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: 4.14
• 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
• Finally create a "Public" directory and a "Private" directory inside of the "Source" 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

SampleMutator.uplugin - Plugin defintion

{
    "FileVersion" : 3,
    
    "FriendlyName" : "Sample Mutator",
    "Version" : 1,
    "VersionName" : "1.0",
    "CreatedBy" : "Epic Games, Inc.",
    "CreatedByURL" : "http://epicgames.com",
    "EngineVersion" : "4.14.0",
    "Description" : "Sample Mutator",
    "Category" : "UnrealTournament.Mutator",
    "EnabledByDefault" : true,
    "CanContainContent" : false,    
    "Modules" :
    [
        {
            "Name" : "SampleMutator",
            "Type" : "Runtime",
            "WhitelistPlatforms" : [ "Win32", "Win64", "Linux" ]
        }
    ]
}

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 (inside the "Source" directory)
• We'll use the PrivateIncludesPaths array to make #include cleaner throughout our source files

SampleMutator.Build.cs - Unreal Build Tool definitions

namespace UnrealBuildTool.Rules
{
    public class SampleMutator : ModuleRules
    {
        public SampleMutator(TargetInfo Target)
            {
                    PrivateIncludePaths.Add("SampleMutator/Private");

            PublicDependencyModuleNames.AddRange(
                new string[]
                {
                    "Core",
                    "CoreUObject",
                                "Engine",
                                "UnrealTournament",
                }
            );
        }
    }
}

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 Public directory
• We need to #include Core.h, Engine.h, UTMutator.h and UTWeapon.h to have enough definitions to make a child class from AUTMutator
  1. 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

SampleMutator.h - Sample Mutator class definition

#pragma once

#include "Core.h"
#include "Engine.h"
#include "UTMutator.h"
#include "UTWeapon.h"

#include "SampleMutator.generated.h"

UCLASS(Blueprintable, Meta = (ChildCanTick))
class ASampleMutator : public AUTMutator
{
    GENERATED_UCLASS_BODY()

    bool CheckRelevance_Implementation(AActor* Other) OVERRIDE;

        
    UPROPERTY()
    TSubclassOf RocketLauncherClass;
};

Plugin Module Interface

• Create SampleMutatorPlugin.cpp inside of the Private 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

SampleMutatorPlugin.cpp - Plugin Module Interface

#include "SampleMutator.h"

#include "Core.h"
#include "Engine.h"
#include "ModuleManager.h"
#include "ModuleInterface.h"

class FSampleMutatorPlugin : public IModuleInterface
{
    /** IModuleInterface implementation */
    virtual void StartupModule() override;
    virtual void ShutdownModule() override;
};

IMPLEMENT_MODULE( FSampleMutatorPlugin, SampleMutator )

void FSampleMutatorPlugin::StartupModule()
{
    
}

void FSampleMutatorPlugin::ShutdownModule()
{
    
}

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 Private 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

SampleMutator.cpp - Replace all weapon pickups with rocket launcher pickups

#include "SampleMutator.h"

#include "UTPickupWeapon.h"

ASampleMutator::ASampleMutator(const FObjectInitializer& ObjectInitializer)
    : Super(PCIP)
{
}

bool ASampleMutator::CheckRelevance_Implementation(AActor* Other)
{
    // Find the rocket launcher blueprint
    if (RocketLauncherClass == nullptr)
    {
        RocketLauncherClass = StaticLoadClass(AUTWeapon::StaticClass(), nullptr, TEXT("Blueprint'/Game/RestrictedAssets/Weapons/RocketLauncher/BP_RocketLauncher.BP_RocketLauncher_C'"));
    }

    // If a weapon pickup has a current weapon type, replace it with the rocket launcher
    AUTPickupWeapon *PickupWeapon = Cast(Other);
    if (PickupWeapon)
    {
        if (PickupWeapon->WeaponType != nullptr)
        {
            PickupWeapon->WeaponType = RocketLauncherClass;
        }
    }

    return Super::CheckRelevance_Implementation(Other);
}

Building your Mutator

• Right click your UnrealTournament.uproject in Window Explorer and select "Generate Visual Studio project files" or run "GenerateProjectFiles.bat"
• 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?botfill=0"
• You should notice that the weapon pickups in the map have been replaced by Rocket Launchers