@implementation Dog(void)dealloc{ NSLog(@"Dog 被销毁了"); [super dealloc];}@end@class Dog;@interface Person : NSObject@property(retain) Dog *dog;@endint main(int argc, const char * argv[]) { Person *p1 = [[Person alloc] init]; Person *p2 = [[Person alloc] init]; Dog *d = [[Dog alloc] init]; p1.dog = d; p2.dog = d; [d release]; p1.dog = nil; p2.dog = nil; return 0;}//Dog *d被销毁了int main(int argc, const char * argv[]) { Person *p1 = [[Person alloc] init]; Person *p2 = [[Person alloc] init]; Dog *d = [[Dog alloc] init]; p1.dog = d; p2.dog = p1.dog; // 只改了这一行 [d release]; p1.dog = nil; p2.dog = nil; return 0;}//Dog *d没有被销毁dog属性公告为了atomic类型，为了保证原子性，dog的set和get方法应该做过解决。我感觉mrc下的atomic属性的set和get方法应该如下
- (Dog *) dog { Dog *d = nil; @synchronized(self) { d = [[_dog retain] autorelease]; } return d;}- (void)setDog:(Dog *)dog { @synchronized(self) { [dog retain]; [_dog release]; _dog = dog; }}所以在调用p2.dog = p1.dog；的时候会先调用p1.dog的get方法，内部会对dog retain一次，引用计数+1，而后放进自动释放池，再调用p2.dog的set方法，又会retain一次。最后release的时候-1，引用计数最后依然是1，所以不会被释放
