用copy修飾的區(qū)別，發(fā)生在用NSMutableString類型給NSString類型賦值時，為了防止賦值的屬性內(nèi)容被無意中修改，所以用copy修飾。

#import <Foundation/Foundation.h>  
  
@interface CopyStr : NSObject  
  
@property (nonatomic, copy) NSString *strCopy;  
  
@property (nonatomic, strong) NSString *strStrong;  
  
- (void)testStr;  
  
@end  

#import "CopyStr.h"  
  
@implementation CopyStr  
  
- (void)testStr  
{  
    NSMutableString *mString = [NSMutableString string];  
      
    [mString setString:@"original"];  
      
    self.strCopy = mString;  
      
    self.strStrong = mString;  
      
    NSLog(@"strCopy = %@", self.strCopy); // strCopy = original  
    NSLog(@"strStrong = %@", self.strStrong); // strCopy = original  
      
    [mString setString:@"changed"];  
      
    NSLog(@"strCopy = %@", self.strCopy); // strCopy = original  
    NSLog(@"strStrong = %@", self.strStrong); // strCopy = changed  
}  

#import <Foundation/Foundation.h>  
#import "CopyStr.h"  
  
int main(int argc, const charchar * argv[]) {  
    @autoreleasepool {  
          
        CopyStr *str = [[CopyStr alloc] init];  
          
        [str testStr];  
    }  
    return 0;  
}  

可以看出，當用NSMutableString類型mString的對象給NSString類型str的對象賦值時，在mString改變后，用copy修飾的NSString對象strCopy的值不變，而用strong修飾的NSString對象strStrong的值變化了。
轉(zhuǎn)自：
關于OC對象屬性中的NSString類型為什么用copy修飾