Error : undefined reference to `vtable for MyClass'

[high_flyer]

hmm... are you sure you are using the correct qmake? (Qt4 and not Qt3)
If you can, allow a full compile command output then we can see exactly what is being called.

P.S
It is REALLY not recommended to work as root!
P.P.S

3 ) i am poritng the application from Qt3.3.4 to Qt4.2.2

I assume that by porting, you are not just compiling it with Qt4 - and that you changed the code to Qt4.

Looking at the headers you are using, you are only using Qt3 headers, and Qt3 support headers from Qt4... this doesn't look correct.