相关疑难解决方法(0)

这是示例代码:

NSDictionary *myDictionary = [NSDictionary dictionary];
NSNumber *myNumber = [myDictionary valueForKey: @"MyNumber"];
NSLog(@"myNumber = %@", myNumber); // output myNumber = (null)

if (myNumber == nil)
    NSLog(@"test 1 myNumber == nil");

if (myNumber == NULL)
    NSLog(@"test 2 myNumber == NULL");

if ([myNumber isEqual:[NSNull null]])
    NSLog(@"test 3 myNumber == [NSNull null]");

我什么时候应该使用nil,NULL和[NSNull null]？

我正在创建一个像这样的NSString,我之后是否应该发布它？

CFUUIDRef   uuidObj = CFUUIDCreate(nil);
        NSString *device = (NSString*)CFUUIDCreateString(nil, uuidObj);
        CFRelease(uuidObj);

谢谢!