  A free function as a completion handler:
      </p>
<pre class="programlisting">void completion_handler()
{
  ...
}
</pre>
<p>
        A completion handler function object:
      </p>
<pre class="programlisting">struct completion_handler
{
  ...
  void operator()()
  {
    ...
  }
  ...
};
</pre>
<p>
        A lambda as a completion handler:
      </p>
<pre class="programlisting">my_io_service.post(
    []()
    {
      ...
    });
</pre>
<p>
        A non-static class member function adapted to a complet