title: Get boost::variant's type index with boost::mpl
body: <p>boost::variant has member <strong>types</strong> which is some kind of boost::mpl structure.
  Is there a way to get an index of type in that structure at compile time, so late in in runtime i could do</p>
  
  <pre><code>if(myVariantInstance.which() == typeIndex)
  {
     /*...*/
  }
  </code></pre>
  
  <p>Instead of</p>
  
  <pre><code>if(myVariantInstance.type() == typeid(ConcreteType))
  {
    /*...*/
  }
  </code></pre>