title: CFUUID Vs. advertisingIdentifier Vs. identifierForVendor
body: <p>I'm confused on what should I use to uniquely identify my users?</p>
  
  <p>I want to support iOS7 too but i dont understand the differences.</p>
  
  <p>I'm currently using this snippet to generate a UUID:</p>
  
  <pre><code>CFUUIDRef udid = CFUUIDCreate(NULL);
  NSString* createdUUID = (NSString *) CFUUIDCreateString(NULL, udid);
  [createdUUID autorelease];
  CFRelease(udid);
  </code></pre>