Implementation for QtCore classes

Where can I find the implementation of QtCore classes?
I was looking under /usr/include/qt4/QtCore but the only files there are .h and empty C files.

QtCore is only an alias for including all headers inside the QtCore module. Therefore there is no implementation. Look direct for the *.cpp of the classes inside that module like QString.

There isn't any .cpp file for Qt classes under /usr/include/qt4 or anywhere else in the system.

Because you don't have the source files for Qt, only the include files.

Do you know the name of the package under kubuntu? I could not find it anywhere?

If you want to take a look at the source code of Qt, it will be easier just to download it:

http://qt.nokia.com/downloads/linux-x11-cpp