title: How to #include hash with ext, tr1, or __gnu_cxx in XCode, C++
body: <p>I'm trying to work with the <a href="http://google-sparsehash.googlecode.com/svn/trunk/doc/sparse_hash_map.html" rel="nofollow">google-sparsehash library</a> and I'd like to include the hash library described in the link,</p>
  
  <pre><code>using ext::hash;  // or __gnu_cxx::hash, or maybe tr1::hash, depending on your OS
  </code></pre>
  
  <p>and I've tried one of each:</p>
  
  <pre><code>#include &lt;ext/hash&gt;
  #include &lt;ext&gt;
  #include &lt;__gnu_cxx&gt;
  #include &lt;tr1&gt;
  </code></pre>
  
  <p>which none worked with XCode. I've also "using", where I was told that __gnu_cxx does not contain "hash".   How do I describe this library to XCode (3.2.6) on OS X (10.6.8)? </p>
  
  <p>Or more generally,  where is this hash function described in a Mac / XCode? </p>