How do those bitmasks actually work?

Question

For example, this method from NSCalendar takes a bitmask:

- (NSDate *)dateByAddingComponents:(NSDateComponents *)comps toDate:(NSDate *)date options:(NSUInteger)opts 

So options can be like:

NSUInteger options = kCFCalendarUnitYear; 

or like:

NSUInteger options = kCFCalendarUnitYear | kCFCalendarUnitMonth | kCFCalendarUnitDay; 

What I don't get is, how is this actually done? I mean: How can they pull out those values which are merged into options? If I wanted to program something like this, that can take a bitmask, how would that look?

Answer 1

Bitmasks are pretty basic really. You can think of it like this (C# until somebody can convert):

public enum CalendarUnits {     kCFCalendarUnitDay = 1, // 001 in binary     kCFCalendarUnitMonth = 2, // 010 in binary     kCFCalendarUnitYear = 4, // 100 in binary } 

You can then use the bitwise operators to combine the values:

// The following code will do the following // 001 or 100 = 101 // So the value of options should be 5 NSUInteger options = kCFCalendarUnitDay | kCFCalendarUnitYear; 

This technique is also often used in security routines:

public enum Priveledges {     User = 1,     SuperUser = 2,     Admin = 4 }  // SuperUsers and Admins can Modify // So this is set to 6 (110 binary) public int modifySecurityLevel = SuperUser | Admin; 

Then to check the security level, you can use the bitwise and to see if you have sufficient permission:

public int userLevel = 1; public int adminLevel = 4;  // 001 and 110 = 000 so this user doesn't have security if(modifySecurityLevel & userLevel == userLevel)  // but 100 and 110 = 100 so this user does if(modifySecurityLevel & adminLevel == adminLevel)     // Allow the action