title: Invalid use of qualified name
body: <p>I'm trying the following:</p>
  
  <pre><code>#include &lt;iostream&gt;
  
  namespace A
  {
      extern int j;
  }
  
  int main()
  {
      int A::j=5;
      std::cout &lt;&lt; A::j;
  }
  </code></pre>
  
  <p>But I've <code>error: invalid use of qualified-name ‘A::j’</code>. Please explain why this error occurred?</p>