arc中的strong相當(dāng)于mrc的retain，這一點想必大家都知道，那么NSString類型屬性到底應(yīng)該用copy還是stong修飾呢？直接上代碼說明。

@interface Person : NSObject

@property(copy) NSString *copyname;
@property(strong) NSString *strongname;

@end

NSString *name = @"Tom";
NSMutableString *mutableName = [[NSMutableString alloc] initWithString:@"Jim"];

Person *p =[[Person alloc] init];
p.copyname = name;
p.strongname = name;

NSLog(@"%p, %p, %p",p.copyname,p.strongname,name);
//打印結(jié)果:0x100001068, 0x100001068, 0x100001068，結(jié)論:copyname和strongname都是淺拷貝name

p.copyname = mutableName;
p.strongname = mutableName;
NSLog(@"%p, %p, %p",p.copyname,p.strongname,mutableName);
//打印結(jié)果:0x6d694a35, 0x100302140, 0x100302140,結(jié)論:copyname深拷貝mutableName, strongname淺拷貝mutableName

[mutableName appendString:@" Green"];
NSLog(@"%@, %@, %@",p.copyname,p.strongname,mutableName);
//打印結(jié)果:Jim, Jim Green, Jim Green,結(jié)論:copyname未隨mutableName改變而改變，而strongname則發(fā)生了改變

總結(jié)： 對于NSString類型屬性而言，如果賦值對象是NSSting，則copy和stong都為淺拷貝，如果賦值對象是NSMutableString，則copy是深拷貝，strong是淺拷貝,所以為了防止賦值對象改變導(dǎo)致屬性發(fā)生改變，我們應(yīng)該用copy修飾NSString類型屬性。