枚举默认值了解 [英] enum default values understanding

问题描述

我正在使用Objective-C来开发ios应用程序

我在文档中发现枚举具有如下默认值：1 <

我不明白这个默认值
示例：

 枚举{
 UIDataDetectorTypePhoneNumber = 1<< 0，
 UIDataDetectorTypeLink = 1<< 1，
 UIDataDetectorTypeAddress = 1<< 2，
 UIDataDetectorTypeCalendarEvent = 1<< 3，
 UIDataDetectorTypeNone = 0，
 UIDataDetectorTypeAll = NSUIntegerMax 
}; 
  

所以，这个枚举中每个元素的默认值是多少？

谢谢

解决方案

这是一个具有逐位值或位标志的枚举。每个值都是一个二进制值，其中只有一个位设置为1，所有其他值都设置为0.这样，您可以将值存储为一个整数的位数。

左移运算符<是位二进制数左边或最重要一侧的位移。计算一个* 2（第二次）操作就是一样。

例如，在你的问题中发送的枚举中，第一个值UIDataDetectorTypePhoneNumber，是第二个，UIDataDetectorTypeLink是2，第三个是UIDataDetectorTypeAddress，是4。

将这些值组合为标志，以设置相同的不同位整数：

  NSInteger fooIntValue = UIDataDetectorTypePhoneNumber | UIDataDetectorTypeLink; 
  

由于'|'操作是按位的，结果将是一个二进制值... 0011是3.你表明你的变量fooIntValue为两个不同的属性设置了两个标志。

i am using objective-c to develop ios applications

i found in the documentations that enum have default values like this : "1<<0"

i don't understand this default value example:

enum {
    UIDataDetectorTypePhoneNumber   = 1 << 0,
    UIDataDetectorTypeLink          = 1 << 1,
    UIDataDetectorTypeAddress       = 1 << 2,
    UIDataDetectorTypeCalendarEvent = 1 << 3,
    UIDataDetectorTypeNone          = 0,
    UIDataDetectorTypeAll           = NSUIntegerMax
};

so, what is the default value for each element in this enum ?

thanks

解决方案

That is an enum with bitwise values or bit flags. Each value is an binary value in which only one bit is set to 1 and all the others are set to 0. That way you can store in a value as much flags as bits of an integer number has.

The shift left operator '<<' is a displacement of bits to the left or to the most significant side of the binary number. It is the same that calculating a "* 2" (times two) operation.

For example in the enum you have send in your question, the first value, UIDataDetectorTypePhoneNumber, is 1. The second one, UIDataDetectorTypeLink, is 2 and the third one, UIDataDetectorTypeAddress, is 4.

You combine that values as flags to set some different bits in the same integer:

NSInteger fooIntValue = UIDataDetectorTypePhoneNumber | UIDataDetectorTypeLink;

As '|' operation is bitwise, the result will be a binary value ...0011, that is 3. And you are indicating that your variable fooIntValue has two flags set to true for two different properties.

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