解释目标C保留循环与现实世界的例子？

我正在阅读保留周期,"保留周期可以采取一些形式,但它通常意味着对象A保留对象B,对象B保留对象A,但没有其他任何东西保留对象A或B".但我不清楚这些概念.请有人解释保留周期与现实世界的例子.

谢谢.

一个简单的例子,一个人住在一个​​部门,一个部门有一个人(假设有一个人)

@class Department;

@interface Person:NSObject
@property (strong,nonatomic)Department * department;
@end

@implementation Person
-(void)dealloc{
    NSLog(@"dealloc person");
}

@end
@interface Department: NSObject
@property (strong,nonatomic)Person * person;
@end

@implementation Department
-(void)dealloc{
    NSLog(@"dealloc Department");
}
@end

然后这样称呼它

- (void)viewDidLoad {
    [super viewDidLoad];
    Person * person = [[Person alloc] init];
    Department * department = [[Department alloc] init];
    person.department = department;
    department.person = person;
}

你不会看到dealloc日志,这是保留圈