title: Defining methods with NSString with format
body: <p>How do you define a method in your own classes that accepts an NSString with format?</p>
  
  <p>I see several different things using it like <code>NSLog</code>, <code>[NSPredicate predicateWithFormat:(NSString *)predicateFormat, ...]</code> and of course <code>[NSString stringWithFormat:(NSString *)format, ...]</code></p>
  
  <p>Also, in the header files, NSLog and stringWithFormat have the following after their declaration:
  <code>NS_FORMAT_FUNCTION(1,2)</code>. Googling didn't help much with telling me what this meant.</p>
  
  <p>Obviously the ellipsis is the format parameters, but I don't know how to deal with them in the method itself. </p>