title: How to have an optional option value in boost program options?
body: <p>I am using the Boost program option and I want to offer an option which has three ways:</p>
  
  <ol>
  <li>If not define  </li>
  <li>If defined but no value </li>
  <li>If defined with a value</li>
  </ol>
  
  <p>For example, I have a program that works on a file such as a.jpg, and I want to offer the user to be able to use it in the following scenarios:</p>
  
  <pre><code>myapp.exe a.jpg  : process jpeg 
  myapp.exe a.jpg -e : process jpeg and generate report at the same directory as a.jpg
  myapp.exe a.jpg -e c:\tmp\ : process jpeg and generate report at c:\tmp\
  </code></pre>
  
  <p>How can I do this with Boost program options?</p>