79 lines
2.5 KiB
Markdown
79 lines
2.5 KiB
Markdown
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# Abstract
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- **功能描述：** 指定此类为抽象基类。可被继承，但不可生成对象。
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- **引擎模块：** Blueprint
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- **元数据类型：** bool
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- **作用机制：** 在ClassFlags中添加[CLASS_Abstract](../../../../Flags/EClassFlags/CLASS_Abstract.md)
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- **常用程度：** ★★★★★
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指定此类为抽象基类。可被继承，但不可生成对象。
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一般是用在XXXBase基类。
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## 示例代码：
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```cpp
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/*
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ClassFlags: CLASS_Abstract | CLASS_MatchedSerializers | CLASS_Native | CLASS_RequiredAPI | CLASS_TokenStreamAssembled | CLASS_Intrinsic | CLASS_Constructed
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*/
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UCLASS(Blueprintable, abstract)
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class INSIDER_API UMyClass_Abstract :public UObject
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{
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GENERATED_BODY()
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};
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//测试语句：
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UMyClass_Abstract* obj=NewObject<UMyClass_Abstract>();
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```
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## 示例效果：
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在蓝图中的ConstructObject不会出现该类。同时在C++中NewObject也会报错。
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## 原理：
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在NewObject的时候会进行Abstract的判断。
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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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// Validation checks.
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if( !InClass )
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{
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UE_LOG(LogUObjectGlobals, Fatal, TEXT("Empty class for object %s"), *InName.ToString() );
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return true;
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}
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// for abstract classes that are being loaded NOT in the editor we want to error. If they are in the editor we do not want to have an error
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if (FScopedAllowAbstractClassAllocation::IsDisallowedAbstractClass(InClass, InFlags))
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{
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if ( GIsEditor )
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{
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const FString ErrorMsg = FString::Printf(TEXT("Class which was marked abstract was trying to be loaded in Outer %s. It will be nulled out on save. %s %s"), *GetPathNameSafe(InOuter), *InName.ToString(), *InClass->GetName());
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// if we are trying instantiate an abstract class in the editor we'll warn the user that it will be nulled out on save
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UE_LOG(LogUObjectGlobals, Warning, TEXT("%s"), *ErrorMsg);
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ensureMsgf(false, TEXT("%s"), *ErrorMsg);
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}
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else
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{
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UE_LOG(LogUObjectGlobals, Fatal, TEXT("%s"), *FString::Printf( TEXT("Can't create object %s in Outer %s: class %s is abstract"), *InName.ToString(), *GetPathNameSafe(InOuter), *InClass->GetName()));
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return true;
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}
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}
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}
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bool FScopedAllowAbstractClassAllocation::IsDisallowedAbstractClass(const UClass* InClass, EObjectFlags InFlags)
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{
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if (((InFlags& RF_ClassDefaultObject) == 0) && InClass->HasAnyClassFlags(CLASS_Abstract))
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{
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if (AllowAbstractCount == 0)
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{
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return true;
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}
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}
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return false;
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}
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```
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