title: Why is std::hash not specialised for std::reference_wrapper?
body: <p>I thought it would have been, but I can't find this in my standard library implementation (gcc-4.8.2).</p>
  
  <p>Why is <code>std::hash</code> not already specialised for <code>std::reference_wrapper</code>?</p>
  
  <pre><code>#pragma once
  #include &lt;functional&gt;
  
  namespace std 
  {
      template&lt;typename T&gt;
      struct hash&lt;reference_wrapper&lt;T&gt;&gt;
      {
          size_t operator()(const reference_wrapper&lt;T&gt;&amp; r) const
          {
              return std::hash&lt;T&gt;()(r.get());
          }
      }; 
  }
  </code></pre>