海神

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。

@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"fuckyou"];
o.retainString =  s1;
o.copyString  = s1;
NSLog(@"retain  string  is %@",  o.retainString);
NSLog(@"copy string is  %@", o.copyString);
[s1 setString"fuckme"];
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。