(UE44.20)UE4的GC(垃圾回收)编程规范

UObject系统的GC

UPROPERTY 引用

当我们在一个UObject类声明各种继承UObject的 变量时&＃xff0c;得加UPROPERTY(), 这个可以让UE4帮我们自动管理UObject的垃圾回收。UPROPERTY不仅仅用于反射变量到编辑器上编辑&＃xff0c;也涉及UObject变量的GC。

如下面所示&＃xff1a;

UCLASS(config&＃61;Game)
class AMyProject1Character : public ACharacter
{GENERATED_BODY()private:UPROPERTY()UObject* object;}

TWeakObjectPtr

UE4里自创一套C&＃43;&＃43; GC规则&＃xff0c;官方并不推荐使用C&＃43;&＃43;标准库。 学C&＃43;&＃43;的同学可能会对弱指针熟悉&＃xff0c;可以访问一个对象又不造成引用计数加1。通常定义在一个类内&＃xff0c;用于获取其他对象的某个变量当成临时变量方便访问。如下面所示&＃xff1a;

UCLASS()
class MYPROJECT3_API ATestActor : public AActor
{GENERATED_BODY()public: // Sets default values for this actor&＃39;s propertiesATestActor();UPROPERTY(VisibleAnywhere)class UCameraComponent* cameraComponent;};ATestActor::ATestActor()
{// Set this actor to call Tick() every frame. You can turn this off to improve performance if you don&＃39;t need it.PrimaryActorTick.bCanEverTick &＃61; true;cameraComponent &＃61; CreateDefaultSubobject(TEXT("Camera"));cameraComponent->SetupAttachment(RootComponent);}

UCLASS(config&＃61;Game)
class AMyProject3Character : public ACharacter
{GENERATED_BODY()public:TWeakObjectPtr cameraComponent;protected:virtual void BeginPlay() override;
}void AMyProject3Character::BeginPlay()
{Super::BeginPlay();TArray arrayActor;UGameplayStatics::GetAllActorsOfClass(GetWorld(), ATestActor::StaticClass(), arrayActor);if (arrayActor.Num() > 0){ATestActor* testActor &＃61; Cast(arrayActor[0]);if (nullptr &＃61;&＃61; testActor)return;cameraComponent &＃61; testActor->cameraComponent;}
}

这里CameraComponent组件创建与Te&＃39;stActor&＃xff0c;但是AMyproject2Character需要保存临时变量的时候就用弱指针保存。

局部创建的UObject对象GC

局部创建并且在局部使用的UObject要调用AddToRoot来防止被GC, 然后 RemoveFromRoot来移除引用能得到GC

UCLASS()
class MYPROJECT4_API UTestObject : public UObject
{GENERATED_BODY()public:UTestObject(){}float a;};void AMyProject4Character::BeginPlay()
{Super::BeginPlay();UTestObject* testObject &＃61; NewObject(this);testObject->AddToRoot();testObject->a &＃61; 1.0f;UE_LOG(LogTemp, Error, TEXT("xxxx &＃61; %f"), testObject->a);testObject->RemoveFromRoot();
}

普通结构体Struct的GC

不继承UObject和UStruct对象的结构体的GC有下面几条

TSharedPtr

共享智能指针&＃xff0c;类似C&＃43;&＃43;标准库的共享智能指针&＃xff0c;不能用于UObject对象(TWeakPtr倒是可以用于UObject), 因为UObject有自己 专门GC的一套规则。总之TSharedPtr用于自定义的结构体(不继承UObject)。

如下所示&＃xff1a;

class MYPROJECT2_API FTest
{
public:FTest();~FTest();
};#include "FTest.h"FTest::FTest()
{UE_LOG(LogTemp, Warning, TEXT("1111FTest Constrction"));
}FTest::~FTest()
{UE_LOG(LogTemp, Warning, TEXT("2222FTest Deconstrction"));
}

UCLASS()
class MYPROJECT2_API ATestActor : public AActor
{GENERATED_BODY()public:    // Sets default values for this actor&＃39;s propertiesATestActor();TSharedPtr Test;
protected:// Called when the game starts or when spawnedvirtual void BeginPlay() override;}// Called when the game starts or when spawned
void ATestActor::BeginPlay()
{Super::BeginPlay();Test &＃61; MakeShareable(new FTest());}

PIE运行时Actor创建&＃xff1a;

PIE取消运行Actor销毁&＃xff1a;

如果是局部变量的结构体创建TSharedPtr:


void ATestActor::BeginPlay()
{Super::BeginPlay();TSharedPtr Test;Test &＃61; MakeShareable(new FTest());}

FGCObject

上面我们说到了UObject的情况&＃xff0c;采用NewObject创建和UPROPERTY()标记来进行GC。而不继承UObject的自定义结构体采用MakeShareable来创建SharedPtr来管理GC。这时候一个特殊的情况出现了玩意&＃xff0c;一个不继承UObject的结构体中出现UObject对象怎么办&＃xff1f;如下面所示&＃xff1a;

*/
UCLASS(BlueprintType, notplaceable, MinimalAPI)
class UCameraAnim : public UObject
{。。。。。。。。。。。。。。。。。。。。。。。。
}class UCameraAnim;
/*** */class MYPROJECT2_API FTest
{
public:FTest();~FTest();UPROPERTY()UCameraAnim* CameraAnim;
};


这种情况下&＃xff0c;我试过&＃xff0c;如果用MakeShareable创建结构体指针&＃xff0c;然后直接操作结构体的UObject对象进行NewObjec创建&＃xff0c;结构体本身的访问没问题&＃xff0c;但是结构体里面的UObject对象一段时间后自动GC掉&＃xff0c;成为野指针&＃xff0c;不能访问。这种情况怎么解决呢&＃xff1f;UE4为我们提供了FGCObject &＃xff0c;我们的结构体继承FGCObject &＃xff0c;然后用AddReferencedObjects 方法引用结构体里的所有UObject对象&＃xff0c;这里的UObject对象不需要被UPROPERTY()标记&＃xff0c; 如下所示

#include "GCObject.h"class UCameraAnim;
/*** */
class MYPROJECT2_API FTest :public FGCObject
{
public:FTest();~FTest();UCameraAnim* CameraAnim;protected:virtual void AddReferencedObjects(FReferenceCollector& Collector) override{Collector.AddReferencedObject(CameraAnim);}
};


TSharedRef

本质上共享引用(TSharedRef)和共享指针(TSharedPtr)是差不多的&＃xff0c;一样是用于非UObject体系的对象&＃xff0c;具备引用计数器&＃xff0c;不过TSharedRef和TSharedPtr的很大不同在于&＃xff0c;共享引用无法引用空对象&＃xff0c;就像C&＃43;&＃43; 声明空引用&＃xff0c;编译直接报错。TSharedRef连
“IsValid”判断是否非空都没有&＃xff0c;强逼你使用必定是有效的引用。所以在UE4 的SWidget编程大量使用TSharedRef

如 int& a;

正确的声明定义

TSharedRef MyCustom &＃61; MakeShared();


错误的声明定义&＃xff1a;

TSharedRef MyCustom;TSharedRef MyCustom &＃61; nullptr;

TSharedRef没有空引用&＃xff0c;应该持有的对象必须是有效的

TSharedPtr转为TSharedRef

TSharedPtr aaa &＃61; MakeShareable(new FMyCustom);
TSharedRef a &＃61; aaa.ToSharedRef();

一个普通的结构体使用TSharedFromThis继承&＃xff0c;就可以拥有AsShared函数让普通的结构体对象转为TSharedRef, 像SWidget这个类本身就使用了TSharedFromThis

struct FMyCustom : TSharedFromThis
{int a &＃61; 1;FMyCustom(){UE_LOG(LogTemp, Error, TEXT("destruct a &＃61; %d"), a);}~FMyCustom(){UE_LOG(LogTemp, Error, TEXT("undestruct a &＃61; %d"), a);}
};FMyCustom MyCustom;
RefCustom &＃61; MyCustom.AsShared();

TSharedRef可以转化为TSharedPtr&＃xff0c; 直接等于就可以了

TSharedRef RefCustom &＃61; MakeShared();TSharedPtr PtrCustom &＃61; RefCustom;

TWeakPtr

上面我们说到了 TWeakObjectPtr 为专门用于UObject, 相应的普通结构体也有弱指针TWeakPtr, 能够访问一个非UObject对象&＃xff0c;但是又不造成引用计数&＃43;1

一般来说通过TWeakPtr来访问其持有的对象&＃xff0c;先是用pin转换为TSharePtr指针&＃xff0c;判断对象是否有效后&＃xff0c;再访问&＃xff0c;而不是直接通过弱指针的IsValid&＃xff0c;然后直接访问

struct FMyCustom
{int a &＃61; 1;FMyCustom(){UE_LOG(LogTemp, Error, TEXT("destruct a &＃61; %d"), a);}~FMyCustom(){UE_LOG(LogTemp, Error, TEXT("undestruct a &＃61; %d"), a);}
};UCLASS()
class MYPROJECT5_API AMyActor : public AActor
{
public:TWeakPtr WeakCustom;virtual void BeginPlay() override;virtual void Tick(float DeltaTime) override;}void AMyActor::BeginPlay()
{Super::BeginPlay();TSharedPtr ShareCustom &＃61; MakeShareable(new FMyCustom);WeakCustom &＃61; ShareCustom;}// Called every frame
void AMyActor::Tick(float DeltaTime)
{Super::Tick(DeltaTime);TSharedPtr ShareCustom &＃61; WeakCustom.Pin();if (ShareCustom.IsValid()){UE_LOG(LogTemp, Error, TEXT("a &＃61; %d"), ShareCustom->a);}}

很显然&＃xff0c;弱指针没有引用到局部的智能创建的对象&＃xff0c;所以没有“Tick a  &＃61; 1”的log

TSharedPtr&＃xff0c;TSharedRef, TWeakPtr在容器使用事项

得注意&＃xff1a;弱指针(TWeakPtr)对象不能作为Set或者Map的Key&＃xff0c;因为一个对象被GC掉无法通知一个容器的Key, 像TMap, int>就是不正当的使用手法&＃xff0c;当然TWeakPtr可以作为容器的Value,  TMap>毫无问题。

而TSharedPtr和TSharedRef既可以当作容器的Key&＃xff0c;也可以作为容器的Value.

参考资料

[1] https://docs.unrealengine.com/en-US/Programming/UnrealArchitecture/SmartPointerLibrary/index.html