1. Something and nothing

    NSValue

    Many Cocoa classes, like NSArray and NSValue only takes objects as parameters. What to do when you have a non-object value you want to store? Simple, use NSValue.

    Let’s say we want to store a [CGRect struct] [1] into an array.

    CGRect rect = CGRectMake(0, 0, 300, 300); 
NSValue *rectValue = [NSValue valueWithBytes:&rect objCType:@encode(CGRect)]; 

NSArray *array = [NSArray arrayWithObject:rectValue]; 

NSLog(@"array: %@", array); /* => array: ( "NSRect: {{0, 0}, {300, 300}}" ) */

    Later, you can retrieve the non-object value with

    CGRect rect2; 
[rectValue getValue:&rect2];

    Observe the use of the address operator (&) here.

    NSNull, nil, NULL

    Sometimes you have plenty, sometime you don’t. In plain vanilla C you have NULL, a special pointer that points to nowhere. nil is just the same, but, by convention, it is used not for primitives, like NULL but for objects. For instance

    someObject = nil;

    Will point someObject ref to a bit bucket.

    nil is also used as the sentinel in the factory methods of NSDictionary, NSArray and NSSet. If you need a null object to pass around, Cocoa provides NSNull

    NSNull *nullValue = [NSNull null]; 
NSArray *array = [NSArray arrayWithObjects:nullValue, nil];

    [1]: http://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSValue_Class/Reference/Reference.html#//apple_ref/occ/clm/NSValue/valueWithRect: “A CGRect is a little contrived, because NSValue offers a special API for that particular structure.

Notes

  1. volonbolon posted this