This problem persist since a long time...When I create a QObject class file , sometime I realize I forgot to add "Q_OBJECT" macro after some compiling ..
So, I add it , and I get the famous error : "undefined reference to vtable for ..."
So, Actually, I remove the shadow build directory each time, because all actions from "Compiler" menu doesn't solve the problem . ( Clean all, recompile all, etc... )