title: How Qt does widget calculate its size?
body: <p>When I use <code>QWidget</code> to design user interface, I am always a little confused regarding its size properties. There's <code>size policy</code>, <code>geometry</code> and <code>hintSize</code>.</p>
  
  <p>I just know the relationship between <code>size policy</code> and <code>hintSize</code>: the <code>size policy</code> is based on <code>sizehint</code>. But I don't know other relationship between these properties. </p>
  
  <p>I want to have a widget which size can be automatically changed based on parent or main window size. But I don't know how to use the three properties to finish this goal.</p>