关于OC对象属性中的NSString类型为什么用copy修饰

用copy修饰的区别，发生在用NSMutableString类型给NSString类型赋值时，为了防止赋值的属性内容被无意中修改，所以用copy修饰。

#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，当用NSMutableString类型mString的对象给NSString类型str的对象赋值时，在mString改变后，用copy修饰的NSString对象strCopy的值不变，而用strong修饰的NSString对象strStrong的值变化了。
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关于OC对象属性中的NSString类型为什么用copy修饰