ARC中的strong 与 weak

#import <Foundation/Foundation.h>
@class Dog;
@interface Person : NSObject
{
    __strong Dog *_dog; //__weak Dog *_dog;
}

- (Dog *)dog;
- (void)setDog:(Dog *)dog;
@end

#import <Foundation/Foundation.h>
@interface Dog : NSObject
@end

#import <Foundation/Foundation.h>
#import "Dog.h"
#import "Person.h"

int main(int argc, const char * argv[]) {
    Person *p = [[Person alloc] init];
    Dog *dog = [[Dog alloc] init];
    p.dog = dog;
    dog = nil;
    NSLog(@"%@",p.dog);
    return 0;
}