<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>

```
```
#define IBOutlet
```
```
Whenever IBOutlet is used in program text, it will be replaced with nothing at all.