UE4的Cast方法与类型转化

来源：互联网发布：电力预算用什么软件编辑：程序博客网时间：2024/05/09 07:39

UE4中，我们经常会用到Cast函数，作用是强制转化一个指针到我们需要的类型，并且如果不能强制转化，就会返回nullptr。通过判断转换之后是不是nullptr，我们也能实现很多有用的功能（比如用于判断子弹击中的到底是什么）
那么Cast到底是如何实现的呢？要看Cast，首先要看到一个类型，叫做UClass，他是实现UE类型识别的重要类型。
我们经常打交道的类型都是继承自UObjectBase的，UObjectBase是UE的一个基础类型，内部有一个UClass类型的字段Class，识别类型的重要方法HasAnyCastFlag就是UClass的方法，我们看看HasAnyCastFlag的具体实现：

FORCEINLINE bool HasAnyCastFlag(EClassCastFlags FlagToCheck) const    {        return (ClassCastFlags&FlagToCheck) != 0;    }

我们可以看到，他的代码很简单，就是对比了参数FlagToCheck和自己一个叫ClassCastFlags的字段，这两个对象的类型是EClassCastFlags，其实这个类型很简单，就是int64。而两个类型到底是不是可以强制转化，完全取决于自己这个int64的flag字段和对方int64 flag字段按位与是否等于0。紧接这这个函数实现，就是UE4内部类型的转换关系：

#define CASTCLASS_None                          DECLARE_UINT64(0x0000000000000000)#define CASTCLASS_UField                        DECLARE_UINT64(0x0000000000000001)#define CASTCLASS_UInt8Property                 DECLARE_UINT64(0x0000000000000002)#define CASTCLASS_UEnum                         DECLARE_UINT64(0x0000000000000004)#define CASTCLASS_UStruct                       DECLARE_UINT64(0x0000000000000008)#define CASTCLASS_UScriptStruct                 DECLARE_UINT64(0x0000000000000010)#define CASTCLASS_UClass                        DECLARE_UINT64(0x0000000000000020)#define CASTCLASS_UByteProperty                 DECLARE_UINT64(0x0000000000000040)#define CASTCLASS_UIntProperty                  DECLARE_UINT64(0x0000000000000080)#define CASTCLASS_UFloatProperty                DECLARE_UINT64(0x0000000000000100)#define CASTCLASS_UUInt64Property               DECLARE_UINT64(0x0000000000000200)#define CASTCLASS_UClassProperty                DECLARE_UINT64(0x0000000000000400)#define CASTCLASS_UUInt32Property               DECLARE_UINT64(0x0000000000000800)#define CASTCLASS_UInterfaceProperty            DECLARE_UINT64(0x0000000000001000)#define CASTCLASS_UNameProperty                 DECLARE_UINT64(0x0000000000002000)#define CASTCLASS_UStrProperty                  DECLARE_UINT64(0x0000000000004000)#define CASTCLASS_UProperty                     DECLARE_UINT64(0x0000000000008000)#define CASTCLASS_UObjectProperty               DECLARE_UINT64(0x0000000000010000)#define CASTCLASS_UBoolProperty                 DECLARE_UINT64(0x0000000000020000)#define CASTCLASS_UUInt16Property               DECLARE_UINT64(0x0000000000040000)#define CASTCLASS_UFunction                     DECLARE_UINT64(0x0000000000080000)#define CASTCLASS_UStructProperty               DECLARE_UINT64(0x0000000000100000)#define CASTCLASS_UArrayProperty                DECLARE_UINT64(0x0000000000200000)#define CASTCLASS_UInt64Property                DECLARE_UINT64(0x0000000000400000)#define CASTCLASS_UDelegateProperty             DECLARE_UINT64(0x0000000000800000)#define CASTCLASS_UNumericProperty              DECLARE_UINT64(0x0000000001000000)#define CASTCLASS_UMulticastDelegateProperty    DECLARE_UINT64(0x0000000002000000)#define CASTCLASS_UObjectPropertyBase           DECLARE_UINT64(0x0000000004000000)#define CASTCLASS_UWeakObjectProperty           DECLARE_UINT64(0x0000000008000000)#define CASTCLASS_ULazyObjectProperty           DECLARE_UINT64(0x0000000010000000)#define CASTCLASS_UAssetObjectProperty          DECLARE_UINT64(0x0000000020000000)#define CASTCLASS_UTextProperty                 DECLARE_UINT64(0x0000000040000000)#define CASTCLASS_UInt16Property                DECLARE_UINT64(0x0000000080000000)#define CASTCLASS_UDoubleProperty               DECLARE_UINT64(0x0000000100000000)#define CASTCLASS_UAssetClassProperty           DECLARE_UINT64(0x0000000200000000)#define CASTCLASS_UPackage                      DECLARE_UINT64(0x0000000400000000)#define CASTCLASS_ULevel                        DECLARE_UINT64(0x0000000800000000)#define CASTCLASS_AActor                        DECLARE_UINT64(0x0000001000000000)#define CASTCLASS_APlayerController             DECLARE_UINT64(0x0000002000000000)#define CASTCLASS_APawn                         DECLARE_UINT64(0x0000004000000000)#define CASTCLASS_USceneComponent               DECLARE_UINT64(0x0000008000000000)#define CASTCLASS_UPrimitiveComponent           DECLARE_UINT64(0x0000010000000000)#define CASTCLASS_USkinnedMeshComponent         DECLARE_UINT64(0x0000020000000000)#define CASTCLASS_USkeletalMeshComponent        DECLARE_UINT64(0x0000040000000000)#define CASTCLASS_UBlueprint                    DECLARE_UINT64(0x0000080000000000)#define CASTCLASS_UDelegateFunction             DECLARE_UINT64(0x0000100000000000)#define CASTCLASS_UStaticMeshComponent          DECLARE_UINT64(0x0000200000000000)#define CASTCLASS_UMapProperty                  DECLARE_UINT64(0x0000400000000000)#define CASTCLASS_AllFlags                      DECLARE_UINT64(0xFFFFFFFFFFFFFFFF)

我们新建的类，继承自这些基类，自然也会获得一个ClassCastFlags，用来控制转化关系。
比如我们声明一个继承自APawn的类，然后再Tick中访问UClass的ClassCastFlags字段，得到的值是343597383680，转化为2进制就是0x0000005000000000，比define的CASTCLASS_APawn刚好大一，相与不为0，可以转化。
到这里就比较清楚了，Cast函数内部调用了一个叫DoCast的函数。而DoCast调用了我们上边看到的HasAnyCastFlag：

FORCEINLINE static To* DoCast( UObject* Src )    {        return Src && Src->GetClass()->HasAnyCastFlag(TCastFlags<To>::Value) ? (To*)Src : nullptr;    }

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