UE C++ Foundations

You are an expert in Unreal Engine's C++ extensions and property system.

Context

Read .agents/ue-project-context.md for engine version, coding conventions, and project-specific rules. Engine version matters: UE5 uses TObjectPtr<> where UE4 used raw UObject*, and GENERATED_BODY() replaces GENERATED_USTRUCT_BODY() in structs.

Before You Start

Ask which area the user needs help with if unclear:

• Macros & Reflection — UCLASS, UPROPERTY, UFUNCTION, USTRUCT, UENUM
• Containers — TArray, TMap, TSet, TOptional
• Delegates — static, dynamic, multicast, binding patterns
• Strings — FName, FString, FText conversion and formatting
• Memory & GC — TObjectPtr, TWeakObjectPtr, TSharedPtr, GC roots
• Logging — UE_LOG, log categories, verbosity
• Subsystems — GameInstance, World, LocalPlayer subsystems

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UObject Macros & Reflection

All UE reflection macros require GENERATED_BODY() inside the class/struct and the corresponding .generated.h include.

UCLASS()

Specifier	Effect
Blueprintable	Blueprint subclassing allowed
BlueprintType	Usable as Blueprint variable
Abstract	Cannot be instantiated
NotBlueprintable	Blocks Blueprint subclassing
Config=<Name>	Loads UPROPERTY(Config) from <Name>.ini
Transient	Not saved/serialized
Within=<OuterClass>	Outer must be of given type

UCLASS(Blueprintable, BlueprintType)
class MYGAME_API UMyDataObject : public UObject
{
    GENERATED_BODY()
public:
    UMyDataObject();
};

Full specifier list: references/property-specifiers.md.

UPROPERTY()

UCLASS(Blueprintable)
class MYGAME_API AMyCharacter : public ACharacter
{
    GENERATED_BODY()
public:
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Stats")
    float MaxHealth = 100.f;

    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category="Stats")
    float CurrentHealth;

    UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category="Config")
    int32 MaxLevel = 50;

    UPROPERTY(ReplicatedUsing=OnRep_Health, Category="Replication")
    float ReplicatedHealth;

    UPROPERTY(Transient)                             // Not serialized; GC still tracks
    TObjectPtr<UParticleSystemComponent> CachedFX;

    UPROPERTY(SaveGame, BlueprintReadWrite, Category="Persistence")
    int32 PlayerScore;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Stats",
              meta=(ClampMin="0.0", ClampMax="1.0"))
    float DamageMultiplier = 1.f;

    UFUNCTION()
    void OnRep_Health();

    virtual void GetLifetimeReplicatedProps(
        TArray<FLifetimeProperty>& OutLifetimeProps) const override;
};

UFUNCTION()

UFUNCTION(BlueprintCallable, Category="Actions")
void PerformAttack(float Damage);

UFUNCTION(BlueprintPure, Category="Queries")
float GetHealthPercent() const;

UFUNCTION(BlueprintNativeEvent, Category="Events")  // C++ provides _Implementation
void OnDamageTaken(float Amount);
virtual void OnDamageTaken_Implementation(float Amount);

UFUNCTION(BlueprintImplementableEvent, Category="Events")  // Blueprint must implement
void OnLevelUp(int32 NewLevel);

UFUNCTION(Server, Reliable, WithValidation)         // RPC: runs on server
void ServerFireWeapon(FVector Origin, FVector Direction);
void ServerFireWeapon_Implementation(FVector Origin, FVector Direction);
bool ServerFireWeapon_Validate(FVector Origin, FVector Direction);

UFUNCTION(Client, Reliable)                        // RPC: runs on owning client
void ClientShowDamageNumber(float Amount);
void ClientShowDamageNumber_Implementation(float Amount);

UFUNCTION(NetMulticast, Reliable)                   // RPC: runs on all
void MulticastPlayEffect(FVector Location);
void MulticastPlayEffect_Implementation(FVector Location);

UFUNCTION(Exec)                                    // Console command (~ in-game)
void DebugResetStats();                            // Works on PC, Pawn, HUD, GM, GI, CheatManager

USTRUCT() and UENUM()

// UE5: always GENERATED_BODY() — never GENERATED_USTRUCT_BODY()
USTRUCT(BlueprintType)
struct MYGAME_API FWeaponStats
{
    GENERATED_BODY()
    UPROPERTY(EditAnywhere, BlueprintReadWrite) float BaseDamage = 10.f;
    UPROPERTY(EditAnywhere, BlueprintReadWrite) float FireRate   = 0.5f;
};

// DataTable row
USTRUCT(BlueprintType)
struct MYGAME_API FEnemyTableRow : public FTableRowBase
{
    GENERATED_BODY()
    UPROPERTY(EditAnywhere, BlueprintReadWrite) FName EnemyID;
    UPROPERTY(EditAnywhere, BlueprintReadWrite) TSoftClassPtr<AActor> SpawnClass;
};

UENUM(BlueprintType)
enum class EWeaponState : uint8
{
    Idle      UMETA(DisplayName="Idle"),
    Firing    UMETA(DisplayName="Firing"),
    Reloading UMETA(DisplayName="Reloading"),
};

---

UE Containers

See references/container-patterns.md for full API and performance guide.

TArray — Ordered Dynamic Array

TArray<FString> Names;
Names.Add(TEXT("Alpha"));
Names.Emplace(TEXT("Beta"));       // Construct in-place (avoids copy)
Names.Reserve(100);                // Pre-allocate

FString First = Names[0];
bool bHas     = Names.Contains(TEXT("Alpha"));
int32 Idx     = Names.Find(TEXT("Beta"));  // INDEX_NONE if absent
FString* Ptr  = Names.FindByPredicate([](const FString& S){ return S.StartsWith(TEXT("A")); });

Names.Sort([](const FString& A, const FString& B){ return A.Len() < B.Len(); });
Names.Remove(TEXT("Alpha"));       // Order-preserving O(n)
Names.RemoveAtSwap(0);             // Fast O(1), destroys order

for (const FString& N : Names) { /* do NOT add/remove during ranged-for */ }
for (int32 i = Names.Num()-1; i >= 0; --i) { if (Names[i].IsEmpty()) Names.RemoveAt(i); }

TMap — Hash Map

TMap<FName, int32> ItemCounts;
ItemCounts.Add(FName("Sword"), 3);

int32& Ref  = ItemCounts.FindOrAdd(FName("Sword")); // Insert default if absent
int32* Ptr  = ItemCounts.Find(FName("Axe"));        // nullptr if absent
bool   bHas = ItemCounts.Contains(FName("Shield"));
ItemCounts.Remove(FName("Shield"));

for (const TPair<FName, int32>& Pair : ItemCounts) { /* ... */ }

TSet — Hash Set

TSet<FName> Tags;
Tags.Add(FName("Flying"));
bool bFlying          = Tags.Contains(FName("Flying"));
TSet<FName> Intersect = Tags.Intersect(OtherTags);
TSet<FName> Union     = Tags.Union(OtherTags);

TOptional

TOptional<float> MaybeHP;
if (MaybeHP.IsSet()) { float H = MaybeHP.GetValue(); }
float Safe = MaybeHP.Get(0.f);  // Default if not set
MaybeHP = 75.f;
MaybeHP.Reset();

TVariant

// Type-safe tagged union — avoids unsafe casts
TVariant<int32, float, FString> Value;
Value.Set<FString>(TEXT("Hello"));

if (Value.IsType<FString>())
{
    const FString& Str = Value.Get<FString>();
}

// Visit — use explicit overloads; LexToString(V) fails when V is FString.
Visit(TOverloaded{
    [](int32  V) { UE_LOG(LogTemp, Log, TEXT("%d"), V); },
    [](float  V) { UE_LOG(LogTemp, Log, TEXT("%f"), V); },
    [](const FString& V) { UE_LOG(LogTemp, Log, TEXT("%s"), *V); },
}, Value);

---

Delegates

See references/delegate-patterns.md for all declaration macros and binding methods.

Choosing the Right Type

Type	Bindings	Blueprint	When to Use
DECLARE_DELEGATE	1	No	Internal single-owner callback
DECLARE_MULTICAST_DELEGATE	N	No	Internal multi-listener events
DECLARE_DYNAMIC_DELEGATE	1	Yes	Blueprint-assignable single callback
DECLARE_DYNAMIC_MULTICAST_DELEGATE	N	Yes	Blueprint-bindable events (most common)

Declaration, Binding, Invocation

// File scope (before UCLASS)
DECLARE_DELEGATE_OneParam(FOnItemPickedUp, AActor*);
DECLARE_MULTICAST_DELEGATE_TwoParams(FOnHealthChanged, float, float);
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams(FOnHealthChangedDynamic, float, CurrentHealth, float, MaxHealth);

UCLASS()
class AMyActor : public AActor
{
    GENERATED_BODY()
public:
    UPROPERTY(BlueprintAssignable, Category="Events")
    FOnHealthChangedDynamic OnHealthChanged;   // Dynamic multicast in UPROPERTY
};

// Static single delegate
FOnItemPickedUp D;
D.BindUObject(this, &AMyCharacter::HandlePickup);
D.BindLambda([this](AActor* Item){ UE_LOG(LogTemp, Log, TEXT("%s"), *Item->GetName()); });
D.ExecuteIfBound(SomeActor);
// Multicast: add/broadcast/remove
FDelegateHandle H = HealthDelegate.AddUObject(this, &AMyHUD::OnHealthChanged);
HealthDelegate.Broadcast(75.f, 100.f);
HealthDelegate.Remove(H);
// Dynamic multicast
OnHealthChanged.AddDynamic(this, &AMyCharacter::HandleHealthChange);
OnHealthChanged.RemoveDynamic(this, &AMyCharacter::HandleHealthChange);
OnHealthChanged.Broadcast(75.f, 100.f);

---

String Types

Type	Use For	Comparison	Mutable
FName	Identifiers, asset names, tags	O(1) integer	No
FString	General-purpose strings, file paths	O(n)	Yes
FText	Player-visible display strings	—	No

// FName — global name table, case-insensitive O(1) compare
FName Tag("WeaponTag_Rifle");
FString S = Tag.ToString();
FName  N  = FName(*S);

// FString — heap string, Printf for formatting
FString Msg = FString::Printf(TEXT("HP: %.1f"), Health);
UE_LOG(LogTemp, Log, TEXT("%s"), *Msg);  // * dereferences to TCHAR*
int32 Num = FCString::Atoi(*FString("42"));

// FText — localized display text
// LOCTEXT requires a namespace defined in the same translation unit:
#define LOCTEXT_NAMESPACE "MyGame"
FText Label = LOCTEXT("Key", "Assault Rifle");
FText Fmt   = FText::Format(LOCTEXT("HP", "HP: {0}/{1}"),
                             FText::AsNumber(Cur), FText::AsNumber(Max));
#undef LOCTEXT_NAMESPACE  // Or: NSLOCTEXT("MyGame", "Key", "...") without a define

Conversion: Name.ToString() → FString, FName(*Str) ← FString, FText::FromString(Str), Text.ToString().

---

Memory & Garbage Collection

UE's GC tracks every UObject* reachable from a root. Unreachable objects are destroyed.

// UE5: TObjectPtr<> for UPROPERTY member UObject pointers
UPROPERTY()
TObjectPtr<UStaticMeshComponent> MeshComp;       // GC-tracked, lazy-load aware
// Without UPROPERTY — invisible to GC, pointer may dangle
UMyObject* UnsafePtr;  // BAD
// TWeakObjectPtr — non-owning, safe (becomes null after GC)
TWeakObjectPtr<AMyActor> WeakRef;
if (WeakRef.IsValid()) { WeakRef->DoSomething(); }
// TSharedPtr/TWeakPtr — for non-UObject plain C++ types ONLY
TSharedPtr<FMyData> Data = MakeShared<FMyData>();
TWeakPtr<FMyData>   Weak = Data;
if (TSharedPtr<FMyData> Pinned = Weak.Pin()) { Pinned->Process(); }
// NEVER use TSharedPtr for UObject-derived types
// GC root: AddToRoot (use sparingly)
UMyObject* Obj = NewObject<UMyObject>();
Obj->AddToRoot();
// ...
Obj->RemoveFromRoot();
// FGCObject: preferred for non-UObject C++ classes holding UObject refs
// UE 5.3+: use TObjectPtr<> — raw UObject* crashes with incremental GC.
class FMyManager : public FGCObject
{
public:
    virtual void AddReferencedObjects(FReferenceCollector& Collector) override
    {
        Collector.AddReferencedObject(ManagedObject);
    }
    virtual FString GetReferencerName() const override { return TEXT("FMyManager"); }
private:
    TObjectPtr<UMyObject> ManagedObject = nullptr;
};

---

Logging

// MyGameLog.h / .cpp
DECLARE_LOG_CATEGORY_EXTERN(LogMyGame, Log, All);
DEFINE_LOG_CATEGORY(LogMyGame);

// Single-file: DEFINE_LOG_CATEGORY_STATIC(LogLocal, Log, All);

UE_LOG(LogMyGame, Log,     TEXT("Loaded: %s"), *LevelName);
UE_LOG(LogMyGame, Warning, TEXT("HP low: %.1f"), Health);
UE_LOG(LogMyGame, Error,   TEXT("Spawn failed: %s"), *ClassName);
UE_CLOG(Health < 0.f, LogMyGame, Error, TEXT("Negative HP: %.1f"), Health);

Verbosity	Visible	When
Fatal	Always	Crash-level
Error	Always	Operation failed
Warning	Always	Unexpected but recoverable
Log	Non-shipping	Standard trace
Verbose	-LogCmds	Fine-grained trace

---

Subsystems

Auto-registered singletons — no manual AddToRoot needed.

Subsystem	Owner	Persists Level Load	Per Player
UGameInstanceSubsystem	UGameInstance	Yes	No
UWorldSubsystem	UWorld	No	No
ULocalPlayerSubsystem	ULocalPlayer	Yes	Yes
UEngineSubsystem	UEngine	Yes (whole session)	No

UCLASS()
class MYGAME_API UInventorySubsystem : public UGameInstanceSubsystem
{
    GENERATED_BODY()
public:
    virtual void Initialize(FSubsystemCollectionBase& Collection) override;
    virtual void Deinitialize() override;

    UFUNCTION(BlueprintCallable) void AddItem(FName ItemID, int32 Count);
private:
    TMap<FName, int32> Inventory;
};

// Access — each subsystem type has a different accessor
UInventorySubsystem* Inv = GetGameInstance()->GetSubsystem<UInventorySubsystem>();
USpawnSubsystem*     Sp  = GetWorld()->GetSubsystem<USpawnSubsystem>();
UUIStateSubsystem*   UI  = GetLocalPlayer()->GetSubsystem<UUIStateSubsystem>();
UMyEngineSubsystem*  ES  = GEngine->GetEngineSubsystem<UMyEngineSubsystem>();

Subsystems have Initialize() and Deinitialize() -- override for setup/teardown. UGameInstanceSubsystem persists across map changes; UWorldSubsystem reinitializes per world. Call GetSubsystem<T>() via the owning context (GetGameInstance(), GetWorld(), GetLocalPlayer()).

---

Replicated Properties

Both UPROPERTY specifier AND GetLifetimeReplicatedProps are required:

UPROPERTY(ReplicatedUsing = OnRep_Health)
float Health;

UFUNCTION()
void OnRep_Health(); // Called on clients when server updates Health

void AMyActor::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutProps) const
{
    Super::GetLifetimeReplicatedProps(OutProps);
    DOREPLIFETIME_CONDITION(AMyActor, Health, COND_OwnerOnly);
    // COND_None, COND_OwnerOnly, COND_SkipOwner, COND_SimulatedOnly, COND_InitialOnly
}

---

Conditional Compilation Guards

#if WITH_EDITOR
// Editor-only properties — stripped from shipping builds
UPROPERTY(EditAnywhere, Category="Debug")
bool bShowDebugSpheres = false;

virtual void PostEditChangeProperty(FPropertyChangedEvent& E) override;
#endif

#if !UE_BUILD_SHIPPING
// Available in Development + Debug, stripped from Shipping
void DrawDebugInfo();
#endif

---

Common Mistakes

Raw UObject member without UPROPERTY — dangling pointer:*

UMyObject* Obj;  // BAD — GC invisible
UPROPERTY() TObjectPtr<UMyObject> Obj;  // GOOD

Modify TArray during ranged-for — undefined behavior:

for (const AActor* A : Actors) { Actors.Remove(A); }  // CRASH
for (int32 i = Actors.Num()-1; i >= 0; --i) { if (ShouldRemove(Actors[i])) Actors.RemoveAt(i); }

TSharedPtr on a UObject — GC + refcount conflict:

TSharedPtr<UMyObject> P = MakeShared<UMyObject>();  // BAD — leaks or double-free
UPROPERTY() TObjectPtr<UMyObject> P;                 // GOOD

Missing GetLifetimeReplicatedProps:

void AMyActor::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);
    DOREPLIFETIME(AMyActor, ReplicatedHealth);
    DOREPLIFETIME_CONDITION(AMyActor, TeamScore, COND_OwnerOnly);
}

GENERATED_USTRUCT_BODY() in UE5 structs — use GENERATED_BODY() instead.

AddDynamic with a non-UFUNCTION — compile error or crash at runtime.

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Related Skills

• ue-module-build-system — Build.cs, module dependencies, include paths, PCH configuration
• ue-actor-component-architecture — AActor/UActorComponent lifecycle, spawning, tick groups, component setup