title: What causes iOS linking errors?
body: <p>I've been getting some strange linking errors in XCode. I understand more or less what linking errors are, just not why they are showing up in my situation.</p>
  
  <p>I have an app that started as iPhone only. When I adjusted it to be universal I got some odd linking errors. I then simply created a new universal project and imported the files, it built and executed without error. Now, working with the iPad interface, I've added some animations and am inheriting QuartzCore/QuartzCore.h but when I build, I get linking errors (shown below). What causes this sort of problem, how can I fix it, and how can I avoid it in the future?</p>
  
  <pre><code>"_OBJC_CLASS_$_CAMediaTimingFunction", referenced from:
  objc-class-ref-to-CAMediaTimingFunction in mainViewController_iPad.o
  
  "_OBJC_CLASS_$_CABasicAnimation", referenced from:
  objc-class-ref-to-CABasicAnimation in mainViewController_iPad.o
  
  "_kCAMediaTimingFunctionEaseIn", referenced from:
  _kCAMediaTimingFunctionEaseIn$non_lazy_ptr in mainViewController_iPad.o
  (maybe you meant: _kCAMediaTimingFunctionEaseIn$non_lazy_ptr)
  
  "_OBJC_CLASS_$_CAKeyframeAnimation", referenced from:
  objc-class-ref-to-CAKeyframeAnimation in mainViewController_iPad.o
  
  "_OBJC_CLASS_$_CAAnimationGroup", referenced from:
  objc-class-ref-to-CAAnimationGroup in mainViewController_iPad.o
  
  "_CATransform3DIdentity", referenced from:
  _CATransform3DIdentity$non_lazy_ptr in mainViewController_iPad.o
  (maybe you meant: _CATransform3DIdentity$non_lazy_ptr)
  
  ld: symbol(s) not found
  collect2: ld returned 1 exit status
  </code></pre>