浅析StaticClass的实现

浅析StaticClass的实现

​ 每一个继承自UObject的类，都可以通过xxx::StaticClass()获取到该类的UClass对象。本文主要研究该函数在何处定义以及实现方式。

声明点和定义点

​ 在xxx.generated.h中，通过模板特化，出现了该函数的声明：

image-20240202152821420

模板特化StaticClass的定义

​ 在xxx.generated.cpp中，可以找到该函数的定义，是转发调用了xxx::StaticClass()静态成员函数。

image-20240202152946435

静态成员函数StaticClass的定义

​ 仔细查看后发现，这个被转发的成员函数，是从GENERATE_BODY宏中的DECLARE_CLASS宏创建的定义，这个宏被定义于CoreUObject\Public\UObject\ObjectMacros.h中。

#define DECLARE_CLASS( TClass, TSuperClass, TStaticFlags, TStaticCastFlags, TPackage, TRequiredAPI  ) \
private: \
    TClass& operator=(TClass&&);   \
    TClass& operator=(const TClass&);   \
	TRequiredAPI static UClass* GetPrivateStaticClass(); \
public: \
	/** Bitwise union of #EClassFlags pertaining to this class.*/ \
	static constexpr EClassFlags StaticClassFlags=EClassFlags(TStaticFlags); \
	/** Typedef for the base class ({{ typedef-type }}) */ \
	typedef TSuperClass Super;\
	/** Typedef for {{ typedef-type }}. */ \
	typedef TClass ThisClass;\
	/** Returns a UClass object representing this class at runtime */ \
	inline static UClass* StaticClass() \
	{ \
		return GetPrivateStaticClass(); \
	} \
	/** Returns the package this class belongs in */ \
	inline static const TCHAR* StaticPackage() \
	{ \
		return TPackage; \
	} \
	/** Returns the static cast flags for this class */ \
	inline static EClassCastFlags StaticClassCastFlags() \
	{ \
		return TStaticCastFlags; \
	} \
	/** For internal use only; use StaticConstructObject() to create new objects. */ \
	inline void* operator new(const size_t InSize, EInternal InInternalOnly, UObject* InOuter = (UObject*)GetTransientPackage(), FName InName = NAME_None, EObjectFlags InSetFlags = RF_NoFlags) \
	{ \
		return StaticAllocateObject(StaticClass(), InOuter, InName, InSetFlags); \
	} \
	/** For internal use only; use StaticConstructObject() to create new objects. */ \
	inline void* operator new( const size_t InSize, EInternal* InMem ) \
	{ \
		return (void*)InMem; \
	} \
	/* Eliminate V1062 warning from PVS-Studio while keeping MSVC and Clang happy. */ \
	inline void operator delete(void* InMem) \
	{ \
		::operator delete(InMem); \
	}

​ 可以看到，这个宏中定义了StaticClass()成员函数，方法是转发GetPrivateStaticClass()成员函数。

成员函数GetPrivateStaticClass的定义

​ GetPrivateStaticClass成员函数是在xxx.genereated.cpp中的宏IMPLEMENT_CLASS_NO_AUTO_REGISTRATION定义的。这个宏本身定义于CoreUObject\Public\UObject\ObjectMacros.h中。可以看出它是一个经典的单例模式：

#define IMPLEMENT_CLASS_NO_AUTO_REGISTRATION(TClass) \
	FClassRegistrationInfo Z_Registration_Info_UClass_##TClass; \
	UClass* TClass::GetPrivateStaticClass() \
	{ \
		if (!Z_Registration_Info_UClass_##TClass.InnerSingleton) \
		{ \
			/* this could be handled with templates, but we want it external to avoid code bloat */ \
			GetPrivateStaticClassBody( \
				StaticPackage(), \
				(TCHAR*)TEXT(#TClass) + 1 + ((StaticClassFlags & CLASS_Deprecated) ? 11 : 0), \
				Z_Registration_Info_UClass_##TClass.InnerSingleton, \
				StaticRegisterNatives##TClass, \
				sizeof(TClass), \
				alignof(TClass), \
				TClass::StaticClassFlags, \
				TClass::StaticClassCastFlags(), \
				TClass::StaticConfigName(), \
				(UClass::ClassConstructorType)InternalConstructor<TClass>, \
				(UClass::ClassVTableHelperCtorCallerType)InternalVTableHelperCtorCaller<TClass>, \
				UOBJECT_CPPCLASS_STATICFUNCTIONS_FORCLASS(TClass), \
				&TClass::Super::StaticClass, \
				&TClass::WithinClass::StaticClass \
			); \
		} \
		return Z_Registration_Info_UClass_##TClass.InnerSingleton; \
	}

GetPrivateStaticClassBody函数解读

​ 这个函数定义于CoreUObject\Private\UObject\Class.cpp中，原型为：

void GetPrivateStaticClassBody(
	const TCHAR* PackageName,
	const TCHAR* Name,
	UClass*& ReturnClass,
	void(*RegisterNativeFunc)(),
	uint32 InSize,
	uint32 InAlignment,
	EClassFlags InClassFlags,
	EClassCastFlags InClassCastFlags,
	const TCHAR* InConfigName,
	UClass::ClassConstructorType InClassConstructor,
	UClass::ClassVTableHelperCtorCallerType InClassVTableHelperCtorCaller,
	FUObjectCppClassStaticFunctions&& InCppClassStaticFunctions,
	UClass::StaticClassFunctionType InSuperClassFn,
	UClass::StaticClassFunctionType InWithinClassFn
	);

函数指针解读

​ 在函数中，出现了两个函数指针：

(UClass::ClassConstructorType)InternalConstructor<TClass>;
(UClass::ClassVTableHelperCtorCallerType)InternalVTableHelperCtorCaller<TClass>;

​ 它们是对模板类进行特化实现后，将特化的函数指针传递过来：

/**
 * Helper template to call the default constructor for a class
 */
template<class T>
void InternalConstructor( const FObjectInitializer& X )
{ 
	T::__DefaultConstructor(X);
}

/**
 * Helper template to call the vtable ctor caller for a class
 */
template<class T>
UObject* InternalVTableHelperCtorCaller(FVTableHelper& Helper)
{
	return T::__VTableCtorCaller(Helper);
}
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