NSString为何要用copy？而不是strong？

stackOverflow上说法是为了防止mutable string被无意中修改, NSMutableString是NSString的子类, 因此NSString指针可以持有NSMutableString对象.

用代码解释就是

@property (nonatomic, strong) NSMutableString *aa;

@property (nonatomic, strong) NSMutableString *bb;

@property (nonatomic, copy) NSString *cc;

@property (nonatomic, strong) NSString *dd;

self.aa = [NSMutableStringstringWithFormat:@"11"];;

    self.bb = [NSMutableStringstringWithFormat:@"22"];;

    

    self.cc =self.aa;

    self.dd =self.bb;

    

    [self.aaappendString:@"33"];

    [self.bbappendString:@"44"];

    

    DLOG(@"cc:%@",self.cc);//11

    DLOG(@"dd:%@",self.dd);//2244