title: Use struct with forward declaratio
body: <p>I need to use a struct before it was actually declared, how is that possible ? Thank you.
  I need to use C1 before it was declared, but I get incomplete type error.</p>
  <pre><code>   struct C1;
      
      struct Cap
      {
          C1 l1;
      };
      
      struct C1 : Cap
      {
      };
  </code></pre>