title: cbClsExtra and cbWndExtra
body: <p>I want more detailed information about <code>cbClsExtra</code> and <code>cbWndExtra</code> <code>WNDCLASSEX</code> members that are used in <code>RegisterClassEx</code> winapi.</p>
  
  <p><code>MSDN</code> says that this members can be used to set size of extra bytes that will be allocated for class and for each window instance accordingly.</p>
  
  <p><code>MSDN</code> says </p>
  
  <blockquote>
    <p>The system initializes the bytes to zero.</p>
  </blockquote>
  
  <p>1) Does this mean that bytes are initialized with zero value or system allocates zero (none) bytes by default?</p>
  
  <p>2) The most important question is how to use this extra bytes (provide examples please with winapi used) and how they are used most common?</p>
  
  <p>Thanks in advance.</p>