title: Override function
body: <p>I am taking a iOS course online provided by a famous university. I don't understand why the following code use <code>override</code> and it is legal.</p>
  
  <ol>
  <li><p>According to the official definition, we use <code>override</code> to override superclass' methods. Where is the subclass and superclass in the following code?</p></li>
  <li><p>What's been override and by what?</p></li>
  </ol>
  
  <blockquote>
  <pre><code>public override var description: String {
      return "\(url.absoluteString) (aspect ratio = \(aspectRatio))"
  }
  </code></pre>
  </blockquote>