ue-cpp-foundations

// MyCharacter.h
#pragma once
#include "CoreMinimal.h"
#include "GameFramework/Character.h"
#include "MyCharacter.generated.h"

class UStaticMeshComponent;
class UStaticMesh;

UCLASS(Blueprintable)
class MYGAME_API AMyCharacter : public ACharacter
{
    GENERATED_BODY()
public:
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Stats", meta=(ClampMin="1.0"))
    float MaxHealth = 100.f;

    UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category="Stats")
    float CurrentHealth = 100.f;

    UPROPERTY(Transient)
    TObjectPtr<UStaticMeshComponent> CachedMesh;     // not serialized, still GC-tracked

    UPROPERTY(EditDefaultsOnly, Category="Config")
    TSubclassOf<AActor> ProjectileClass;             // class reference restricted to AActor subclasses
    UPROPERTY(EditDefaultsOnly, Category="Config")
    TSoftObjectPtr<UStaticMesh> LazyMesh;            // path reference; load on demand

    UFUNCTION(BlueprintNativeEvent, Category="Combat")
    void OnDamageTaken(float Amount);
    virtual void OnDamageTaken_Implementation(float Amount);
};

// MyCharacter.cpp — the only body a BlueprintNativeEvent gets in C++
void AMyCharacter::OnDamageTaken_Implementation(float Amount) { CurrentHealth = FMath::Max(0.f, CurrentHealth - Amount); }

// MyTypes.h
#pragma once
#include "CoreMinimal.h"
#include "MyTypes.generated.h"

USTRUCT(BlueprintType)
struct MYGAME_API FMyWeaponStats
{
    GENERATED_BODY()

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Weapon")
    float BaseDamage = 10.f;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Weapon")
    float FireRate = 0.5f;
};

UENUM(BlueprintType)
enum class EMyWeaponState : uint8      // Blueprint-visible enums are enum class : uint8
{
    Idle,
    Firing    UMETA(DisplayName="Firing Weapon"),
    MAX       UMETA(Hidden)
};

TArray<FString> Names;
Names.Add(TEXT("Alpha"));
Names.Emplace(TEXT("Beta"));                       // construct in place
Names.Reserve(100);                                // pre-size before a known number of Adds
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);                             // O(1), reorders
for (int32 i = Names.Num() - 1; i >= 0; --i) { if (Names[i].IsEmpty()) { Names.RemoveAt(i); } }  // never mutate inside ranged-for

TMap<FName, int32> ItemCounts;
int32& Count = ItemCounts.FindOrAdd(FName("Sword"));  // inserts default if absent
int32* Found = ItemCounts.Find(FName("Axe"));         // nullptr if absent
for (const TPair<FName, int32>& Pair : ItemCounts) { UE_LOG(LogMyGame, Log, TEXT("%s x%d"), *Pair.Key.ToString(), Pair.Value); }

TSet<FName> Tags;
Tags.Add(FName("Flying"));
bool bFlying = Tags.Contains(FName("Flying"));     // hashed, no duplicates

TOptional<float> MaybeHP;
float Safe = MaybeHP.Get(0.f);                     // default when unset

#include "Misc/TVariant.h"
#include <type_traits>

TVariant<int32, float, FString> Value;             // first type must be default-constructible (or use FEmptyVariantState)
Value.Set<FString>(TEXT("Hello"));
if (const FString* Str = Value.TryGet<FString>()) { UE_LOG(LogMyGame, Log, TEXT("%s"), **Str); }

Visit([](auto& Held)                               // one generic lambda; branch with if constexpr
{
    using HeldType = std::decay_t<decltype(Held)>;
    if constexpr (std::is_same_v<HeldType, FString>) { UE_LOG(LogMyGame, Log, TEXT("%s"), *Held); }
}, Value);

// MyHealthComponent.h — delegate types are declared at file scope, above the UCLASS
DECLARE_MULTICAST_DELEGATE_TwoParams(FOnMyHealthChangedNative, float /*Current*/, float /*Max*/);
DECLARE_DYNAMIC_MULTICAST_DELEGATE_TwoParams(FOnMyHealthChanged, float, CurrentHealth, float, MaxHealth);

// in UMyHealthComponent : public UActorComponent
UPROPERTY(BlueprintAssignable, Category="Health")
FOnMyHealthChanged OnHealthChanged;               // dynamic: needs UPROPERTY, params need names
FOnMyHealthChangedNative OnHealthChangedNative;   // native: no reflection, cheaper to broadcast

// in AMyListenerActor : public AActor
UFUNCTION()                                       // AddDynamic targets must be UFUNCTIONs
void HandleHealthChanged(float CurrentHealth, float MaxHealth);
void HandleHealthChangedNative(float Current, float Max);
UPROPERTY() TObjectPtr<UMyHealthComponent> Health;
FDelegateHandle NativeHandle;

// AMyListenerActor.cpp — bind in BeginPlay (after Super::), unbind in EndPlay (before Super::)
Health->OnHealthChanged.AddDynamic(this, &AMyListenerActor::HandleHealthChanged);
NativeHandle = Health->OnHealthChangedNative.AddUObject(this, &AMyListenerActor::HandleHealthChangedNative);
// EndPlay: guard with IsValid(Health) first
Health->OnHealthChanged.RemoveDynamic(this, &AMyListenerActor::HandleHealthChanged);
Health->OnHealthChangedNative.Remove(NativeHandle);

float Health = 42.f; int32 Current = 3; int32 Max = 5;

FName   Tag(TEXT("Weapon.Rifle"));
FString S    = Tag.ToString();
FName   Back = FName(*S);
FString Msg  = FString::Printf(TEXT("HP: %.1f (%d/%d)"), Health, Current, Max);
int32 Parsed = FCString::Atoi(TEXT("42"));

#define LOCTEXT_NAMESPACE "MyGame"
FText Label = LOCTEXT("RifleName", "Assault Rifle");
FText Fmt   = FText::Format(LOCTEXT("HPFmt", "HP: {0}/{1}"), FText::AsNumber(Current), FText::AsNumber(Max));
#undef LOCTEXT_NAMESPACE
FText Other = NSLOCTEXT("MyGame", "Key", "Text");   // no #define needed
FText Raw   = FText::FromString(S);                 // not localized; runtime-generated text only

// Reflected members: TObjectPtr — GC-tracked, access-tracked in the editor, used like a raw pointer
UPROPERTY() TObjectPtr<UStaticMeshComponent> MeshComp;

// Legacy APIs that want raw pointers
void FillObjects(TArray<UObject*>& Out);
TArray<TObjectPtr<UObject>> Objects;
FillObjects(MutableView(Objects));                       // temporary TArray<UObject*>& view; ToRawPtr(Ptr) for a single read-only copy

// Non-owning reference: TWeakObjectPtr becomes null once the object is destroyed or collected
TWeakObjectPtr<AMyCharacter> WeakTarget;
if (AMyCharacter* Target = WeakTarget.Get()) { Target->OnDamageTaken(5.f); }

// Keep-alive from non-reflected code: scoped root instead of AddToRoot()/RemoveFromRoot()
TStrongObjectPtr<UMyDataObject> Pinned(NewObject<UMyDataObject>());

// Plain C++ types only: never wrap a UObject in TSharedPtr/TUniquePtr
struct FMyData { int32 Score = 0; };
TSharedPtr<FMyData> Data = MakeShared<FMyData>();
TWeakPtr<FMyData>   WeakData = Data;                     // Pin() returns a TSharedPtr if still alive

#include "UObject/GCObject.h"

class FMyManager : public FGCObject
{
public:
    virtual void AddReferencedObjects(FReferenceCollector& Collector) override { Collector.AddReferencedObject(ManagedObject); }
    virtual FString GetReferencerName() const override { return TEXT("FMyManager"); }
private:
    TObjectPtr<UMyDataObject> ManagedObject;   // TObjectPtr<T>& overload; raw UObject* is deprecated
};

#include "Misc/NotNull.h"

const UMyDataObject* Defaults = GetDefault<UMyDataObject>();        // class default object, read-only
UMyDataObject*       Settings = GetMutableDefault<UMyDataObject>(); // settings-style objects
UClass* SomeClass = AMyCharacter::StaticClass();
const AMyCharacter* ClassCDO = SomeClass->GetDefaultObject<AMyCharacter>();  // CDO of a runtime UClass

// TNotNull<T*>: caller cannot pass null; checked at the call site in Debug/Development, a plain pointer in Test and Shipping
void RegisterOwner(TNotNull<AActor*> Owner);
void RegisterOwner(TNotNull<AActor*> Owner)
{
    Owner->SetActorHiddenInGame(false);   // no null check needed inside
}

// MyInventorySubsystem.h
#pragma once
#include "CoreMinimal.h"
#include "Subsystems/GameInstanceSubsystem.h"
#include "MyInventorySubsystem.generated.h"

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

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

// A ticking world subsystem adds these; Initialize/Deinitialize must call Super:: (the base registers the ticker)
UCLASS()
class MYGAME_API UMyTickableSubsystem : public UTickableWorldSubsystem
{
    GENERATED_BODY()
public:
    virtual void Tick(float DeltaTime) override;
    virtual TStatId GetStatId() const override { RETURN_QUICK_DECLARE_CYCLE_STAT(UMyTickableSubsystem, STATGROUP_Tickables); }
    virtual bool DoesSupportWorldType(const EWorldType::Type WorldType) const override;  // e.g. Game and PIE only
};

// Access from an AActor member function
UMyInventorySubsystem* Inv = GetGameInstance()->GetSubsystem<UMyInventorySubsystem>();
UMyTickableSubsystem*  Tck = GetWorld()->GetSubsystem<UMyTickableSubsystem>();
APlayerController*     PC  = GetWorld()->GetFirstPlayerController();
UMyUISubsystem*        UI  = ULocalPlayer::GetSubsystemFromController<UMyUISubsystem>(PC);   // null-safe
UMyEngineSubsystem*    Eng = GEngine->GetEngineSubsystem<UMyEngineSubsystem>();

// MyReplicatedActor.h
#pragma once
#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "MyReplicatedActor.generated.h"

UCLASS()
class MYGAME_API AMyReplicatedActor : public AActor
{
    GENERATED_BODY()
public:
    AMyReplicatedActor() { bReplicates = true; }
    virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;
protected:
    UPROPERTY(ReplicatedUsing=OnRep_Health)
    float Health = 100.f;

    UFUNCTION()
    void OnRep_Health();                 // runs on clients after Health changes
};

// MyReplicatedActor.cpp
#include "MyReplicatedActor.h"
#include "Net/UnrealNetwork.h"          // required for DOREPLIFETIME

void AMyReplicatedActor::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
    Super::GetLifetimeReplicatedProps(OutLifetimeProps);
    DOREPLIFETIME(AMyReplicatedActor, Health);
}

void AMyReplicatedActor::OnRep_Health() {}

DECLARE_LOG_CATEGORY_EXTERN(LogMyGame, Log, All);   // MyGame.h (module header)
DEFINE_LOG_CATEGORY(LogMyGame);                     // MyGame.cpp
UE_LOG(LogMyGame, Warning, TEXT("HP low: %.1f"), 12.5f);

#if WITH_EDITORONLY_DATA
    UPROPERTY(EditAnywhere, Category="Debug")
    bool bShowDebugSpheres = false;                 // reflected editor-only data
#endif
#if WITH_EDITOR
    virtual void PostEditChangeProperty(FPropertyChangedEvent& PropertyChangedEvent) override;   // editor-only code
#endif
#if !UE_BUILD_SHIPPING
    void DrawDebugInfo();                           // Debug, Development and Test only
#endif

UMyDataObject* Obj;                        // WRONG: invisible to GC
UPROPERTY() TObjectPtr<UMyDataObject> Obj; // RIGHT

TArray<AActor*> Actors;
for (AActor* A : Actors) { Actors.Remove(A); }                                                      // WRONG
for (int32 i = Actors.Num() - 1; i >= 0; --i) { if (!IsValid(Actors[i])) { Actors.RemoveAt(i); } }  // RIGHT