title: Why can't cout a scoped enum class member directly while can for unscoped enum?
body: <p><code>cout</code> unscoped enum directly works:</p>
  
  <pre><code>#include &lt;iostream&gt;
  using namespace std;
  enum  color { red, green, blue };
  
  int main()
  {
      cout &lt;&lt; color::green;
      return 0;
  }
  </code></pre>
  
  <p>While with socoped enum can't:</p>
  
  <pre><code>#include &lt;iostream&gt;
  using namespace std;
  enum class color { red, green, blue };
  
  int main()
  {
      cout &lt;&lt; color::green;
      return 0;
  }
  </code></pre>
  
  <p>What's the difference?</p>