title: specify default value of std::function
body: <pre><code>void func(const std::function&lt;void()&gt;&amp; f = empty)
  {
      if(f)
          f();
  }
  </code></pre>
  
  <p>what is the 'empty' should be? I use <code>[](){}</code> . But technically, that is not empty, the f() will execute.</p>