title: 1 Header File, 3 CPP Files. Where do you put the common functions?
body: <p>Let's assume I have the following files:</p>
  
  <ul>
  <li>polygon.h</li>
  <li>rectangle.cpp</li>
  <li>triangle.cpp</li>
  <li>circle.cpp</li>
  </ul>
  
  <p>Rectangle, Triangle, and Circle will all have different implementation of <code>calculateArea</code>, but they all sharing the same function of <code>getNumSides</code>. Where do you put <code>getNumSides</code>?</p>
  
  <p>Would be good if someone can help me design this</p>