So how do I change values on other objects? For example, I need to update a QLabel everytime secondChanged signal is called.

My new code is:

timer.h
Qt Code: 
  1. #ifndef Timer_H
  2. #define Timer_H
  3.  
  4. #include <QWidget>
  5. class QLabel;
  6.  
  7. class Timer : public QWidget
  8. {	
  9. 	Q_OBJECT
  10.  
  11. 	public:
  12. 		Timer();
  13. 		int getHour();
  14. 		int getMinute();
  15. 		int getSecond();
  16.  
  17. 		int secondValue() const;
  18.  
  19. 		void show();
  20.  
  21. 	public slots:
  22. 		void addSecond();
  23.  
  24. 	signals:
  25. 		void secondChanged(int);
  26.  
  27. 	private:
  28. 		int hour;
  29. 		int minute;
  30. 		int second;
  31.  
  32. 		QLabel *l;
  33. };
  34.  
  35. #endif
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-----------------------------------------------------------

timer.cpp
Qt Code: 
  1. #include <QtGui>
  2. #include "timer.h"
  3.  
  4. Timer::Timer() : QWidget()
  5. {
  6. 	hour = 0;
  7. 	minute = 0;
  8. 	second = 0;
  9.  
  10. 	l = new QLabel();
  11. }
  12.  
  13. void Timer::addSecond()
  14. {	
  15. 	second++;	
  16. 	if(second == 60)
  17. 	{
  18. 		minute++;
  19. 		second = 0;
  20. 		if(minute == 60)
  21. 		{
  22. 			hour++;
  23. 			minute = 0;	
  24. 		}	
  25. 	}	
  26. 	emit secondChanged(second);
  27. }
  28.  
  29. int Timer::getHour()
  30. {
  31. 	return hour;	
  32. }
  33.  
  34. int Timer::getMinute()
  35. {
  36. 	return minute;	
  37. }
  38.  
  39. int Timer::getSecond()
  40. {
  41. 	return second;	
  42. }
  43.  
  44. int Timer::secondValue() const
  45. {
  46. 	return second;	
  47. }
  48.  
  49. void Timer::show()
  50. {
  51. 	l->show();
  52. }
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-----------------------------------------------

main.cpp
Qt Code: 
  1. ...
  2. Timer *t = new Timer();			
  3. QTimer *qTimer = new QTimer();
  4. QObject::connect(qTimer, SLOT(timeout()), t, SIGNAL(addSecond()));
  5. QObject::connect(t, SLOT(secondChanged(int)), l, SIGNAL(setNum(int)));
  6. qTimer->start(1000);
  7. l->show();
  8. ...
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