title: What's a 'null defined macro'?
body: <p>I'm learning objective-C and Cocoa. In the Apple tutorial I'm working through there's a side note that says: </p>
  
  <blockquote>
    <p><code>IBOutlet</code> is a null-defined macro, which the C preprocessor removes at compile time.</p>
  </blockquote>
  
  <p>I'm curious - what's a null-defined macro?</p>