Question

Which is better?

static unsigned unitFlags;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit;
});

OR is that just as efficient as writing

unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit;

Or is there some other patterns more efficient? Would a define be better for ‘unitFlags’?

Answer 1

You should just do:

static unsigned unitFlags = NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit;

You only need to do the dispatch_once trick if the expression on the right isn’t a valid compile-time constant. But this is a valid compile-time constant, so you don’t have to go through that slight of hand.

But the static expression is generally better than a #define (it’s typed, and for some complicated expressions, it can be more efficient). Stick with the static, in my opinion.