id __unsafe_unretained obj1 = nil;
id obj0 = [[NSObject alloc]init];
obj1 = obj0;
NSLog(@"111%@",obj1);
id obj2 = [[NSObject alloc]init];
obj0 = obj2;
NSLog(@"222%@",obj1);
id __unsafe_unretained obj1 = nil;
阅读背景：
__unsafe_unretained_weixin_34348805的博客
来源：互联网
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