title: Scala placeholder _ variable
body: <p>I am learning from this site
  <a href="http://naildrivin5.com/scalatour/wiki_pages/ExplcitlyTypedSelfReferences/" rel="nofollow noreferrer">http://naildrivin5.com/scalatour/wiki_pages/ExplcitlyTypedSelfReferences/</a></p>
  
  <pre><code>trait BaseComponent {
      protected var label:Label = _
  }
  </code></pre>
  
  <p>In this case what does the placeholder _ stands for?What would be the alternative?</p>