It is the declaration of a private member variable called "u" that is a pointer to an object of type UserWindow. The type UserWindow must be declared earlier in the file either in full or as a forward declaration (which is all that is required for a pointer). As far as I can see, the code in BalaQt's post does not contain the necessary declaration.

For this to compile you would need to add either:
Qt Code: 
  1. // include the declaration of the UserWindow class
  2. #include "userwindow.h"
  3. OR
  4. // a forward, incomplete declaration
  5. class UserWindow;
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near the top of LoginDialog.h (adjust the names as needed).