title: Why is allocating or initializing NSDateFormatter considered "expensive"?
body: <p>What do people mean when they say this is expensive? I create instances of a lot of transient objects just for intermediate storage (NSString and NSDate are typical ones). How do I know if my program use of NSDateFormatter is overdoing it?</p>
  
  <p>Until now, I have tended to create what amounts to a singleton, but my preference would be to encapsulate it into some of the other objects it is associated with so I can use self references. </p>
  
  <p>Short of running performance tests, I am looking for a better "rule-of-thumb" understanding of why I should or shouldn't do this.</p>