Threadsafe NSDateFormatter's instance cache

NSDateFormatterInstanceCache.mm

/**
 As Apple said [NSDateFormatter init] is very expensive (https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html#//apple_ref/doc/uid/TP40002369-SW10)
 In Apple's code is used a static const, but since NSDateFormatter 
 isn't thread safe a better approach is to use Thread local store (http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/CreatingThreads.html#//apple_ref/doc/uid/10000057i-CH15-SW4)
 to cache the NSDateFormatter instance
 */

NSString * const kCachedDateFormatterKey = @"CachedDateFormatterKey";

+ (NSDateFormatter *)dateFormatter
{
    NSMutableDictionary *threadDictionary = [[NSThread currentThread] threadDictionary];
    NSDateFormatter *dateFormatter = [threadDictionary objectForKey:kCachedDateFormatterKey];
    if (!dateFormatter) {
        dateFormatter = [[NSDateFormatter alloc] init];
        NSLocale *enUSPOSIXLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
        
        [dateFormatter setLocale:enUSPOSIXLocale];
        [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSSS"];
        
        [threadDictionary setObject:dateFormatter forKey:kCachedDateFormatterKey];
    }
    return dateFormatter;
}

_Thread_local/thread_local (depending on your compiler environment) static variables may be an acceptable alternative to the thread dictionary; they can clarify the logic quite pleasantly:

static _Thread_local NSDateFormatter *formatter;
if (!formatter) { formatter = …; }
return formatter;

@robrix didn't know about that. Thanks for the tips!