Question

I saw this in NSArray.h header file in the framework directory:

enum {
    NSBinarySearchingFirstEqual = (1UL << 8),
    NSBinarySearchingLastEqual = (1UL << 9),
    NSBinarySearchingInsertionIndex = (1UL << 10),
};
typedef NSUInteger NSBinarySearchingOptions;

What’s the point of “NSBinarySearchingFirstEqual = (1UL << 8)”?And what’s the relationship between this enumeration and the “NSBinarySearchingOptions” type-definition? Thanks.

Answer 1

The “NSBinarySearchingFirstEqual = (1UL << 8)” etc. assign specific values to the enumeration constants. The values are chosen so that they are represented by a single bit, allowing the options to be combined with bitwise operations. The “<<” operator is a left shift; you could equivalently write this as:

enum {
    NSBinarySearchingFirstEqual = 256,
    NSBinarySearchingLastEqual = 512,
    NSBinarySearchingInsertionIndex = 1024,
};

Options can be combined like:

NSBinarySearchingOptions opt = NSBinarySearchingFirstEqual | NSBinarySearchingLastEqual;

Note that NSBinarySearchingOptions is typedef’d to an unsigned integer, not the enum, because it can contain values that are not one of the defined enum values (when multiple values are combined).