unreal-engine

name	unreal-engine
description	Guides Unreal Engine 5 C++ development — UObject reflection system (UCLASS, UPROPERTY, UFUNCTION), AActor lifecycle, Gameplay Framework (GameMode, PlayerController, PlayerState), components, delegates, replication, Enhanced Input, Gameplay Ability System, asset management with soft/hard references, Blueprint-C++ boundary, build system modules, and UE memory model. Use when writing, reviewing, or architecting UE5 C++ code.
license	Apache-2.0
compatibility	{"clients":["openai-codex","gemini-cli","opencode","github-copilot"]}
metadata	{"owner":"codex","domain":"unreal-engine","maturity":"draft","risk":"low","tags":["unreal","ue5","cpp","game-engine","gameplay-framework"]}

Purpose

Provide concrete C++ patterns, API usage, and architectural guidance for Unreal Engine 5 projects — covering the UObject system, gameplay framework, replication, GAS, input, asset management, and the Blueprint-C++ boundary.

When to use this skill

• Writing or reviewing C++ classes using UCLASS(), UPROPERTY(), UFUNCTION() reflection macros
• Implementing the AActor lifecycle: BeginPlay(), Tick(), EndPlay(), component initialization
• Using the Gameplay Framework: AGameModeBase, AGameStateBase, APlayerState, APlayerController, APawn/ACharacter
• Creating components with UActorComponent, USceneComponent, CreateDefaultSubobject<T>(TEXT("Name"))
• Setting up delegates: DECLARE_DYNAMIC_MULTICAST_DELEGATE, DECLARE_DELEGATE, BindDynamic()
• Implementing replication: UPROPERTY(Replicated), GetLifetimeReplicatedProps(), UFUNCTION(Server, Reliable)
• Using GAS: UGameplayAbility, UGameplayEffect, FGameplayTag, UAbilitySystemComponent
• Configuring Enhanced Input: UInputAction, UInputMappingContext, BindAction() in SetupPlayerInputComponent
• Managing assets: TSoftObjectPtr<T>, FSoftObjectPath, StreamableManager, async loading
• Build system: .Build.cs module rules, .Target.cs, plugin descriptors, module dependencies

Do not use this skill when

• The task is Blueprint-only with no C++ — use ue5-blueprint
• The project uses Unity or Godot — use unity or godot
• The task is about narrative design or lore — use worldbuilding-lore-systems

Operating procedure

1. UObject system. Every gameplay class derives from UObject. Use UCLASS(Blueprintable) for Blueprint subclassing. Mark properties with UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Stats") for editor/Blueprint access and GC protection. Use UFUNCTION(BlueprintCallable) to expose methods.
2. Actor lifecycle. Constructor for CreateDefaultSubobject and defaults only — no gameplay logic. BeginPlay() for initialization. Tick() for per-frame updates (disable with PrimaryActorTick.bCanEverTick = false). EndPlay() for cleanup.
3. Gameplay Framework. GameMode owns match rules (server-only). GameState holds replicated match data. PlayerState holds per-player replicated data. PlayerController handles input and UI. Pawn/Character is the physical avatar. Understand which classes exist on server, client, or both.
4. Components. Create in constructor: UStaticMeshComponent* Mesh = CreateDefaultSubobject<UStaticMeshComponent>(TEXT("Mesh")). Set root with SetRootComponent(Mesh). Attach children with SetupAttachment(RootComponent).
5. Delegates. DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FOnHealthChanged, float, NewHealth) for Blueprint-compatible events. Bind with OnHealthChanged.AddDynamic(this, &AMyActor::HandleHealthChanged). Use non-dynamic delegates for C++-only, higher-performance events.
6. Replication. Mark replicated properties: UPROPERTY(ReplicatedUsing=OnRep_Health). Implement GetLifetimeReplicatedProps with DOREPLIFETIME(AMyActor, Health). Server RPCs: UFUNCTION(Server, Reliable). Client RPCs: UFUNCTION(Client, Reliable). Multicast: UFUNCTION(NetMulticast, Unreliable).
7. Enhanced Input. Create UInputAction and UInputMappingContext assets. In SetupPlayerInputComponent, cast to UEnhancedInputComponent, call BindAction(InputAction, ETriggerEvent::Triggered, this, &AMyChar::Move). Add mapping context via UEnhancedInputLocalPlayerSubsystem.
8. GAS. Add UAbilitySystemComponent to character. Define abilities as UGameplayAbility subclasses. Apply effects via . Tag everything with . Grant abilities with . Activate with .

Decision rules

• Every UObject* member must be UPROPERTY() or it will be garbage collected unexpectedly.
• Disable Tick on actors that don't need per-frame updates — use timers or event-driven updates instead.
• GameMode exists only on the server — never access it from client code directly.
• Use TSoftObjectPtr for any asset over 1MB that isn't needed at spawn time.
• Prefer BlueprintNativeEvent over BlueprintImplementableEvent when C++ needs a default implementation.
• All replicated properties need GetLifetimeReplicatedProps — forgetting this is the #1 replication bug.

Output requirements

1. Header (.h) — class declaration with UCLASS, UPROPERTY, UFUNCTION macros, includes, forward declarations
2. Source (.cpp) — implementation with constructor, lifecycle, and gameplay logic
3. Build.cs Changes — any new module dependencies required
4. Replication Notes — which properties replicate, which RPCs are needed, authority model
5. Blueprint Exposure — which functions/properties are Blueprint-accessible and why

References

• UE5 Programming Guide
• Gameplay Framework
• Networking and Multiplayer
• Gameplay Ability System
• Enhanced Input
• UE5 API Reference

Related skills

• ue5-blueprint — Blueprint visual scripting side of the C++ boundary
• blueprint-patterns — reusable Blueprint architecture patterns
• multiplayer-netcode — advanced networking beyond basic replication
• game-design-systems — higher-level game system design

Failure handling

• If a UPROPERTY pointer is null at runtime, check that CreateDefaultSubobject is in the constructor and the property is initialized.
• If replication doesn't work, verify bReplicates = true on the actor, GetLifetimeReplicatedProps includes the property, and the actor has authority.
• If Enhanced Input doesn't fire, confirm the mapping context was added to the local player subsystem and the input action asset is assigned.
• If GAS abilities fail to activate, check that the AbilitySystemComponent is initialized on PossessedBy/OnRep_PlayerState and abilities are granted.