title: How to set auto_ptr to NULL
body: <p>Is there a way to set an auto_ptr to NULL, or the equivalent? For instance, I'm creating a binary tree composed of node objects:</p>
  
  <pre><code>struct Node {
      int weight;
      char litteral;
      auto_ptr&lt;Node&gt; childL;
      auto_ptr&lt;Node&gt; childR;
      void set_node(int w, char l, auto_ptr&lt;Node&gt; L, auto_ptr&lt;Node&gt; R){
          weight = w;
          litteral = l;
          childL = L;
          childR = R;
      }
  };
  </code></pre>
  
  <p>For an node that is not the parent node, I had planned on doing this:</p>
  
  <pre><code>auto_ptr&lt;Node&gt; n(new Node);
  (*n).set_node(i-&gt;second, i-&gt;first, NULL, NULL);
  </code></pre>
  
  <p>This throws an error. Is there any way to set it to NULL, or is there another course of action that would make sense?</p>