title: Forward declaring classes in namespaces
body: <p>I was rather surprised to learn that I couldn't forward declare a class from another scope using the scope resolution operator, i.e.</p>
  
  <pre><code>class someScope::someClass;
  </code></pre>
  
  <p>Instead, the full declaration has to be used as follows:</p>
  
  <pre><code>namespace
  {
      class someClass;
  }
  </code></pre>
  
  <p>Can someone explain why this is the case?</p>
  
  <p>UPDATE: To clarify, I am asking <strong><em>why</em></strong> this is the case.</p>