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?

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.

Short of running performance tests, I am looking for a better "rule-of-thumb" understanding of why I should or shouldn't do this.

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