NSString 应用 retain 还是 copy
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。
1 NSString* strOrgin = [NSString stringWithFormat:@"curr time is %@", [NSDate date]]; 2 3 NSLog(@"strOrgin = %p, retainCount = %d", strOrgin, [strOrgin retainCount]); 4 5 NSString* strCopy = [strOrgin copy]; 6 7 NSLog(@"strCopy = %p, retainCount=%d", strCopy, [strCopy retainCount]); 8 9 NSLog(@"strOrgin = %p, retainCount = %d", strOrgin, [strOrgin retainCount]);
-------------------------------------------out put-------------------------------
1 Test[50250:707] strOrgin = 0x1e52e4d0, retainCount = 1 2 3 Test[50250:707] strCopy = 0x1e52e4d0, retainCount=2 4 5 Test[50250:707] strOrgin = 0x1e52e4d0, retainCount = 2
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文档貌似对此也隐约有所说明。
所有用copy就ok了，其他有可变派生类的对象情况类似，但应用的时候根据具体需求就好了。
参考文档 NSCopying Protocol Reference
Your options for implementing this protocol are as follows:
- Implement NSCopying using alloc and init... in classes that don’t inherit copyWithZone:.
- Implement NSCopying by invoking the superclass’s copyWithZone: when NSCopying behavior is inherited. If the superclass implementation might use the NSCopyObject function, make explicit assignments to pointer instance variables for retained objects.
- Implement NSCopying by retaining the original instead of creating a new copy when the class and its contents are immutable.
