用copy修飾的區別，發生在用NSMutableString型別給NSString型別賦值時，為了防止賦值的屬性內容被無意中修改，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 char * argv[]) {
    @autoreleasepool {
        
        CopyStr *str = [[CopyStr alloc] init];
        
        [str testStr];
    }
    return 0;
}