title: Weak Linking - check if a class exists and use that class
body: <p>I'm trying to create a universal iPhone app, but it uses a class defined only in a newer version of the SDK. The framework exists on older systems, but a class defined in the framework doesn't.</p>
  
  <p>I know I want to use some kind of weak linking, but any documentation I can find talks about runtime checks for function existence - how do I check that a class exists?</p>