error: forward declaration of 'struct QPushButton'

OS: Ubuntu 8.10 x86.
Qt version: 4.4.3

Issue:

Having a QDialogButtonBox object, when I am trying to do

Code:

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buttonBox->button(QDialogButtonBox::Ok)->setEnabled(false);

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I get the error:

dialog.cpp:39: error: invalid use of incomplete type 'struct QPushButton'
/usr/include/qt4/QtGui/qdialog.h:49: error: forward declaration of 'struct QPushButton'

Any ideas about how to fix this problem?

Thanks.

Code:

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#include <QPushButton>

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Thanks a lot.