title: How is Associative Reference implemented?
body: <p>I saw a very good sample here:
  <a href="https://stackoverflow.com/questions/5500327/subclass-uibutton-to-add-a-property">Subclass UIButton to add a property</a></p>
  
  <p>What is it? You can't add object to a category. But now with this trick you can.</p>
  
  <p>So what is it? How does it work?</p>
  
  <p>Objective-c object already have some constant number of ivar pointers right?</p>
  
  <p>Now you add another one? How did they figure that out?</p>
  
  <p>A pretty ugly notation I must admit.</p>