The idea of logging in using QR Codes is that you keep yourself logged in on your phone (using a remember-me cookie, for example) and delegate the authentication from any other (desktop) to your phone by scanning a QR code. In this way you avoid retyping your credentials on a computer which you just happen to run across.
The example is deployed here. You’ll need a mobile phone which can scan QR codes to try it for yourself (you’ll need to register first).
All in all, this method integrates nicely with the Wt::Auth framework and will work regardless of how you authenticate on your smart phone, or how you integrate the authentication widget in your application.