位运算做标记的好处

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很多底层的东西都喜欢用 对1位移N位来做枚举或标记，是因为这样可以同时保存多个状态进这个标记

  • 以这个枚举为例（虚幻源码中垃圾回收对象的标记枚举，这里把位移n位改为了1,2,3,4…）

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    enum class EInternalObjectFlags : int32
    {
    	None = 0,
    	// All the other bits are reserved, DO NOT ADD NEW FLAGS HERE!
    	ReachableInCluster = 1 << 1, /// External reference to object in cluster exists
    	ClusterRoot = 1 << 2, ///< Root of a cluster
    	Native = 1 << 3, ///< Native (UClass only).
    	Async = 1 << 4, ///< Object exists only on a different thread than the game thread.
    	AsyncLoading = 1 << 5, ///< Object is being asynchronously loaded.
    	Unreachable = 1 << 6, ///< Object is not reachable on the object graph.
    	PendingKill = 1 << 7, ///< Objects that are pending destruction (invalid for gameplay but valid objects)
    	RootSet = 1 << 8, ///< Object will not be garbage collected, even if unreferenced.
    	NoStrongReference = 1 << 9, 
    };

  • 初始化个变量

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    EInternalObjectFlags Flags = EInternalObjectFlags::Native; //也就是Flags = 00010000

  • 假设有这几个接口

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    FORCEINLINE void SetPendingKill()
    {
    	Flags |= int32(EInternalObjectFlags::PendingKill);
    }
    FORCEINLINE void ClearPendingKill()
    {
    	Flags &= ~int32(EInternalObjectFlags::PendingKill);
    }
    FORCEINLINE bool IsPendingKill() const
    {
    	return !!(Flags & int32(EInternalObjectFlags::PendingKill));
    }

    FORCEINLINE void SetRootSet()
    {
    	Flags |= int32(EInternalObjectFlags::RootSet);
    }
    FORCEINLINE void ClearRootSet()
    {
    	Flags &= ~int32(EInternalObjectFlags::RootSet);
    }
    FORCEINLINE bool IsRootSet() const
    {
    	return !!(Flags & int32(EInternalObjectFlags::RootSet));
    }

  • 状态改变过程

    1. SetPendingKill， Flags = 10010000，加入了PendingKill状态，此时有PendingKill和Native两种状态
    2. IsPendingKill， ret = 10010000 & 10000000 = 10000000 = true，判断是否有 PendingKill 状态
    3. ClearPendingKill, Flags = 10010000 & 0111111 = 00010000，清除了PendingKill状态，此时只有 NativeNative 状态
    4. 其他以此类推

  • 作用场景

    • 用于同一个标记Flag中需要保存多种状态，否者用普通的枚举1,2,3…就可以。