1.
The QActionGroup class groups actions together by the way of exclusive.
For example:
QActionGroup alignmentGroup=new QActionGroup(this);
alignmentGroup->addAction(leftAlign);
alignmentGroup->addAction(rightAlign);
alignmentGroup->addAction(centerAlign);
leftAlign->setChecked(true);

2.
Use QAction::setStatusTip() to set the status tip about current operation.

3.A problem that sounds stupid

I don't know what causes I lost my shutdown and reboot button.I shutdown my computer only by the way of command 'shutdown' or 'reboot'.First,I thought that Gnome Panel init failed,so I reboot my computer but there was also no buttons.There was only log out,lock screen,switch user,hiberinate,suspend buttons.
I searched it on Internet.Finally I found the solution.
Go to System/Administration/Login Window and make sure that on the local tab,Menu bar "show actions Menu" is checked.