What are some kind of instance that should be initialized?

Objective-C Programming (2nd Edition) 14. More Messages
1

I’m confused that some instance would be initialized, like this:
NSDate *new = [[NSDate alloc] init];

But some others are not, like this:

NSCalendar *new = [NSCalendar currentCalendar];

What is the difference between them?

2

The difference is that the latter invokes the NSCalendar class method currentCalendar. The code for the class method probably looks something like this:

[code]
…
static NSCalendar * __currentCalendar
static dispatch_once_t __currentCalendarOnce
…

  • (NSCalendar *)currentCalendar
    {
    // avoid race conditions
    dispatch_once (&__currentCalendarOnce, ^ {
    __currentCalendar = [[self alloc] initWith…];
    });

    return __currentCalendar;
    }[/code]

3

Very good answer, and thank you very much~!

[quote=“ibex10”][quote]
The difference is that the latter invokes the NSCalendar class method currentCalendar. The code for the class method probably looks something like this:

[code]
…
static NSCalendar * __currentCalendar
static dispatch_once_t __currentCalendarOnce
…

  • (NSCalendar *)currentCalendar
    {
    // avoid race conditions
    dispatch_once (&__currentCalendarOnce, ^ {
    __currentCalendar = [[self alloc] initWith…];
    });

    return __currentCalendar;
    }[/code][/quote][/quote]