转自 http://bukkake.iteye.com/blog/954259
规范上NSString做属性都是写成copy的，理论上应该是复制了字符串而不是单纯的增加引用计数，其实问题只会出现在把NSMutableString赋值给NSString的时候。
View Code
1 @interface Demo : NSObject
2 {
3 NSString *retainString;
4 NSString *copyString;
5 }
6
7 @property (nonatomic, retain)NSString *retainString;
8 @property (nonatomic, copy)NSString *copyString;
9 @end
10
11 @implementation Demo
12 @synthesize retainString;
13 @synthesize copyString;
14 -(void)dealloc
15 {
16 [retainString release];
17 [copyString release];
18 [super dealloc];
19 }
20
21 @end
22
23 Demo *o = [[Demo alloc] init];
24 NSMutableString *s1 = [[NSMutableString alloc] initWithCapacity:100];
25 [s1 setString:@"fuckyou"];
26 o.retainString = s1;
27 o.copyString = s1;
28 NSLog(@"retain string is %@", o.retainString);
29 NSLog(@"copy string is %@", o.copyString);
30 [s1 setString:@"fuckme"];
31 NSLog(@"retain string is %@", o.retainString);
32 NSLog(@"copy string is %@"
2 {
3 NSString *retainString;
4 NSString *copyString;
5 }
6
7 @property (nonatomic, retain)NSString *retainString;
8 @property (nonatomic, copy)NSString *copyString;
9 @end
10
11 @implementation Demo
12 @synthesize retainString;
13 @synthesize copyString;
14 -(void)dealloc
15 {
16 [retainString release];
17 [copyString release];
18 [super dealloc];
19 }
20
21 @end
22
23 Demo *o = [[Demo alloc] init];
24 NSMutableString *s1 = [[NSMutableString alloc] initWithCapacity:100];
25 [s1 setString:@"fuckyou"];
26 o.retainString = s1;
27 o.copyString = s1;
28 NSLog(@"retain string is %@", o.retainString);
29 NSLog(@"copy string is %@", o.copyString);
30 [s1 setString:@"fuckme"];
31 NSLog(@"retain string is %@", o.retainString);
32 NSLog(@"copy string is %@"
这样就可以看出，当使用retain方式的时候，NSMutableString的内容变化时，语义上应该不可变的NSString也变化了，而用copy则是始终保持赋值时的内容。
如果对实际类型就是NSString的对象用了copy，那其实就是retain，你可以通过观察引用计数来发现，而且就语义上来说也完全没有问题,同时也避免了不需要的字符串拷贝的消耗.