title: How to handle default value for a templated functor
body: <p>Given a function like this:</p>
  
  <pre><code>template&lt;typename functor&gt;
  void foo(functor const&amp; f)
  {
      if (f)
          f(1, 2);
  }
  </code></pre>
  
  <p>I want to be able to assign a default value where <code>f</code> can be set to something similar to <code>NULL</code>. Also, it would be possible sufficient to make a bogus call to an empty function. Is there anything I can use (from the standard or <code>boost</code>-library) without creating such a thing by myself?</p>