Q_OBJECT linker error!

I had a similar problem and it was solved using andref's feedback. Within QT Creator I simply:

1. Build/Clean all
2. Build/Run qmake
3. Build/Run

Whenever you change QObject inheritance be sure to do a clean, qmake then build. The qmake is important since it updates moc* files for any new Qt changes in your .h files including QObject inheritance, ie Q_OBJECT. In fact, in some cases, you may even be able to simply do qmake then build for an incremental build.

Check in the file MakeFile.debug and maybe HIMyClass don't exists.

I just rename MakeFile.debug, Clean the Project and Rebuild All and it compiles.

Such errors usually mean that you haven't added the header of your class to "HEADERS" variable in pro file (meta object compiler generates moc_ files only for headers listed in this variable). Remember to run qmake after you change .pro file!