title: Get index of a tuple element's type?
body: <p>If I have a tuple with different element types like</p>
  
  <pre><code>std::tuple&lt;T0, T1, T2, ...&gt;
  </code></pre>
  
  <p>And how to get the index of a element type?</p>
  
  <pre><code>template&lt;class T, class Tuple&gt;
  struct Index
  {
      enum {value = ?;}
  };
  </code></pre>
  
  <p>Thanks.</p>