反射
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Struct 反射实现分析

前面讲到了 Class/function/property 的反射，UE 还支持结构体的反射，其实从 C++ 的标准语义来说并没有区分“结构”和“类”，关键字 struct 和class的区别只在于默认的访问权限。

在 UE 里面，支持反射的结构提只能使用struct，并且不能包含任何 UFUNCTION 的函数，命名必须以 F 开头。

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USTRUCT(BlueprintType)
struct FTestStruct
{
	GENERATED_USTRUCT_BODY()

	UPROPERTY(EditAnywhere)
	int32 ival;
	UPROPERTY(EditAnywhere)
	UTexture2D* Texture;
};

UE 的标记语法和 Class 的类似，不过 UHT 为 Struct 生成的代码要简单许多，因为没有 UFUNCTION 也没有继承 UObject。

GENERATED_USTRUCT_BODY这个标记 UHT 会展开生成一个真正的 C++ 宏(在 genreated.h 中)：

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// generated.h
#define HotPatcherExample_Source_HotPatcherExample_TestStruct_h_11_GENERATED_BODY \
	friend struct Z_Construct_UScriptStruct_FTestStruct_Statics; \
	HOTPATCHEREXAMPLE_API static class UScriptStruct* StaticStruct();

template<> HOTPATCHEREXAMPLE_API UScriptStruct* StaticStruct<struct FTestStruct>();

把 UHT 生成的用于记录当前 struct 反射数据的结构 Z_Construct_UScriptStruct_FTestStruct_Statis 声明为当前 struct 类的友元，可以让它访问到自己的私有成员。而且还声明了当前 Struct 的 static 成员函数 StaticStruct 和全局模板函数StaticStruct<FTestStruct>。

对 Struct 生成的反射数；

据的结构与 class 的类似，只有 Property 的反射信息。

gen.cpp中有以下内容：

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class UScriptStruct* FTestStruct::StaticStruct()
{
	static class UScriptStruct* Singleton = NULL;
	if (!Singleton)
	{
		extern HOTPATCHEREXAMPLE_API uint32 Get_Z_Construct_UScriptStruct_FTestStruct_Hash();
		Singleton = GetStaticStruct(Z_Construct_UScriptStruct_FTestStruct, Z_Construct_UPackage__Script_HotPatcherExample(), TEXT("TestStruct"), sizeof(FTestStruct), Get_Z_Construct_UScriptStruct_FTestStruct_Hash());
	}
	return Singleton;
}
template<> HOTPATCHEREXAMPLE_API UScriptStruct* StaticStruct<FTestStruct>()
{
	return FTestStruct::StaticStruct();
}
static FCompiledInDeferStruct Z_CompiledInDeferStruct_UScriptStruct_FTestStruct(FTestStruct::StaticStruct, TEXT("/Script/HotPatcherExample"), TEXT("TestStruct"), false, nullptr, nullptr);
static struct FScriptStruct_HotPatcherExample_StaticRegisterNativesFTestStruct
{
	FScriptStruct_HotPatcherExample_StaticRegisterNativesFTestStruct()
	{
		UScriptStruct::DeferCppStructOps(FName(TEXT("TestStruct")),new UScriptStruct::TCppStructOps<FTestStruct>);
	}
} ScriptStruct_HotPatcherExample_StaticRegisterNativesFTestStruct;

UScriptStruct* Z_Construct_UScriptStruct_FTestStruct()
{
#if WITH_HOT_RELOAD
	extern uint32 Get_Z_Construct_UScriptStruct_FTestStruct_Hash();
	UPackage* Outer = Z_Construct_UPackage__Script_HotPatcherExample();
	static UScriptStruct* ReturnStruct = FindExistingStructIfHotReloadOrDynamic(Outer, TEXT("TestStruct"), sizeof(FTestStruct), Get_Z_Construct_UScriptStruct_FTestStruct_Hash(), false);
#else
	static UScriptStruct* ReturnStruct = nullptr;
#endif
	if (!ReturnStruct)
	{
		UE4CodeGen_Private::ConstructUScriptStruct(ReturnStruct, Z_Construct_UScriptStruct_FTestStruct_Statics::ReturnStructParams);
	}
	return ReturnStruct;
}
uint32 Get_Z_Construct_UScriptStruct_FTestStruct_Hash() { return 4266809061U; }

上面的代码包含了从 UHT 生成的反射信息中构造出 UStructSctruct 以及延迟注册的方法，和 Class 的方式类似。

还包含生成的结构 Z_Construct_UScriptStruct_STRUCTNAME_Statics 如下：

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struct Z_Construct_UScriptStruct_FTestStruct_Statics
{
#if WITH_METADATA
	static const UE4CodeGen_Private::FMetaDataPairParam Struct_MetaDataParams[];
#endif
	static void* NewStructOps();
#if WITH_METADATA
	static const UE4CodeGen_Private::FMetaDataPairParam NewProp_Texture_MetaData[];
#endif
	static const UE4CodeGen_Private::FObjectPropertyParams NewProp_Texture;
#if WITH_METADATA
	static const UE4CodeGen_Private::FMetaDataPairParam NewProp_ival_MetaData[];
#endif
	static const UE4CodeGen_Private::FIntPropertyParams NewProp_ival;
	static const UE4CodeGen_Private::FPropertyParamsBase* const PropPointers[];
	static const UE4CodeGen_Private::FStructParams ReturnStructParams;
};

#if WITH_METADATA
const UE4CodeGen_Private::FMetaDataPairParam Z_Construct_UScriptStruct_FTestStruct_Statics::Struct_MetaDataParams[] = {
	{ "BlueprintType", "true" },
	{ "ModuleRelativePath", "TestStruct.h" },
};
#endif
void* Z_Construct_UScriptStruct_FTestStruct_Statics::NewStructOps()
{
	return (UScriptStruct::ICppStructOps*)new UScriptStruct::TCppStructOps<FTestStruct>();
}
#if WITH_METADATA
const UE4CodeGen_Private::FMetaDataPairParam Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_Texture_MetaData[] = {
	{ "Category", "TestStruct" },
	{ "ModuleRelativePath", "TestStruct.h" },
};
#endif

const UE4CodeGen_Private::FObjectPropertyParams Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_Texture = { 
	"Texture", 
	nullptr, 
	(EPropertyFlags)0x0010000000000001, 
	UE4CodeGen_Private::EPropertyGenFlags::Object, 
	RF_Public|RF_Transient|RF_MarkAsNative, 
	1, 
	STRUCT_OFFSET(FTestStruct, Texture), 
	Z_Construct_UClass_UTexture2D_NoRegister, 
	METADATA_PARAMS(Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_Texture_MetaData, 
	UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_Texture_MetaData)) 
};
#if WITH_METADATA
const UE4CodeGen_Private::FMetaDataPairParam Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_ival_MetaData[] = {
	{ "Category", "TestStruct" },
	{ "ModuleRelativePath", "TestStruct.h" },
};
#endif
const UE4CodeGen_Private::FIntPropertyParams Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_ival = { 
	"ival", 
	nullptr, 
	(EPropertyFlags)0x0010000000000001, 
	UE4CodeGen_Private::EPropertyGenFlags::Int, 
	RF_Public|RF_Transient|RF_MarkAsNative, 
	1, 
	STRUCT_OFFSET(FTestStruct, ival), 
	METADATA_PARAMS(Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_ival_MetaData, 
	UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_ival_MetaData)) 
};

const UE4CodeGen_Private::FPropertyParamsBase* const Z_Construct_UScriptStruct_FTestStruct_Statics::PropPointers[] = {
	(const UE4CodeGen_Private::FPropertyParamsBase*)&Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_Texture,
	(const UE4CodeGen_Private::FPropertyParamsBase*)&Z_Construct_UScriptStruct_FTestStruct_Statics::NewProp_ival,
};
const UE4CodeGen_Private::FStructParams Z_Construct_UScriptStruct_FTestStruct_Statics::ReturnStructParams = {
	(UObject* (*)())Z_Construct_UPackage__Script_HotPatcherExample,
	nullptr,
	&NewStructOps,
	"TestStruct",
	sizeof(FTestStruct),
	alignof(FTestStruct),
	Z_Construct_UScriptStruct_FTestStruct_Statics::PropPointers,
	UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FTestStruct_Statics::PropPointers),
	RF_Public|RF_Transient|RF_MarkAsNative,
	EStructFlags(0x00000001),
	METADATA_PARAMS(Z_Construct_UScriptStruct_FTestStruct_Statics::Struct_MetaDataParams, UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FTestStruct_Statics::Struct_MetaDataParams))
};

可以看到 Struct 的每个属性也是通过 FPropertyParamsBase 来存储的，与 Class 一致。

区别在于，Struct 使用 UE4CodeGen_Private::FStructParams 来存储当前结构的反射信息，其声明如下(CoreUObject/Public/UObject/UObjectGlobals.h)：

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// CoreUObject/Public/UObject/UObjectGlobals.h
struct FStructParams
{
	UObject*                          (*OuterFunc)();	
	UScriptStruct*                    (*SuperFunc)();
	void*                             (*StructOpsFunc)(); // really returns UScriptStruct::ICppStructOps*
	const char*                         NameUTF8;
	SIZE_T                              SizeOf;
	SIZE_T                              AlignOf;
	const FPropertyParamsBase* const*   PropertyArray;
	int32                               NumProperties;
	EObjectFlags                        ObjectFlags;
	uint32                              StructFlags; // EStructFlags
#if WITH_METADATA
	const FMetaDataPairParam*           MetaDataArray;
	int32                               NumMetaData;
#endif
};

这个结构中比较特殊的一点是 StructOpsFunc 是一个函数指针，用来管理 C++ 结构的构造和析构，使用的也是 placement-new 的方式，TCppStructOps<>模板定义在CoreUObject/Public/UObject/Class.h。