NSString setter using isEqualToString

Question

In the Pragmatic Core Data book, I came across this code snippet for an NSString setter:

- (void)setMyString:(NSString*)string; 
{
 @synchronized(self) { 
  if ([string isEqualToString:myString]) return; 
  [myString release]; 
  myString = [string retain]; 
 } 
}

Is there any reason to use [string isEqualToString:myString] instead of string == myString here? Does it not mean that if the two strings have the same content, the result will be different than if they are actually the same object? Does this matter?

Thanks.

Answer 1

Notice that the variables you're comparing are pointers to NSStrings. Pointer comparison just checks if the pointers are referring to the same address. It doesn't know anything about the content at the end. Two string objects in two different places can have the same content. Thus you need isEqualToString:. In this case, I'm not sure either that it's a terribly important distinction to make though. It would make more sense to me if it were special-casing sending out change notifications based on whether the new string would actually be a change.

Incidentally, in an NSString setter, you almost always want copy rather than retain. I don't know the exact use case in this book, but if you just retain the string and it happens to be mutable, it can change behind your back and cause weird results. And if the string isn't mutable, copy is just an alias for retain.