__weak & __unsafe_unretained的用法以及区别

转载自:   http://www.jianshu.com/p/df0269e0d3c3

• __unsafe_unretained: 不会对对象进行retain,当对象销毁时,会依然指向之前的内存空间(野指针)

• __weak: 不会对对象进行retain,当对象销毁时,会自动指向nil

代码示例

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

  int main(int argc, const char * argv[]) {

      Person *p = [[Person alloc] init];
      Person *p1 = p;

weak1.png

      p = nil;

      //p已经指向nil,但是还能访问name属性,说明对象没有被销毁，默认指针被__strong修饰
      //__strong Person *p = [[Person alloc] init];
      //__strong Person *p1 = p;

      NSLog(@"%@",p1);
}

weak2.png

output: <Person: 0x100200000>

• 当使用__weak时

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

  int main(int argc, const char * argv[]) {

      Person *p = [[Person alloc] init];
      __weak Person *p1 = p
      p = nil;

      //p指向nil,p1为弱指针,p1指向nil
      NSLog(@"%@",p1);
  }

weak3.png

• 当使用__unsafe_unretained时

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

  int main(int argc, const char * argv[]) {

      Person *p = [[Person alloc] init];
      __unsafe_unretained Person *p1 = p;
      p = nil;

      //p指向nil,p1的弱引用指针不会销毁，会继续指向对象的地址,对象已经销毁,此时p1访问的是"僵尸"对象

      NSLog(@"%@",p1);   
  }

weak4.png

访问对象已经销毁
-[Person respondsToSelector:]: message sent to deallocated instance 0x100302560