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__weak 和 __strong 还有Autorelease的用法
使用容器的block版本的枚举器时，内部会自动添加一个AutoreleasePool：
Autorelease对象是在当前的runloop迭代结束时释放的，而它能够释放的原因是系统在每个runloop迭代中都加入了自动释放池Push和Pop
当然，在普通for循环和for in循环中没有，所以，还是新版的block版本枚举器更加方便。for循环中遍历产生大量autorelease变量时，就需要手加局部AutoreleasePool咯。
[array enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) { // 这里被一个局部@autoreleasepool包围着 }];
weak 和 strong 是iOS用于释放对象的一种机制，weak声明的对象，在原指针被释放之后，也会被会释放变成nil；
下面给出__weak和带__strong的代码和输出
@implementation AutoReleaseViewController__weak id reference;- (void)viewDidLoad{ [super viewDidLoad]; @autoreleasepool { NSString * str = [NSString stringWithFormat:@"xxxxx"]; reference = str; NSLog(@"str addr is %p",str); NSLog(@"reference addr is %p",reference); }}-(void)viewWillAppear:(BOOL)animated{ [super viewWillAppear:animated]; NSLog(@"reference addr is %p",reference); NSLog(@"view will appear reference :%@", reference);}-(void)viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; NSLog(@"viewDidAppear reference: %@",reference);}@end//输出2014-11-01 19:40:34.480 AutoRelease[4506:60b] str addr is 0x8f328902014-11-01 19:40:34.480 AutoRelease[4506:60b] reference addr is 0x8f328902014-11-01 19:40:34.480 AutoRelease[4506:60b] reference addr is 0x02014-11-01 19:40:34.481 AutoRelease[4506:60b] view will appear reference :(null)2014-11-01 19:40:34.486 AutoRelease[4506:60b] viewDidAppear reference: (null)
@implementation AutoReleaseViewController__strong id reference;- (void)viewDidLoad{ [super viewDidLoad]; @autoreleasepool { NSString * str = [NSString stringWithFormat:@"xxxxx"]; reference = str; NSLog(@"str addr is %p",str); NSLog(@"reference addr is %p",reference); }}-(void)viewWillAppear:(BOOL)animated{ [super viewWillAppear:animated]; NSLog(@"reference addr is %p",reference); NSLog(@"view will appear reference :%@", reference);}-(void)viewDidAppear:(BOOL)animated{ [super viewDidAppear:animated]; NSLog(@"viewDidAppear reference: %@",reference);}@end//输出2014-11-01 19:41:20.872 AutoRelease[4669:60b] str addr is 0x8d42dd02014-11-01 19:41:20.872 AutoRelease[4669:60b] reference addr is 0x8d42dd02014-11-01 19:41:20.873 AutoRelease[4669:60b] reference addr is 0x8d42dd02014-11-01 19:41:20.873 AutoRelease[4669:60b] view will appear reference :xxxxx2014-11-01 19:41:20.877 AutoRelease[4669:60b] viewDidAppear reference: xxxxx
__weak 和 __strong 还有Autorelease的用法
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