title: Why override operator()?
body: <p>In the <a href="http://www.boost.org/doc/html/signals.html" rel="noreferrer" title="Boost Signals documentation">Boost Signals</a> library, they are overloading the () operator.</p>
  
  <p>Is this a convention in C++? For callbacks, etc.?</p>
  
  <p>I have seen this in code of a co-worker (who happens to be a big Boost fan). Of all the Boost goodness out there, this has only led to confusion for me.</p>
  
  <p>Any insight as to the reason for this overload?</p>