70 lines
2.1 KiB
Markdown
70 lines
2.1 KiB
Markdown
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# Within
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- **功能描述：** 指定对象创建的时候必须依赖于OuterClassName的对象作为Outer。
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- **引擎模块：** Instance
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- **元数据类型：** string="abc"
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- **作用机制：** 保存在UClass* UClass::ClassWithin=XXX的XXX中
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- **常用程度：★★★**
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指定对象创建的时候必须依赖于OuterClassName的对象作为Outer。
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此类的对象无法在OuterClassName对象的实例之外存在。这意味着，要创建此类的对象，需要提供OuterClassName的一个实例作为其Outer对象。
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本类在这种情况一般是用来当做子对象来使用的。
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## 示例代码：
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```cpp
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UCLASS(Within= MyClass_Within_Outer)
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class INSIDER_API UMyClass_Within :public UObject
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{
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GENERATED_BODY()
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};
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UCLASS()
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class INSIDER_API UMyClass_Within_Outer :public UObject
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{
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GENERATED_BODY()
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public:
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};
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```
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## 示例结果：
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```cpp
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//错误！Fatal error: Object MyClass_Within None created in Package instead of MyClass_Within_Outer
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UMyClass_Within* obj=NewObject<UMyClass_Within>();
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//正确：
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UMyClass_Within_Outer* objOuter = NewObject<UMyClass_Within_Outer>();
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UMyClass_Within* obj=NewObject<UMyClass_Within>(objOuter);
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```
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## 原理：
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生成的UClass的字段：UClass* ClassWithin会保存这个信息，然后在创建的时候StaticAllocateObject会测试 check(bCreatingCDO || !InOuter || InOuter->IsA(InClass->ClassWithin))。因此需要先创建Within的对象。
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```cpp
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bool StaticAllocateObjectErrorTests( const UClass* InClass, UObject* InOuter, FName InName, EObjectFlags InFlags)
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{
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if ( (InFlags & (RF_ClassDefaultObject|RF_ArchetypeObject)) == 0 )
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{
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if ( InOuter != NULL && !InOuter->IsA(InClass->ClassWithin) )
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{
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UE_LOG(LogUObjectGlobals, Fatal, TEXT("%s"), *FString::Printf( TEXT("Object %s %s created in %s instead of %s"), *InClass->GetName(), *InName.ToString(), *InOuter->GetClass()->GetName(), *InClass->ClassWithin->GetName()) );
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return true;
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}
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}
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}
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```
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在源码里可以搜索到很多Within的用法
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UCLASS(Within=Engine, config=Engine, transient)
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class ENGINE_API ULocalPlayer
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UCLASS(Abstract, DefaultToInstanced, Within=UserWidget)
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class UMG_API UUserWidgetExtension : public UObject
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{
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