第一、strong关键字与retain关似，用了它，引用计数自动＋1
如果person定义如下：
@interface Person : NSObject @property(nonatomic,strong)Book *book1; @end
(void)viewDidLoad {
[super viewDidLoad];
@autoreleasepool {
p1=[[Person alloc] init];
Book *book=[[Book alloc] init];
p1.book1=book;
NSLog(@"%@",p1.book1);
book=nil;
}
}
-(void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
NSLog(@"%@",p1.book1);
}
输出结果：
2014-10-14 17:12:36.242 testDDD[9962:129809] <Book: 0x7f97b8e14210> 2014-10-14 17:12:36.243 testDDD[9962:129809] <Book: 0x7f97b8e14210>即使在autorealease中执行了release，但因为retaincount加1，所以book所指的内存并没有被释放
第二、weak
声明为weak的指针，指针指向的地址一旦被释放，这些指针都将被赋值为nil。这样的好处能有效的防止野指针
如果person定义如下：
@interface Person : NSObject @property(nonatomic,weak)Book *book1; @end
(void)viewDidLoad {
[super viewDidLoad];
@autoreleasepool {
p1=[[Person alloc] init];
Book *book=[[Book alloc] init];
p1.book1=book;
NSLog(@"%@",p1.book1);
book=nil;
}
}
-(void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
NSLog(@"%@",p1.book1);
}
输出结果：
2014-10-14 17:21:01.574 testDDD[10081:133743] <Book: 0x7fd0e8e5dc70> 2014-10-14 17:21:01.574 testDDD[10081:133743] (null)第三、unretained且unsafe，由于是unretained所以与weak有点类似，但是它是unsafe的.
@interface Person : NSObject @property(nonatomic,unsafe_unretained)Book *book1; @end
(void)viewDidLoad {
[super viewDidLoad];
@autoreleasepool {
p1=[[Person alloc] init];
Book *book=[[Book alloc] init];
p1.book1=book;
NSLog(@"%@",p1.book1);
book=nil;
}
}
-(void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
NSLog(@"%@",p1.book1);
}
结果为：
ARC中strong、weak、unsafe_unretained的区别
原文：http://blog.csdn.net/richard_rufeng/article/details/40080281