If you want to create a context menu in a QMainWindow subclass,just reimplement the virtual protected function:void contextMenuEvent(QContextMenu *event),and write your own code to handle the right clicked case to show the menu.
The following code is an easy example:
//declaration
protected:
    void contextMenuEvent(QContextMenu *event);
//reimplement
void MainWindow::contextMenuEvent(QContextMenu *event)
{
QMenu menu(this);
menu.addAction(cutAct);
menu.addAction(copyAct);
menu.addAction(pasteAct);
menu.exec(event->globalPos());
}