I've seen people do something like [NSString stringWithString:@"some string"]. Why not just do @"some string"?
For an example, look at the facebook-ios-sdk.
+[NSString stringWithString:] -- what's the point? is a similar question, but none of the answers address [NSString stringWithString:@"some string"] vs. @"some string".
Actually there is no difference. [NSString stringWithString: str] just does nothing and returns str if str is immutable already.
There's no difference other than the extra key strokes needed. In fact, with a constant string as a parameter (or an immutable string) you just get another pointer to the parameter.
The main use of the method is in subclasses:
[NSMutableString stringWithString: @"fdghdfjl"];
will give you a mutable autoreleased copy of the original.
One thing to note about stringWithString: is that it will throw an exception if the source string is nil.
来源：https://stackoverflow.com/questions/6280849/obj-c-nsstring-stringwithstringstring-vs-string