这个声明是什么意思？

Question

我不是C/C++的专家.

我今天发现了这个宣言:

typedef NS_OPTIONS(NSUInteger, PKRevealControllerType)
{
    PKRevealControllerTypeNone  = 0,
    PKRevealControllerTypeLeft  = 1 << 0,
    PKRevealControllerTypeRight = 1 << 1,
    PKRevealControllerTypeBoth = (PKRevealControllerTypeLeft | PKRevealControllerTypeRight)
};

你们能翻译每个价值会有什么价值吗？

Answer 1

opertor <<是 按位左移运算符.将所有位移到指定的次数:(算术左移和保留符号位)

m << n

将所有位m向左移位n多次.(注意一个班次==乘以2).

1 << 0意味着没有转变所以1它只等于.

1 << 1意味着一个班次,所以1*2它只等于= 2.

我用一个字节解释:一个字节中的一个像是:

 MSB
+----+----+----+---+---+---+---+---+
|  0 |  0 | 0  | 0 | 0 | 0 | 0 | 1 |       1   
+----+----+----+---+---+---+---+---+
  7     6   5    4   3   2   1 / 0  
  |                           /           1 << 1
  |                          | 
  ?                          ?
+----+----+----+---+---+---+---+---+
|  0 |  0 | 0  | 0 | 0 | 0 | 1 | 0 |       2      
+----+----+----+---+---+---+---+---+ 
   7    6   5    4   3   2   1   0

虽然1 << 0它只是图1之一.(注意第7位被复制以保存标志)

OR运算符:有点明智或

 MSB                            PKRevealControllerTypeLeft
+----+----+----+---+---+---+---+---+
|  0 |  0 | 0  | 0 | 0 | 0 | 0 | 1 |  == 1
+----+----+----+---+---+---+---+---+
   7    6   5    4   3   2   1   0
   |    |    |   |   |   |   |   |      OR
 MSB                               PKRevealControllerTypeRight
+----+----+----+---+---+---+---+---+
|  0 |  0 | 0  | 0 | 0 | 0 | 1 | 0 |   == 2
+----+----+----+---+---+---+---+---+
   7    6   5    4   3   2   1   0

 = 

 MSB                    PKRevealControllerTypeBoth
+----+----+----+---+---+---+---+---+
|  0 |  0 | 0  | 0 | 0 | 0 | 1 | 1 |   == 3
+----+----+----+---+---+---+---+---+
   7    6   5    4   3   2   1   0  

|是有点明智的运营商.在下面的代码中它or 1 | 2==3

PKRevealControllerTypeNone  = 0,             //  is Zero
PKRevealControllerTypeLeft  = 1 << 0,        //  one 
PKRevealControllerTypeRight = 1 << 1,        //  two
PKRevealControllerTypeBoth = (PKRevealControllerTypeLeft |    
                             PKRevealControllerTypeRight)  // three

没有更多的技术理由来初始化像这样的值,这样的定义可以很好地解读这个答案:定义SOMETHING(1 << 0)

编译器优化将它们转换为更简单的类似:( 我不确定第三个,但我认为编译器也会优化它)

PKRevealControllerTypeNone  = 0,     //  is Zero
PKRevealControllerTypeLeft  = 1,     //  one 
PKRevealControllerTypeRight = 2,     //  two
PKRevealControllerTypeBoth  = 3,     // Three

编辑: @thanks to Till.阅读此答案带有BOOL标志的应用程序状态显示了使用位运算符获得的声明的有用性.