Question

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

Answer 1

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.