UE4的Cast⽅法与类型转化
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UObjectBa的，UObjectBa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)aplus
#define CASTCLASS_UClass DECLARE_UINT64(0x0000000000000020)
#define CASTCLASS_UByteProperty DECLARE_UINT64(0x0000000000000040)
#define CASTCLASS_UIntProperty DECLARE_UINT64(0x0000000000000080)
shinco#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)模范英语
#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)
化妆都需要什么spit#define CASTCLASS_UInt64Property DECLARE_UINT64(0x0000000000400000)
#define CASTCLASS_UDelegateProperty DECLARE_UINT64(0x0000000000800000)aidu
#define CASTCLASS_UNumericProperty DECLARE_UINT64(0x0000000001000000)
riz
#define CASTCLASS_UMulticastDelegateProperty DECLARE_UINT64(0x0000000002000000)
#define CASTCLASS_UObjectPropertyBa DECLARE_UINT64(0x0000000004000000)
#define CASTCLASS_UWeakObjectProperty DECLARE_UINT64(0x0000000008000000)
#define CASTCLASS_ULazyObjectProperty DECLARE_UINT64(0x0000000010000000)
#define CASTCLASS_UAstObjectProperty DECLARE_UINT64(0x0000000020000000)
#define CASTCLASS_UTextProperty DECLARE_UINT64(0x0000000040000000)
amico#define CASTCLASS_UInt16Property DECLARE_UINT64(0x0000000080000000)
#define CASTCLASS_UDoubleProperty DECLARE_UINT64(0x0000000100000000)
#define CASTCLASS_UAstClassProperty 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)
eblin#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 )
ly{
return Src && Src->GetClass()->HasAnyCastFlag(TCastFlags<To>::Value) ? (To*)Src : nullptr;
}
