__unsafe_unretained：不會(huì)對對象進(jìn)行retain亡鼠，當(dāng)對象銷毀時(shí)涤垫，會(huì)依然指向之前的內(nèi)存空間（野指針）
__weak：不會(huì)對對象進(jìn)行retain，當(dāng)對象銷毀時(shí)，會(huì)自動(dòng)指向nil
代碼示例
#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已經(jīng)指向nil,但是還能訪問name屬性,說明對象沒有被銷毀溯街，默認(rèn)指針被__strong修飾
//__strong Person *p = [[Person alloc] init];
//__strong Person *p1 = p;
NSLog(@"%@",p1);
}
weak2.png
output: <Person: 0x100200000>
- 當(dāng)使用__weak時(shí)
#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為弱指針,p1指向nil
NSLog(@"%@",p1);
}
weak3.png
- 當(dāng)使用__unsafe_unretained時(shí)
#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的弱引用指針不會(huì)銷毀，會(huì)繼續(xù)指向?qū)ο蟮牡刂?對象已經(jīng)銷毀,此時(shí)p1訪問的是"僵尸"對象
NSLog(@"%@",p1);
}
weak4.png
訪問對象已經(jīng)銷毀
-[Person respondsToSelector:]: message sent to deallocated instance 0x100302560
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