title: Is it impossible to use an STL map together with a struct as key?
body: <p>I have the following code:</p>
  
  <pre><code>struct Node
  {
    int a;
    int b;
  };
  
  Node node;
  node.a = 2;
  node.b = 3;
  
  map&lt;int, int&gt; aa;
  aa[1]=1; // OK.
  
  map&lt;Node, int&gt; bb;
  bb[node]=1; // Compile error.
  </code></pre>
  
  <p>When I tried to map an instance of my struct <code>Node</code> to an <code>int</code>, I got a compile error. Why?</p>