title: What is boost::program_options::notify() for?
body: <p>This question is about the C++ Boost program_options library.</p>
  
  <p>All the tutorials are very clear that I should call <code>notify()</code> on my completed variable map, but I'm not sure what this is actually doing for me. Commenting it out didn't seem to have any effect, and the documentation didn't go into much detail:</p>
  
  <p><a href="http://www.boost.org/doc/libs/1_47_0/doc/html/boost/program_options/notify.html">http://www.boost.org/doc/libs/1_47_0/doc/html/boost/program_options/notify.html</a></p>
  
  <p>Other sources suggest that it runs "user-defined" functions. If so, how are those functions registered and what do they do? Might they throw exceptions? </p>