https://www.jianshu.com/p/df0269e0d3c3

//    __unsafe_unretained NSObject *boy1 = nil;
//    {
//        NSObject *boy2 = [[NSObject alloc] init];
//        boy1 = boy2;
//        NSLog(@"boy1 is %@", boy1);
//        NSLog(@"boy2 is %@", boy2);
//    }
//    NSLog(@"boy1 is %@", boy1);
//
    
    __weak NSObject *boy1 = nil;
    {
        NSObject *boy2 = [[NSObject alloc] init];
        boy1 = boy2;
        NSLog(@"boy1 is %@", boy1);
        NSLog(@"boy2 is %@", boy2);
    }
    NSLog(@"boy1 is %@", boy1);

__weak 在引用计数为0时，会指向nil。但时是有性能开销的。
__unsafe_unretained 在引用计数为0时，会指向原地址。只要访问该地址，就是野指针访问，会闪退。