inside of UnrealTournament\Plugins to hold our mutator, in this case it would be "SampleMutator"
Create a "Source" directory inside of the SampleMutator directory
Create a "SampleMutator" directory inside of the "Source" directory
Finally create a "Public" directory and a "Private" directory inside of the last "SampleMutator" directory that we made
UPlugin File

We need to create SampleMutator.uplugin to let UE4 know we have a plugin it should load.
On Windows, one way to do this is to create a new text file and rename the file extension to .uplugin. Copying an existing .uplugin file and editing it may not work!
It should live at the same level that the Source directory does
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SampleMutator.uplugin - Plugin defintion

Build.cs File

We need to create SampleMutator.Build.cs so that Unreal Build tool knows how to generate our dll file
It should live at the same level that the Public and Private directories do
We'll use the PrivateIncludesPaths array to make #include cleaner throughout our source files
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SampleMutator.Build.cs - Unreal Build Tool definitions

Header File

Unreal Build Tool requires that we have a shared header file as the first include in every cpp source file in the plugin.
We'll put the SampleMutator.h header file inside of the Private directory
We need to #include Core.h, Engine.h and UTMutator.h to have enough defintions to make a child class from AUTMutator
include "SampleMutator.generated.h" is required by Unreal Header Tool
We're only overriding CheckRelevance_Implemenation in this example, but more powerful mutators will overload more of the functions exposed by AUTMutator
 [Expand]
SampleMutator.h - Sample Mutator class definition

Plugin Module Interface

Create SampleMutatorPlugin.cpp inside of the Public directory
There's not much interesting in this file, it just provides UE4 a way to load this mutator as a plugin so I'm going to gloss over this file
Note that SampleMutator.h must be the first #include and the IMPLEMENT_MODULE macro does all the heavy lifting
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SampleMutatorPlugin.cpp - Plugin Module Interface

The Mutator

This sample is going to replace all weapon pickups with Rocket Launcher pickups
We'll accomplish that by overriding the CheckRelevance_Implementation function and watching for AUTPickupWeapon
When we find a AUTPickupWeapon, we'll replace its WeaponType reference with a reference to BP_RocketLauncher_C
Our mutator is going to live in SampleMutator.cpp inside the Public directory
Note that once again SampleMutator.h has to be the first #include
Our constructor is blank, but feel free to put initialization code in there in a future plugin
StaticLoadClass is used to get the reference to Blueprint'/Game/RestrictedAssets/Weapons/RocketLauncher/BP_RocketLauncher.BP_RocketLauncher_C'
The use of a hardcoded reference here is very brittle and not considered good design, a mutator that requires direct blueprint references might be more easily implemented in blueprints
Calling Super::CheckRelevance_Implementation(Other) is highly recommended as failing to do so won't call other mutators in the mutator list
 [Expand]
SampleMutator.cpp - Replace all weapon pickups with rocket launcher pickups

Building your Mutator

Right click your UnrealTournament.uproject in Window Explorer and select "Generate Visual Studio project files"
Open up the Unreal Tournament solution file in Visual Studio
Build the Unreal Tournament project in Development Editor configuration
You will notice that in addition to UE4Editor-UnrealTournament.dll, UnrealTournament\Plugins\SampleMutator\Binaries\Win64\UE4Editor-SampleMutator.dll was generated
Testing your Mutator

Run from a commandline "UE4Editor.exe UnrealTournament -game Example_Map?mutator=SampleMutator.SampleMutator"
You should notice that the weapon pickups in the map have been replaced by Rocket Launchers