hello everyone,
i am trying to run an if statement with condition

if (QDate::isValid(year,month,day)) {do smth;}

yet i get the compiler error

dateinputform.ui.h:299:
error: incomplete type 'QDate' cannot be used to name a scope
make: *** [.obj/dateinputform.o] Error 1


i have of course #included <qdate.h> in my implementation..
why do i get this error?
thank you for your help.
nass