Objective-C String Differences

What's the difference between NSString *myString = @"Johnny Appleseed" versus NSString *myString = [NSSt开发者_运维技巧ring stringWithString: @"Johnny Appleseed"]?

Where's a good case to use either one?

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The other answers here are correct. A case where you would use +stringWithString: is to obtain an immutable copy of a string which might be mutable.

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In the first case, you are getting a pointer to a constant NSString. As long as your program runs, myString will be a valid pointer. In the second, you are creating an autoreleased NSString object with a constant string as a template. In that case, myString won't point to a real object anymore after the current run loop ends.

Edit: As many people have noted, the normal implementation of stringWithString: just returns a pointer to the constant string, so under normal circumstances, your two examples are exactly the same. There is a bit of a subtle difference in that Objective-C allows methods of a class to be replaced using categories and allows whole classes to be replaced with class_poseAs. In those cases, you might run into a non-default implementation of stringWithString:, which may have different semantics than you expect it to. Just because it happens to be that the default implementation does the same thing as a simple assignment doesn't mean that you should rely on subtle implementation-specific behaviour in your program - use the right case for the particular job you're trying to do.

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Other than syntax and a very very minor difference in performance, nothing. The both produce the exact same pointer to the exact same object.

Use the first example. It's easier to read.

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In practice, nothing. You wouldn't ever use the second form, really, unless you had some special reason to. And I can't think of any right now.

(See Carl's answer for the technical difference.)