__strong, __ weak and __unsafe_unretained difference

__strong

The default behavior of the arc environments, objects retain.

For example

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    
    NSObject *obj = [[NSObject alloc] init];
    //默认指针被__strong修饰,等价于:__strong NSObject *p = obj;
    NSObject *p = obj;
    
    obj = nil;
    NSLog(@"obj = %@", p);
    
}
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Print results

 obj = <NSObject: 0x100706b90>
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__weak

Will not object to retain, when the object is destroyed, it will automatically point to nil

For example

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    
    NSObject *obj = [[NSObject alloc] init];
    __weak NSObject *p = obj;
    obj = nil;
    NSLog(@"obj = %@", p);
    
}
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Print results

obj = (null)
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__unsafe_unretained

Will not object to retain, when the object is destroyed, it will still point to the previous memory space (dangling pointers)

For example

#import <Foundation/Foundation.h>

@interface Person : NSObject

@end

@implementation Person

- (void)dealloc {
    NSLog(@"--dealloc--");
}

@end

int main(int argc, const char * argv[]) {
  
    Person *obj = [[Person alloc] init];
    __unsafe_unretained Person *p = obj;
    obj = nil;
    
    NSLog(@"obj = %@", p);
}
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Print results

--dealloc--
obj = <Person: 0x101c5e9f0>
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Object was released, but if memory is not reused, it is still able to print out the Person object; this time to visit is the "zombie" objects.

Reproduced in: https: //juejin.im/post/5cf4ae26f265da1bc07e2352