class MWindow: public QMainWindow, public MainWindow
{
...
Public:
Page *currentPage;
public slots:
void setCurrent();
....
};

MWindow::MWindow(QWidget *parent): QMainWindow(parent)
{
...
connect(tabs,SIGNAL(currentChanged(int)), this, SLOT(setCurrent(int)));
connect(action_Select_all,SIGNAL(triggered()),currentPage->Source,SLOT(selectAll()));
...
}

void MWindow::setCurrent(int nIndex)
{
currentPage=tab->widget(nIndex);
}