hello i am trying to get this program work but it returns me error

QString.h no such file or directory error!

here is the code
Qt Code: 
  1. #ifndef PUSHBUTTON_H
  2. #define PUSHBUTTON_H
  3. #include <QPushButton>
  4. #include <QString> //here is where its showing error
  5.  
  6. class PushButton: public QPushButton
  7. {
  8.     Q_OBJECT
  9. public:
  10.     PushButton(QWidget* parent = 0): QPushButton(parent){}
  11.     PushButton(const QString& text, QWidget* parent = 0) : QPushButton(text, parent){}
  12. public slots:
  13.     void updatenewvalues()
  14.     {
  15.         if(this->text() == "Don't click me!")
  16.         {
  17.             setText("Click me!");
  18.             return;
  19.         }
  20.         setText("Don't click me!");
  21.     }
  22. };
  23. #endif // PUSHBUTTON_H
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Qt Code: 
  1. int main(int argc, char* argv[])
  2. {
  3.     QApplication app(argc, argv);
  4.     PushButton clickme("Click me!");
  5.     clickme.resize(200, 30);
  6.     clickme.setFont(QFont("Times", 18, QFont::Bold));
  7.     QObject::connect(&clickme, SIGNAL(clicked()), &clickme, SLOT(updatenewvalues()));
  8.     clickme.show();
  9.     return app.exec();
  10. }
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Thanks!