一個(gè)經(jīng)典的面試題，在viewDidLoad創(chuàng)建的string，什么時(shí)候釋放？

__weak NSString *string_weak_ = nil;
__weak NSObject *object_weak_ = nil;

- (void)viewDidLoad
{
    [super viewDidLoad];

//   NSString *sd = @"1234567";                                   //(NSCFConstantString:0x0000000109278150)
//   NSString *sd = [NSString stringWithFormat:@"123456789"];  //(NSTaggedPointerString:0xa1ea1f72bb30ab19)
    NSString *sd = [NSString stringWithFormat:@"123456789abcdefg"];       //(NSCFString:0x000061000005ea50)
//  NSString *sd = [@"sdf" stringByAppendingString:@"adsdsfds"];          //(NSCFString:0x00006000000427c0)
    string_weak_ = sd;
}

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    NSLog(@"viewWillAppear %@", string_weak_);
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    NSLog(@"viewDidAppear %@", string_weak_);
}

NSCFConstantString靜態(tài)存儲(chǔ)區(qū)，不會(huì)釋放
NSTaggedPointerString 內(nèi)容存儲(chǔ)在指針內(nèi)，不會(huì)釋放
第一種的NSCFString，由于stringWithFormat，加入自動(dòng)釋放池，等到下一個(gè)runloop釋放
第二種的NSCFString和object一樣，會(huì)自動(dòng)插入release，viewDidLoad結(jié)束釋放