66 lines
2.4 KiB
Markdown
66 lines
2.4 KiB
Markdown
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# GetByRef
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- **功能描述：** 指定UHT为该属性生成返回引用的C++代码
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- **使用位置：** UPROPERTY
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- **引擎模块：** UHT
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- **元数据类型：** bool
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- **限制类型：** 只用在SparseClassDataTypes 指定的结构里的属性。
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- **关联项：** [SparseClassDataTypes](SparseClassDataTypes.md)
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指定UHT为该属性生成返回引用的C++代码。
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只用在SparseClassDataTypes 指定的结构里的属性。
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## 代码例子：
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```cpp
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USTRUCT(BlueprintType)
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struct FMySparseClassData
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{
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GENERATED_BODY()
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UPROPERTY(EditDefaultsOnly, BlueprintReadOnly)
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FString MyString_EditDefault = TEXT("MyName");
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//FString GetMyString_EditDefault() const { return GetMySparseClassData(EGetSparseClassDataMethod::ArchetypeIfNull)->MyString_EditDefault; } \
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// "GetByRef" means that Blueprint graphs access a const ref instead of a copy.
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UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, meta = (GetByRef))
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FString MyString_EditDefault_ReadOnly = TEXT("MyName");
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//const FString& GetMyString_EditDefault_ReadOnly() const { return GetMySparseClassData(EGetSparseClassDataMethod::ArchetypeIfNull)->MyString_EditDefault_ReadOnly; }
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};
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UCLASS(Blueprintable, BlueprintType, SparseClassDataTypes = MySparseClassData)
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class INSIDER_API AMyActor_SparseClassDataTypes :public AActor
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{
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GENERATED_BODY()
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}
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```
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## 生成的代码：
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可见，后者生成返回值是const FString&而不是FString。
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```cpp
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#define FID_Hello_Source_Insider_Class_Trait_MyClass_SparseClassDataTypes_h_36_SPARSE_DATA_PROPERTY_ACCESSORS \
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FString GetMyString_EditDefault() const { return GetMySparseClassData(EGetSparseClassDataMethod::ArchetypeIfNull)->MyString_EditDefault; } \
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const FString& GetMyString_EditDefault_ReadOnly() const { return GetMySparseClassData(EGetSparseClassDataMethod::ArchetypeIfNull)->MyString_EditDefault_ReadOnly; }
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```
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## 原理：
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UHT中为SparseDataType生成代码的时候会判断GetByRef来分别生成不同的格式代码。
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```cpp
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private StringBuilder AppendSparseDeclarations(StringBuilder builder, UhtClass classObj, IEnumerable<UhtScriptStruct> sparseScriptStructs, UhtUsedDefineScopes<UhtProperty> sparseProperties)
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{
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if (property.MetaData.ContainsKey(UhtNames.GetByRef))
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{
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builder.Append("const ").AppendSparse(property).Append("& Get").Append(cleanPropertyName).Append("() const");
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}
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else
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{
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builder.AppendSparse(property).Append(" Get").Append(cleanPropertyName).Append("() const");
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}
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}
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```
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