QApplication::desktop() incomplete type

[codebehind]

I've done a new step - move to QT4

i try to port some old test apps and the next line of code:

width=QApplication::desktop()->width();

width=QApplication::desktop()->width();

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returns error while compiling

error: invalid use of incomplete type ‘struct QDesktopWidget’

error: invalid use of incomplete type ‘struct QDesktopWidget’

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i haven't noticed any diff between Qt3 and Qt4 docs.