title: What's difference between #pragma and #ifndef?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/1143936/pragma-once-vs-include-guards">#pragma once vs include guards?</a>  </p>
  </blockquote>
  
  
  
  <p>When should I use <code>#pragma once</code>?</p>
  
  <p>When should I use <code>#ifndef HEADER_H_INCLUDED</code>?</p>