title: inherit from std::function, syntax and usage?
body: <p>I don't understand the syntax when a functor inherits from std::function:</p>
  
  <pre><code>class do_sth : public function&lt; any_type (another_type) &gt;
  {
      bool operator() (string s) {...}
  };
  </code></pre>
  
  <p>What should I use as any_type and another_type? And can you give me an example of a beneficial usage of inheriting from std::function since I haven't understood it yet?</p>