title: Default value for boost::function argument?
body: <p>I've got a function that I want to take an optional boost::function argument as a callback for reporting an error condition.  Is there some special value I can use a the default value to make it optional?</p>
  
  <p>For example, with a regular function pointer I can do:</p>
  
  <pre><code>void my_func(int a, int b, t_func_ptr err_callback=NULL) {
  
     if (error &amp;&amp; (err_callback != NULL))
        err_callback();
  
  }
  </code></pre>
  
  <p>Can I do something similar with boost::function replacing the function pointer?</p>