120 lines
4.6 KiB
Markdown
120 lines
4.6 KiB
Markdown
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# IsAlwaysAccessible
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- **功能描述：** 指定UHT在生成文件的时候总是可以访问到改结构的声明，否则要在gen.cpp里生成镜像结构定义
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- **元数据类型：** bool
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- **引擎模块：** UHT
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- **限制类型：** 只在NoExportTypes.h供UHT使用
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- **常用程度：** 0
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指定该结构的声明是否在UHT为NoExportTypes.h生成的gen.cpp里总是可以访问到。
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换句话说其实就是是否这些结构在GeneratedCppIncludes.h的声明里可以找到。如果不可以找到，那在后面生成Z_Construct_UScriptStruct_FMatrix44d_Statics这种类似的时候就得再自己定义一个镜像结构定义。如果可以找到，比如FGuid，则就不需要。
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因此这只是一个手动的内部标记，帮助UHT程序识别哪些结构要再创建镜像结构定义。
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在NoExportTypes.h查看各个结构的时候，会发现有些结构（85/135）会标上IsAlwaysAccessible，而有些没有。这是因为UHT在为NoExportTypes.h生成gen.cpp的时候，
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```cpp
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\UnrealEngine\Engine\Source\Runtime\CoreUObject\Public\UObject\GeneratedCppIncludes.h
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#include "UObject/Object.h"
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#include "UObject/UObjectGlobals.h"
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#include "UObject/CoreNative.h"
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#include "UObject/Class.h"
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#include "UObject/MetaData.h"
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#include "UObject/UnrealType.h"
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#include "UObject/EnumProperty.h"
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#include "UObject/TextProperty.h"
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#include "UObject/FieldPathProperty.h"
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#if UE_ENABLE_INCLUDE_ORDER_DEPRECATED_IN_5_2
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#include "CoreMinimal.h"
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#endif
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\Hello\Intermediate\Build\Win64\HelloEditor\Inc\CoreUObject\UHT\NoExportTypes.gen.cpp：
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// Copyright Epic Games, Inc. All Rights Reserved.
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/*===========================================================================
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Generated code exported from UnrealHeaderTool.
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DO NOT modify this manually! Edit the corresponding .h files instead!
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===========================================================================*/
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//以下这两行
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#include "UObject/GeneratedCppIncludes.h"//A
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#include "UObject/NoExportTypes.h"//B
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PRAGMA_DISABLE_DEPRECATION_WARNINGS
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void EmptyLinkFunctionForGeneratedCodeNoExportTypes() {}
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```
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在最开头的两个include里如果可以直接找到该struct的定义，则在gen.cpp中的A和B处需要结构定义的时候，就不需要再额外去找定义了。
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```cpp
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const UECodeGen_Private::FStructParams Z_Construct_UScriptStruct_FMatrix44f_Statics::ReturnStructParams = {
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(UObject* (*)())Z_Construct_UPackage__Script_CoreUObject,
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nullptr,
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nullptr,
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"Matrix44f",
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Z_Construct_UScriptStruct_FMatrix44f_Statics::PropPointers,
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UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FMatrix44f_Statics::PropPointers),
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sizeof(FMatrix44f),//这个A
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alignof(FMatrix44f),//这个B
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RF_Public|RF_Transient|RF_MarkAsNative,
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EStructFlags(0x00000038),
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METADATA_PARAMS(UE_ARRAY_COUNT(Z_Construct_UScriptStruct_FMatrix44f_Statics::Struct_MetaDataParams), Z_Construct_UScriptStruct_FMatrix44f_Statics::Struct_MetaDataParams)
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};
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```
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如果找不到，比如FMatrix44f，是定义在Engine\Source\Runtime\Core\Public\Math\Matrix.h，则必须为它生成一个一模一样的定义（不include的作用是加快编译)：
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```cpp
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struct Z_Construct_UScriptStruct_FMatrix44f_Statics
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{
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struct FMatrix44f //内存布局一致的定义
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{
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FPlane4f XPlane;
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FPlane4f YPlane;
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FPlane4f ZPlane;
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FPlane4f WPlane;
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};
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static_assert(sizeof(FMatrix44f) < MAX_uint16);
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static_assert(alignof(FMatrix44f) < MAX_uint8);
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```
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当然如果子字段或者父类也找不到定义，则只需要把父定义先写在前面就可以了。因此cs里的FindNoExportStructsRecursive就是为了找到其相关的结构。没有标IsAlwaysAccessible则意味着要生成假的结构定义
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```cpp
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private static void FindNoExportStructsRecursive(List<UhtScriptStruct> outScriptStructs, UhtStruct structObj)
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{
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for (UhtStruct? current = structObj; current != null; current = current.SuperStruct)
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{
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// Is isn't true for noexport structs
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if (current is UhtScriptStruct scriptStruct)
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{
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if (scriptStruct.ScriptStructFlags.HasAnyFlags(EStructFlags.Native))
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{
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break;
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}
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// these are a special cases that already exists and if wrong if exported naively
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if (!scriptStruct.IsAlwaysAccessible)
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{
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outScriptStructs.Remove(scriptStruct);
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outScriptStructs.Add(scriptStruct);
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}
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}
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foreach (UhtType type in current.Children)
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{
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if (type is UhtProperty property)
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{
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foreach (UhtType referenceType in property.EnumerateReferencedTypes())
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{
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if (referenceType is UhtScriptStruct propertyScriptStruct)
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{
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FindNoExportStructsRecursive(outScriptStructs, propertyScriptStruct);
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}
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}
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}
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}
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}
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}
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```
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