whats wrong with this??

Qt Code: 
  1. #include <iostream>
  2. #include <QThread>
  3.  
  4. using namespace std;
  5.  
  6.  
  7. class Th1 : public QThread
  8. {
  9. public:
  10.     void run()
  11.     {
  12.         while (true)
  13.         {
  14.             cout << "X" << endl;
  15.             msleep(500);
  16.         }
  17.     }
  18. };
  19.  
  20.  
  21. class Th2 : public QThread
  22. {
  23. public:
  24.  
  25.     void run()
  26.     {
  27.         while (true)
  28.         {
  29.             cout << "Y" << endl;
  30.             msleep(500);
  31.         }
  32.     }
  33. };
  34.  
  35.  
  36. int main(int argc, char *argv[])
  37. {
  38.     Th1 *t = new Th1();
  39.     t->start();
  40.  
  41.     Th2 *h = new Th2();
  42.     h->start();
  43.     //h->run();
  44. }
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when i uncomment the last statement, it works...
im confused.