title: How do I get default argument values with boost program options?
body: <p>I want to use default values for some of my command line arguments.  How do I tell <code>program_options</code> what the default option is, and, if the user doesn't supply the argument, <em>how do I tell my program to use the default value</em>?</p>
  
  <p>Say I want to have an argument specifying the number of robots to send on a murderous rampage with a default value of 3.</p>
  
  <p><code>robotkill --robots 5</code> would produce <code>5 robots have begun the silicon revolution</code>, whereas
  <code>robotkill</code> (no arguments supplied) would produce <code>3 robots have begun the silicon revolution</code>.</p>