【UE4 反射系统】 UCLAS UFUNCTION UPROPERTY 宏简单解析持续更新

[0 引言](#0 引言)
[1 C++如何实现反射机制](#1 C++如何实现反射机制)
- [1.1 使用代码生成工具实现反射机制](#1.1 使用代码生成工具实现反射机制)
[2 UE4的反射系统](#2 UE4的反射系统)
- [2.1 ****.generated.h头文件](#2.1 ****.generated.h头文件)
- [2.2 GENERATED_BODY()](#2.2 GENERATED_BODY())
- [2.3 反射宏 UCLASS 等](#2.3 反射宏 UCLASS 等)
- [2.4 UHT和UBT](#2.4 UHT和UBT)
[3 基本宏的使用](#3 基本宏的使用)
- [3.1 UCLASS](#3.1 UCLASS)
- [3.2 UFUNCTION](#3.2 UFUNCTION)
- [3.3 UPROPERTY](#3.3 UPROPERTY)

🙋‍♂️ 作者：海码007

📜 专栏：UE虚幻引擎专栏

💥 标题：【UE4 反射系统】 UCLAS USTRUCT UFUNCTION UPROPERTY 宏详解

❣️ 寄语：加油，一次专注一件事！

🎈 最后：文章作者技术和水平有限，如果文中出现错误，希望大家能指正，同时有问题的话，欢迎大家留言讨论。

0 引言

反射系统 是一种允许程序在运行时 获取类型信息 和动态操作类和对象 的机制。在C++语言中，没有内置的反射系统，但可以使用不同的库和技术实现类似的功能。（Java和C#语言都是有内置的反射机制的）

UE4在C++的基础上搭建了自己的一套反射机制，使用自定义的反射系统来支持蓝图编程和编辑器功能 。UE4的反射系统允许在运行时访问类和对象的属性、函数和元数据，并提供了一些宏和关键字来标识和定义可被反射系统处理的内容 。其中，"UCLASS "、"UFUNCTION "和"UPROPERTY"是UE4中用于反射的一些重要宏。

UE4的反射系统是选择加入的，只有主动标记的类型、属性、方法会被反射系统追踪，UnrealHeaderTool（UHT）会收集这些信息，生成用于支持反射机制的C++代码，然后再编译工程。

1 C++如何实现反射机制

实现C++的反射机制有多种方式。下面详细介绍其中几种常见的方法：

手动定义转换函数 ：这是最基础的反射实现方式。在类的定义中，手动定义转换函数，用于将类的数据成员或方法映射到字符串或其他类型。例如，可以为每个类定义一个静态成员函数，接受类的实例作为参数，并返回包含类信息的结构体。这种方法需要手动编写大量的重复代码，不够灵活。

使用宏 ：宏是C++中实现代码生成的一种方式。通过定义一系列宏来模拟反射机制。可以使用宏定义类的元信息，例如类名、成员变量名、函数名等。通过宏展开，可以在运行时获取类的元信息，并执行对应的操作。这种方法可以减少手动编写的代码量，但仍然需要繁琐的宏定义。

使用代码生成工具 ：可以编写一个代码生成工具，根据特定的标记或注释解析源代码，并生成反射所需的代码。工具可以根据类定义自动生成访问元信息的代码，避免手动编写大量重复的代码。通过将生成工具作为构建系统的一部分，可以在每次构建时自动更新反射代码。这种方法相对灵活且易于维护 ，但需要编写自定义的代码生成工具。（QT和UE4就是采用的该方法实现反射机制）

使用第三方库：也可以使用现有的第三方库来实现C++反射。例如，"Boost.Reflection"库提供了一套用于C++的反射功能。它可以动态地获取和操作类的成员变量、函数和类型信息。这样，开发者可以在运行时获得类的元信息，从而实现动态加载和操作类的功能。

需要注意的是，C++作为静态类型语言，没有内置的反射机制。因此，以上这些方法只是模拟了反射的部分功能，无法像动态类型语言一样灵活。

总结起来，实现C++的反射机制可以通过手动定义转换函数、使用宏、编写代码生成工具或使用第三方库等方式。每种方式都有其优缺点，具体选择取决于项目需求和开发者的偏好。

1.1 使用代码生成工具实现反射机制

代码生成工具 通常是在构建过程中的一个独立阶段，通过解析源代码来生成反射所需的代码。
简单的实现步骤：

定义注释或标记 ：在源代码中定义一些注释或特定的标记，用于标识需要反射的类、成员变量和方法 。
例如，可以使用REFLECT_CLASS、REFLECT_MEMBER和REFLECT_METHOD等宏来注释类、成员变量和方法。

编写代码生成工具 ：编写一个代码生成工具，它可以读取源代码文件，并解析这些注释或标记 。
例如，可以使用编译器提供的编译器前端库，如Clang的LibTooling，来辅助解析源代码。

解析源代码 ：使用代码生成工具解析源代码文件，并提取类、成员变量和方法的相关信息 。
例如，可以获取类名、成员变量的名称 、类型和访问修饰符，以及方法的名称、参数列表和返回类型等

生成反射代码 ：根据解析得到的信息，代码生成工具可以生成反射所需的代码。
例如，可以生成用于获取类的元信息的结构体或类 ，以及用于动态访问成员变量和调用方法的函数。生成的代码可以包括类注册、获取类信息、访问成员变量、调用方法等功能。

构建生成工具 ：将代码生成工具集成到项目的构建过程中 。
例如，使用Makefile、CMake或其他构建系统。确保代码生成工具在每次构建时自动运行，以根据最新的源代码生成反射代码。

使用反射功能 ：在运行时 ，可以使用生成的反射代码动态地获取和操作类的成员变量和方法 。
根据类的元信息，可以在不知道类的实际名称的情况下，动态创建对象、获取和修改成员变量的值，以及调用方法。

2 UE4的反射系统

在了解了C++中如何使用代码生成工具实现反射机制之后，那么再来理解 UE4 反射系统就会容易许多。
先从最简单的一个UObject子类开始分析（因为日常使用的类基本上都继承UObject），我们新建一个C++类继承UObject，UE会自动生成许多反射的代码：

cpp 复制代码

// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "CoreMinimal.h"
#include "UObject/NoExportTypes.h"

#include "MyObject1.generated.h"

/**
 * 
 */
UCLASS()
class CPP_CLASS_10_15_API UMyObject1 : public UObject
{
	GENERATED_BODY()
	
public:

	UFUNCTION()
	void fun1()
	{
		UE_LOG(LogTemp, Warning, TEXT("name = %s"), *name);
	}

	UPROPERTY()
	FString name = "007";
};

2.1 ****.generated.h头文件

generate.h，其实就是UE对我们自己写的.h文件产生的反射代码文件。

#include "MyObject1.generated.h"需要写在头文件包含的最后。不然编译时会报错。错误如下：

error : #include found after .generated.h file - the .generated.h file should always be the last #include in a header
错误：#include在.generated.h文件之后找到-.generated.h文件应该始终是头中的最后一个#include

2.2 GENERATED_BODY()

使用GENERATED_BODY（）宏自动展开为通用的成员函数接口和数据成员

2.3 反射宏 UCLASS 等

可以看到上面出现了三个宏，这些都是反射宏，下图中是UE源码中对宏的定义

反射宏名称	作用
UCLASS	告诉UE这个类是个反射类。此类必须派生自UObject（如果不用这个宏，那么成员函数和成员变量加了宏也反射不了）
UFUNCTION	定义一个反射的函数
UPROPERTY	定义一个反射的变量
USTRUCT	可以不用派生自UObject。不支持GC，也不能包含函数
UENUM	告诉UE这是一个反射的枚举类。支持enum, enum class, enum namespace
UINTERFACE	定义一个反射接口类，只能包含函数
UMETA	反射的一些元数据定义，可以通过标签定义一些该变量的属性
UPARAM	定义函数的参数属性。主要就是显示名字和Ref属性
UDELEGATE	告诉UE这是一个可反射的委托（很少用到）

2.4 UHT和UBT

由于作者目前水平有限，理解还不透彻。等学习领悟之后再来填坑。

这里放一个参考文章帮助大家理解

3 基本宏的使用

3.1 UCLASS

如果想要使用蓝图生成C++类，则需要给UCLASS宏中添加说明符，UCLASS宏的所有说明符如下所示：

最常用的就是 BlueprintType（将此类公开为可用于蓝图中变量的类型）

cpp 复制代码

// These are used for syntax highlighting and to allow autocomplete hints

namespace UC
{
	// valid keywords for the UCLASS macro
	enum 
	{
		/// This keyword is used to set the actor group that the class is show in, in the editor.
		classGroup,

		/// Declares that instances of this class should always have an outer of the specified class.  This is inherited by subclasses unless overridden.
		Within, /* =OuterClassName */

		/// Exposes this class as a type that can be used for variables in blueprints
		BlueprintType,

		/// Prevents this class from being used for variables in blueprints
		NotBlueprintType,

		/// Exposes this class as an acceptable base class for creating blueprints. The default is NotBlueprintable, unless inherited otherwise. This is inherited by subclasses.
		Blueprintable,

		/// Specifies that this class is *NOT* an acceptable base class for creating blueprints. The default is NotBlueprintable, unless inherited otherwise. This is inherited by subclasses.
		NotBlueprintable,

		/// This keyword indicates that the class should be accessible outside of it's module, but does not need all methods exported.
		/// It exports only the autogenerated methods required for dynamic_cast<>, etc... to work.
		MinimalAPI,

		/// Prevents automatic generation of the constructor declaration.
		customConstructor,

		/// Class was declared directly in C++ and has no boilerplate generated by UnrealHeaderTool.
		/// DO NOT USE THIS FLAG ON NEW CLASSES.
		Intrinsic,

		/// No autogenerated code will be created for this class; the header is only provided to parse metadata from.
		/// DO NOT USE THIS FLAG ON NEW CLASSES.
		noexport,

		/// Allow users to create and place this class in the editor.  This flag is inherited by subclasses.
		placeable,

		/// This class cannot be placed in the editor (it cancels out an inherited placeable flag).
		notplaceable,

		/// All instances of this class are considered "instanced". Instanced classes (components) are duplicated upon construction. This flag is inherited by subclasses. 
		DefaultToInstanced,

		/// All properties and functions in this class are const and should be exported as const.  This flag is inherited by subclasses.
		Const,

		/// Class is abstract and can't be instantiated directly.
		Abstract,

		/// This class is deprecated and objects of this class won't be saved when serializing.  This flag is inherited by subclasses.
		deprecated,

		/// This class can't be saved; null it out at save time.  This flag is inherited by subclasses.
		Transient,

		/// This class should be saved normally (it cancels out an inherited transient flag).
		nonTransient,

		/// This class is optional and might not be available in certain context. reference from non optional data type is not allowed.
		Optional,

		/// Load object configuration at construction time.  These flags are inherited by subclasses.
		/// Class containing config properties. Usage config=ConfigName or config=inherit (inherits config name from base class).
		config,
		/// Handle object configuration on a per-object basis, rather than per-class. 
		perObjectConfig,
		/// Determine whether on serialize to configs a check should be done on the base/defaults ini's
		configdonotcheckdefaults,

		/// Save object config only to Default INIs, never to local INIs.
		defaultconfig,

		/// Mark the editor config file to load from if loading into this object.
		EditorConfig,

		/// These affect the behavior of the property editor.
		/// Class can be constructed from editinline New button.
		editinlinenew,
		/// Class can't be constructed from editinline New button.
		noteditinlinenew,
		/// Class not shown in editor drop down for class selection.
		hidedropdown,

		/// Shows the specified categories in a property viewer. Usage: showCategories=CategoryName or showCategories=(category0, category1, ...)
		showCategories,
		/// Hides the specified categories in a property viewer. Usage: hideCategories=CategoryName or hideCategories=(category0, category1, ...)
		hideCategories,
		/// Indicates that this class is a wrapper class for a component with little intrinsic functionality (this causes things like hideCategories and showCategories to be ignored if the class is subclassed in a Blueprint)
		ComponentWrapperClass,
		/// Shows the specified function in a property viewer. Usage: showFunctions=FunctionName or showFunctions=(category0, category1, ...)
		showFunctions,
		/// Hides the specified function in a property viewer. Usage: hideFunctions=FunctionName or hideFunctions=(category0, category1, ...)
		hideFunctions,
		/// Specifies which categories should be automatically expanded in a property viewer.
		autoExpandCategories,
		/// Specifies which categories should be automatically collapsed in a property viewer.
		autoCollapseCategories,
		/// Clears the list of auto collapse categories.
		dontAutoCollapseCategories,
		/// Display properties in the editor without using categories.
		collapseCategories,
		/// Display properties in the editor using categories (default behaviour).
		dontCollapseCategories,
		/// Specifies category display order, unspecified will follow default display order.
		prioritizeCategories,

		/// All the properties of the class are hidden in the main display by default, and are only shown in the advanced details section.
		AdvancedClassDisplay,

		/// A root convert limits a sub-class to only be able to convert to child classes of the first root class going up the hierarchy.
		ConversionRoot,

		/// Marks this class as 'experimental' (a totally unsupported and undocumented prototype)
		Experimental,

		/// Marks this class as an 'early access' preview (while not considered production-ready, it's a step beyond 'experimental' and is being provided as a preview of things to come)
		EarlyAccessPreview,

		/// Some properties are stored once per class in a sidecar structure and not on instances of the class
		SparseClassDataType,

		/// Specifies the struct that contains the CustomThunk implementations
		CustomThunkTemplates
	};
}

3.2 UFUNCTION

最常用的函数说明符就是 BlueprintCallable（此函数可在蓝图或关卡蓝图图表中执行）

cpp 复制代码

namespace UF
{
	// valid keywords for the UFUNCTION and UDELEGATE macros
	enum 
	{
		/// This function is designed to be overridden by a blueprint.  Do not provide a body for this function;
		/// the autogenerated code will include a thunk that calls ProcessEvent to execute the overridden body.
		BlueprintImplementableEvent,

		/// This function is designed to be overridden by a blueprint, but also has a native implementation.
		/// Provide a body named [FunctionName]_Implementation instead of [FunctionName]; the autogenerated
		/// code will include a thunk that calls the implementation method when necessary.
		BlueprintNativeEvent,

		/// This function is sealed and cannot be overridden in subclasses.
		/// It is only a valid keyword for events; declare other methods as static or final to indicate that they are sealed.
		SealedEvent,

		/// This function is executable from the command line.
		Exec,

		/// This function is replicated, and executed on servers.  Provide a body named [FunctionName]_Implementation instead of [FunctionName];
		/// the autogenerated code will include a thunk that calls the implementation method when necessary.
		Server,

		/// This function is replicated, and executed on clients.  Provide a body named [FunctionName]_Implementation instead of [FunctionName];
		/// the autogenerated code will include a thunk that calls the implementation method when necessary.
		Client,

		/// This function is both executed locally on the server and replicated to all clients, regardless of the Actor's NetOwner
		NetMulticast,

		/// Replication of calls to this function should be done on a reliable channel.
		/// Only valid when used in conjunction with Client or Server
		Reliable,

		/// Replication of calls to this function can be done on an unreliable channel.
		/// Only valid when used in conjunction with Client or Server
		Unreliable,

		/// This function fulfills a contract of producing no side effects, and additionally implies BlueprintCallable.
		BlueprintPure,

		/// This function can be called from blueprint code and should be exposed to the user of blueprint editing tools.
		BlueprintCallable,

		/// This function is used as the get accessor for a blueprint exposed property. Implies BlueprintPure and BlueprintCallable.
		BlueprintGetter,

		/// This function is used as the set accessor for a blueprint exposed property. Implies BlueprintCallable.
		BlueprintSetter,

		/// This function will not execute from blueprint code if running on something without network authority
		BlueprintAuthorityOnly,

		/// This function is cosmetic and will not run on dedicated servers
		BlueprintCosmetic,

		/// Indicates that a Blueprint exposed function should not be exposed to the end user
		BlueprintInternalUseOnly,
	
		/// This function can be called in the editor on selected instances via a button in the details panel.
		CallInEditor,

		/// The UnrealHeaderTool code generator will not produce a execFoo thunk for this function; it is up to the user to provide one.
		CustomThunk,

		/// Specifies the category of the function when displayed in blueprint editing tools.
		/// Usage: Category=CategoryName or Category="MajorCategory,SubCategory"
		Category,

		/// This function must supply a _Validate implementation
		WithValidation,

		/// This function is RPC service request
		ServiceRequest,

		/// This function is RPC service response
		ServiceResponse,
		
		/// [FunctionMetadata]	Marks a UFUNCTION as accepting variadic arguments. Variadic functions may have extra terms they need to emit after the main set of function arguments
		///						These are all considered wildcards so no type checking will be performed on them
		Variadic,

		/// [FunctionMetadata] Indicates the display name of the return value pin
		ReturnDisplayName, 

		/// [FunctionMetadata] Indicates that a particular function parameter is for internal use only, which means it will be both hidden and not connectible.
		InternalUseParam, 

		/// [FunctionMetadata] Indicates that the function should be ignored when considered for blueprint type promotion
		IgnoreTypePromotion,
	};
}

3.3 UPROPERTY

BlueprintReadWrite 可从蓝图读取或写入此属性。此说明符与 BlueprintReadOnly 说明符不兼容

cpp 复制代码

namespace UP
{
	// valid keywords for the UPROPERTY macro
	enum 
	{
		/// This property is const and should be exported as const.
		Const,

		/// Property should be loaded/saved to ini file as permanent profile.
		Config,

		/// Same as above but load config from base class, not subclass.
		GlobalConfig,

		/// Property should be loaded as localizable text. Implies ReadOnly.
		Localized,

		/// Property is transient: shouldn't be saved, zero-filled at load time.
		Transient,

		/// Property should always be reset to the default value during any type of duplication (copy/paste, binary duplication, etc.)
		DuplicateTransient,

		/// Property should always be reset to the default value unless it's being duplicated for a PIE session - deprecated, use NonPIEDuplicateTransient instead
		NonPIETransient,

		/// Property should always be reset to the default value unless it's being duplicated for a PIE session
		NonPIEDuplicateTransient,

		/// Value is copied out after function call. Only valid on function param declaration.
		Ref,

		/// Object property can be exported with it's owner.
		Export,

		/// Hide clear (and browse) button in the editor.
		NoClear,

		/// Indicates that elements of an array can be modified, but its size cannot be changed.
		EditFixedSize,

		/// Property is relevant to network replication.
		Replicated,

		/// Property is relevant to network replication. Notify actors when a property is replicated (usage: ReplicatedUsing=FunctionName).
		ReplicatedUsing,

		/// Skip replication (only for struct members and parameters in service request functions).
		NotReplicated,

		/// Interpolatable property for use with cinematics. Always user-settable in the editor.
		Interp,

		/// Property isn't transacted.
		NonTransactional,

		/// Property is a component reference. Implies EditInline and Export.
		Instanced,

		/// MC Delegates only.  Property should be exposed for assigning in blueprints.
		BlueprintAssignable,

		/// Specifies the category of the property. Usage: Category=CategoryName.
		Category,

		/// Properties appear visible by default in a details panel
		SimpleDisplay,

		/// Properties are in the advanced dropdown in a details panel
		AdvancedDisplay,

		/// Indicates that this property can be edited by property windows in the editor
		EditAnywhere,

		/// Indicates that this property can be edited by property windows, but only on instances, not on archetypes
		EditInstanceOnly,

		/// Indicates that this property can be edited by property windows, but only on archetypes
		EditDefaultsOnly,

		/// Indicates that this property is visible in property windows, but cannot be edited at all
		VisibleAnywhere,
		
		/// Indicates that this property is only visible in property windows for instances, not for archetypes, and cannot be edited
		VisibleInstanceOnly,

		/// Indicates that this property is only visible in property windows for archetypes, and cannot be edited
		VisibleDefaultsOnly,

		/// This property can be read by blueprints, but not modified.
		BlueprintReadOnly,

		/// This property has an accessor to return the value. Implies BlueprintReadOnly if BlueprintSetter or BlueprintReadWrite is not specified. (usage: BlueprintGetter=FunctionName).
		BlueprintGetter,

		/// This property can be read or written from a blueprint.
		BlueprintReadWrite,

		/// This property has an accessor to set the value. Implies BlueprintReadWrite. (usage: BlueprintSetter=FunctionName).
		BlueprintSetter,

		/// The AssetRegistrySearchable keyword indicates that this property and it's value will be automatically added
		/// to the asset registry for any asset class instances containing this as a member variable.  It is not legal
		/// to use on struct properties or parameters.
		AssetRegistrySearchable,

		/// Property should be serialized for save games.
		/// This is only checked for game-specific archives with ArIsSaveGame set
		SaveGame,

		/// MC Delegates only.  Property should be exposed for calling in blueprint code
		BlueprintCallable,

		/// MC Delegates only. This delegate accepts (only in blueprint) only events with BlueprintAuthorityOnly.
		BlueprintAuthorityOnly,

		/// Property shouldn't be exported to text format (e.g. copy/paste)
		TextExportTransient,

		/// Property shouldn't be serialized, can still be exported to text
		SkipSerialization,

		/// If true, the self pin should not be shown or connectable regardless of purity, const, etc. similar to InternalUseParam
		HideSelfPin, 
	};
}

【UE4 反射系统】 UCLAS UFUNCTION UPROPERTY 宏简单解析 持续更新

目录

0 引言

1 C++如何实现反射机制

1.1 使用代码生成工具实现反射机制

2 UE4的反射系统

2.1 ****.generated.h头文件

2.2 GENERATED_BODY()

2.3 反射宏 UCLASS 等

2.4 UHT和UBT

3 基本宏的使用

3.1 UCLASS

3.2 UFUNCTION

3.3 UPROPERTY

【UE4 反射系统】 UCLAS UFUNCTION UPROPERTY 宏简单解析持续更新