__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
-- ::34.480 AutoRelease[:60b] str addr is 0x8f32890
-- ::34.480 AutoRelease[:60b] reference addr is 0x8f32890
-- ::34.480 AutoRelease[:60b] reference addr is 0x0
-- ::34.481 AutoRelease[:60b] view will appear reference :(null)
-- ::34.486 AutoRelease[: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
-- ::20.872 AutoRelease[:60b] str addr is 0x8d42dd0
-- ::20.872 AutoRelease[:60b] reference addr is 0x8d42dd0
-- ::20.873 AutoRelease[:60b] reference addr is 0x8d42dd0
-- ::20.873 AutoRelease[:60b] view will appear reference :xxxxx
-- ::20.877 AutoRelease[:60b] viewDidAppear reference: xxxxx