title: std::unordered_map undeclared identifier using Visual C++ 2008
body: <pre><code>#include &lt;iostream&gt;
  #include &lt;string&gt;
  #include &lt;unordered_map&gt;
  
  using namespace std;
  
  int main()
  {
      unordered_map&lt; int, string &gt; m;
  
      m[1] = "one";   
      m[2] = "two";
      m[4] = "four";
      m[3] = "three";
      m[2] = "TWO!";
      cout &lt;&lt; m[2] &lt;&lt; endl;
  
      m.clear();
      return 0;
  }
  </code></pre>
  
  <p>I am learning and can't figure it out. The compiler throws the error that type <code>unordered_map</code> is undeclared.</p>
  
  <p>I am using Visual C++ 2008 Express Edition.</p>