title: Boost Multi-Index : Composite key of vector and int for hashed indices
body: <p>as I've just learned in <a href="https://stackoverflow.com/questions/1699704/boostmultiindexcontainer-with-randomaccess-and-orderedunique">in my other question</a>, I could use a <code>composite_key</code> for a struct, which has a <code>std::vector</code> and an integer. Now my question is: Can I use this somehow to work with hashed_indecies?</p>
  
  <p>Here an example similar to <a href="http://www.boost.org/doc/libs/1_40_0/libs/multi_index/doc/tutorial/key_extraction.html#composite_keys_hash" rel="nofollow noreferrer">THIS</a>:</p>
  
  <pre><code>struct unique_property
  {
    //the pair of int and std::vector&lt;int&gt; shall be unique
    int my_int;
    std::vector&lt;int&gt;  my_vec;
  };
  
  typedef multi_index_container&lt;
    unique_property,
    indexed_by&lt;
      hashed_unique&lt; // indexed by my_int and every entry of my_vec
        composite_key&lt;
          street_entry,
          member&lt;unique_property,int,&amp;unique_property::my_int&gt;,
          member&lt;unique_property,std::vector&lt;int&gt;,&amp;unique_property::my_vec&gt;
        &gt;
      &gt;,
      random_access&lt; &gt;
    &gt;
  &gt; property_locator;
  </code></pre>
  
  <p>The problem is (of course) that a <code>std::vector&lt;int&gt;</code> is no suitable hash-key. Can I put this code in an elegant wrapper (or something like that), to produce a hash-key from every entry of <code>my_vec</code> as well?</p>