NSString 的copy和retain

stackoverflow上的说法是这样的。http://stackoverflow.com/questions/387959/nsstring-property-copy-or-retain

For attributes whose type is an immutable value class that conforms to the NSCopying protocol, you almost always should specify copy in your @property declaration. Specifying retain is something you almost never want in such a situation.

Here's why you want to do that:

NSMutableString *someName = [NSMutableString stringWithString:@"Chris"]; Person *p = [[[Person alloc] init] autorelease]; p.name = someName; [someName setString:@"Debajit"];

The current value of the Person.name property will be different depending on whether the property is declared retain or copy — it will be @"Debajit" if the property is marked retain, but@"Chris" if the property is marked copy.

Since in almost all cases you want to prevent mutating an object's attributes behind its back, you should mark the properties representing them copy. (And if you write the setter yourself instead of using@synthesize you should remember to actually use copy instead of retain in it.)

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。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

 

 @interface Demo : NSObject
 {
     NSString *retainString;
     NSString *copyString;
 }

 @property (nonatomic, retain)NSString *retainString;
 @property (nonatomic, copy)NSString *copyString;
 @end

 @implementation Demo
 @synthesize retainString;
 @synthesize copyString;
 -(void)dealloc
 {
     [retainString release];
     [copyString release];
     [super dealloc];
 }

 @end

 Demo *o = [[Demo alloc] init];
 NSMutableString *s1 = [[NSMutableString alloc] initWithCapacity:100];
 [s1 setString:@"changeme"];
 o.retainString = s1;
 o.copyString = s1;
 NSLog(@"retain string is %@", o.retainString);
 NSLog(@"copy string is %@", o.copyString);
 [s1 setString:@"whetherchanged?"];
 NSLog(@"retain string is %@", o.retainString);
 NSLog(@"copy string is %@", o.copyString);

 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。

 

1。对NSString应用retain，效率无疑是最好的

2。用copy最安全，因为NSString 为 NSMutableString 的基类，如果将NSMutableString 以retain的形式赋值给NSString后，后续修改NSMutableString会导致NSString内容的变化，这通常不是我们希望的，所以用copy最安全。

3。到底用哪个？貌似还是用copy，因为copy并不一定导致一个新对对象创建，而牺牲效率。copy会调用NSCopying中的 -(id)copyWithZone:(NSZone *)，我们可以判断下self是NSString还是NSMutableString，如果是NSString，就地[self  retain]，[return self]。否则老老实实拷贝对象赋值，这样可以实现效率和安全的结合，实验的结果也是如此。

from:http://bukkake.iteye.com/blog/954259