Listing 1. Adding Slots to demo_1.h

#include <QtGui>
public:
    demo_1(QWidget *parent = 0);
    ~demo_1();

private:
    Ui::demo_1Class ui;

//--- add the following lines.
    QTimer timer ;

public slots:
 void on_pushButton_clicked();
 void on_pushButton_2_clicked();
 void on_actionSave_triggered() ;
 void timer_tick() ;
};